Author: Renu Maurya

Structure Of Rna

STRUCTURE OF RNA

Q. 1

All of the following statements about Ribozymes are false, EXCEPT:

 A

They are DNA molecules

 B

They are not present in ribosomes

 C

They plays a key role in RNA synthesis

 D

They play a key role in post-transcriptional conversion of pre-mRNA to mature mRNA

Q. 1

All of the following statements about Ribozymes are false, EXCEPT:

 A

They are DNA molecules

 B

They are not present in ribosomes

 C

They plays a key role in RNA synthesis

 D

They play a key role in post-transcriptional conversion of pre-mRNA to mature mRNA

Ans. D

Explanation:

Ribozymes are RNAs which exhibit highly substrate specific catalytic activity.
They play a key role in the intron transcripts excision and exon transcripts splicing events essential for the conversion of pre mRNA to mature mRNA.
 
Ribozymes catalyzed transesterification, and ultimate hydrolysis of phosphodiester bonds in RNA molecules. 
 
Ref: Textbook Of Medical Biochemistry (3Rd Edn.) By S. Ramakrishnan page 338.

Q. 2

mRNA is a complimentary copy of ‑

 A

TRNA

 B

RRNA

 C

Ribosomal DNA

 D

A single strand of DNA

Q. 2

mRNA is a complimentary copy of ‑

 A

TRNA

 B

RRNA

 C

Ribosomal DNA

 D

A single strand of DNA

Ans. D

Explanation:

D i.e. A single strand of DNA


Q. 3

True about ribozymes are A/E

 A

Catalytic activity

 B

Involved in transesterification

 C

Hammerhead metallo enzyme

 D

Deamination

Q. 3

True about ribozymes are A/E

 A

Catalytic activity

 B

Involved in transesterification

 C

Hammerhead metallo enzyme

 D

Deamination

Ans. D

Explanation:

D i.e. Deamination

Ribozyme is RNA with catalytic activityQ. Examples include selfsplicing group I introns, RNase P, & hammerhead metallo enzymeQ of virusoids (requiring Mg2+). These are involved in transesterification, phosphodiester bond hydrolysis (cleavage), RNA metabolism (splicing & endoribonuclease), peptide bond formation (peptidyl transferases) and site specific RNA cleavage.

Quiz In Between


Q. 4

Shine-Dalgarno sequence in bacterial mRNA is near:

 A

AUG codon

 B

UAA codon

 C

UAG codon

 D

UGA codon

Q. 4

Shine-Dalgarno sequence in bacterial mRNA is near:

 A

AUG codon

 B

UAA codon

 C

UAG codon

 D

UGA codon

Ans. A

Explanation:

A i.e. AUG Codon


Q. 5

Ribozymes is/are :

 A

Splicing of heterogenous RNA (hnRNA) to form mRNA

 B

Splicing of polypeptide chain and mRNA

 C

Transcription of mRNA

 D

All

Q. 5

Ribozymes is/are :

 A

Splicing of heterogenous RNA (hnRNA) to form mRNA

 B

Splicing of polypeptide chain and mRNA

 C

Transcription of mRNA

 D

All

Ans. A

Explanation:

Q. 6

Poly ‘A’ tail attached at 3′ end of mRNA helps in‑

 A

Unwinding of mRNA

 B

Stabilization of mRNA

 C

Polymerization of mRNA

 D

Transcription of mRNA

Q. 6

Poly ‘A’ tail attached at 3′ end of mRNA helps in‑

 A

Unwinding of mRNA

 B

Stabilization of mRNA

 C

Polymerization of mRNA

 D

Transcription of mRNA

Ans. B

Explanation:

Ans. is ‘b’ i.e., Stabilization of mRNA

At the 51-end, mRNA possesses a 7-methylguanosine triphosphate cap which helps in the recognition of mRNA in protein biosynthesis and it helps to stabilize the mRNA by preventing attack of 51-exonuclease.

At its 31-end, there is a poly-A tail made up of several adenylate residues which stabilize mRNA by preventing attack of 31-exonuclease.

Quiz In Between


Q. 7

3′ end of t-RNA posseses ‑

 A

Poly ‘A’ tail

 B

CCA sequence

 C

Anticodon

 D

D arm

Q. 7

3′ end of t-RNA posseses ‑

 A

Poly ‘A’ tail

 B

CCA sequence

 C

Anticodon

 D

D arm

Ans. B

Explanation:

 

t-RNA molecule get folded into a structure that appears like a clover leaf.

There are four arms.

  1. Acceptor arm : It consists of a base paired stem that terminates in the sequence CCA at the 3′ end. This is the attachment site for amino acids.
  2. D arm – It contains the base dihydrouridine (D).
  3.  Anticodon arm – It contains anticodon that base pairs with the codon of coming mRNA. Anticodon has nucleotide sequence complementary to the codon of mRNA and is responsible for the specificity of the t RNA.
  4. Tyr C arm : It contains both ribothymidine (T) and pseudouridine

Q. 8

True about tRNA ‑

 A

80% of total RNA

 B

Contains 50-60 nucleotides

 C

CCA sequence is transcribed

 D

Longest RNA

Q. 8

True about tRNA ‑

 A

80% of total RNA

 B

Contains 50-60 nucleotides

 C

CCA sequence is transcribed

 D

Longest RNA

Ans. C

Explanation:

 

“The CCA tail is a CCA sequence at 3′ end of the tRNA molecule. In prokaryotes, CCA sequence is transcribed. In eukaryotes, the CCA sequence is added during processing”.

“tRNA is the smallest of three major species of RNAs”   — Dinesh puri

tRNA comprises 15% of total RNA in the cell. It contains 73-93 nucleotide residue.


Q. 9

Both DNA and RNA are present in:

 A

Bacteria

 B

Prions

 C

Virioids

 D

Plasmid

Q. 9

Both DNA and RNA are present in:

 A

Bacteria

 B

Prions

 C

Virioids

 D

Plasmid

Ans. A

Explanation:

Ans. a. Bacteria

Both DNA and RNA are pre sent in Bacteria

Micro-organism Genetic Material
Bacteria Contain both DNA and RNA, as well as extra-chromosomal DNA material (plasmids)Q
Plasmid

Extra-choromosomal circular DNA present in cytoplasm of bacteria and capable of autonomous

Viroids Contain low molecular weight RNA (No DNA)Q
Prions

Are misfolded proteinsQ

Devoid of both DNA and RNA

 

Quiz In Between


Q. 10

Synthesis of rRNA takes place in

 A

Cytosol

 B

Nucleus

 C

Nucleolus

 D

Mitochondria

Q. 10

Synthesis of rRNA takes place in

 A

Cytosol

 B

Nucleus

 C

Nucleolus

 D

Mitochondria

Ans. C

Explanation:

Ans. is ‘c i.e., Nucleolus 

Organelle

Function

Nucleolus Site of synthesis of r-RNA
Ribosomes

Site of protein synthesis, translation of mRNA

RER / Site of protein synthesis
Granular ER

(e.g. hormones, proteins found in enzyme)

SER / Agrannlar

Site of steroid synthesis I0

ER detoxification / FA elongation

Golgi Body

Processing / packaging, intracellular sorting of proteins, formation of lysosomes
Lysosomes Contain digestive / lytic enzymes and hydrolases (suicidal bags of cell)
Peroxisomes

Contain oxidases


Q. 11

RNA which contains codon for speicific amino acid ‑

 A

tRNA

 B

rRNA

 C

mRNA

 D

None

Q. 11

RNA which contains codon for speicific amino acid ‑

 A

tRNA

 B

rRNA

 C

mRNA

 D

None

Ans. C

Explanation:

Ans. is ‘c’ i.e., m RNA

The m RNA carries genetic information in the form of codons.

  • Codons are a group of three adjacent nucleotides that code for the amino acids of protein.
  • Each mRNA molecule is a transcript of antisense or template strand of a particular gene.
  • Its nucleotide sequence is complementary to that of antisense or template strand of the gene, i.e. adenine for thyamine, guanine for cytosine, uracil for adenine (as RNA does not contain thymine) and cytosine for guanine.
  • For example, if antisense strand of DNA has a gene with sequence 5′-TTACGTAC-3′, its complementary RNA transcript will be 5 ‘-GUACGUAA-3’.

Q. 12

RNA is present in ‑

 A

Cytoplasm

 B

Nucleus

 C

Ribosome

 D

All of the above

Q. 12

RNA is present in ‑

 A

Cytoplasm

 B

Nucleus

 C

Ribosome

 D

All of the above

Ans. D

Explanation:

Ans. is `d’ i.e., All of the above

  • mRNA is synthesized from DNA by the process of transcription in the nucleus. 
  • After formation mRNA transport out of the nucleus into cytoplasm.
  • t-RNA is also synthesized in nucleus and is transported to cytoplasm.
  • Protein synthesis (translation) occurs in ribosomes, and requires both mRNA and tRNA.
  • rRNA is present in ribosomes.
  • rRNA is synthesized in nucleolus

Thus, RNA can be found in –

  1. Nucleus
  2. Cytoplasm
  3. Ribosome
  4. Nucleolus

Quiz In Between



Structure Of Rna

STRUCTURE OF RNA


STRUCTURE OF RNA

  • RNA is a polymer of ribonucleotides held together by 3’-5’ phosphodiester bridges.
  • Sugar present in RNA is ribose.
  • RNA contains uracil.
  • RNA is a single stranded
  • Mainly seen in cytoplasm.
  • It does not obey Chargaff’s rule.
  • Types of RNA-
  1. Messenger RNA (mRNA)- 5-10%
  2. Transfer RNA (tRNA)- 10-20%
  3. Ribosomal RNA (rRNA)- 50-80%

1. Messenger RNA (mRNA)-

  • The mRNA is synthesized in the nucleus as heterogeneous nuclear RNA (hn RNA) processed into functional m RNA.
  • mRNA carries information from the nucleus to the ribosome.
  • 5’ end capped by 7 Methylguanosine triphosphate.
  • 3’ end contains 20-250 polymer of adenylate residues.

2. Ribosomal RNA (rRNA)-

  • Most abundant RNA & play an important role in protein synthesis and binding of mRNA to ribosomes.
  • The eukaryotic ribosomes are composed of two major nucleoprotein complexes- 60S and 40S subunit.
  • 28S, 18S and 5S are the major varieties.

3. Transfer RNA (tRNA)-

  • It is the smallest tRNA having 73 to 93 nucleotide residues.
  • L shaped tertiary structure.
  • Clover leaf shape in the secondary structure.
  • It transport amino acids in an activated form from cytosol to ribosome for protein synthesis.
  • tRNA contain significant proportion of nucleosides with unusual bases are-
  1. DihydroUridine
  2. Pseudouridine
  3. Inosine
  4. Ribothymidine
  • tRNA is the only RNA that can contain thymine.
  • tRNA mainly contains 4 arms-
  1. Acceptor arm- site of attachment of amino acid. Terminations in the sequence CCA at the 31 end.
  2. Anticodon arm-
  3. D arm

4. TψC arm

  •  miRNA and siRNAMain function- it cause inhibition of gene expression.
  • P bodies- non translating mRNA form ribonucleoprotein particles and they accumulate in cytoplasmic organelle.
  • Ribozymes- some ribosomes have intrinsic catalytic activity.
  • It involves cleavage of nucleic acid.
  • 2 types of ribozymes are-
  1. Peptidyl transferase
  2. Ribozymes involved in RNA splicing.

Exam Important

  • mRNA contains a poly A tail.
  • Poly-A tail made up of several adenylate residues which stabilize mRNA by preventing attack of 3′-exonuclease.
  • tRNA may be modified and known as modified or unusual bases.
  • Modified base is altered purine or pyrimidine.
  • Acceptor arm in tRNA has CCA tail is added during post-transcriptional modification.
  • rRNA is synthesized in nucleolus.
  • Specific Nucleaees lnvolved in the Post Transcriptional modification of miRNA and siRNA is- Drosha-DGCR8 Nucleases
Don’t Forget to Solve all the previous Year Question asked on STRUCTURE OF RNA

Module Below Start Quiz

Alkaptonuria

Alkaptonuria

Q. 1

Alkaptonuria is caused by defect in which of the following enzymes?

 A

Enolase

 B

Homogentisate oxidase

 C

Pyruvate carboxylase

 D

None of the above

Q. 1

Alkaptonuria is caused by defect in which of the following enzymes?

 A

Enolase

 B

Homogentisate oxidase

 C

Pyruvate carboxylase

 D

None of the above

Ans. B

Explanation:

Alkaptonuria was first recognized and described in the 16th century.
Characterized in 1859, it provided the basis for Garrod’s classic ideas concerning heritable metabolic disorders.
The defect is lack of homogentisate oxidase. The urine darkens on exposure to air due to oxidation of excreted homogentisate.
Late in the disease, there is arthritis and connective tissue pigmentation (ochronosis) due to oxidation of homogentisate to benzoquinone acetate, which polymerizes and binds to connective tissue.
Ref: Harper 28th edition, chapter 29.

 


Q. 2

In alkaptonuria there is increased pigmentation in all of the following locations, EXCEPT: –

 A

Eyes

 B

Nose

 C

Ear

 D

Articular cartilage

Q. 2

In alkaptonuria there is increased pigmentation in all of the following locations, EXCEPT: –

 A

Eyes

 B

Nose

 C

Ear

 D

Articular cartilage

Ans. B

Explanation:

Alkaptonuria is an autosomal recessively inherited deficiency of homogentisic acid oxidase enzyme which is involved in the metabolism of phenylalanine and tyrosine. Due to this defect, an oxidation product gets deposited in the cartilage throughout the body. In these patients examination of the skin shows a slight darkish blue color below the skin in areas overlying cartilage such as in the ears, sclera, conjunctiva and cornea. Metabolites also get deposited in the heart valves leading aortic or mitral stenosis. 

 
Patients often develop back pain due to spondylitis. It can be differentiated from ankylosing spondylitis by the absence of fusion of sacroiliac joint.  Diagnosis can be made when urine turns black spontaneously when exposed to air due to the presence of homogentisic acid. 
 
REf: CURRENT Medical Diagnosis & Treatment 2014 chapter 40.

Quiz In Between



Alkaptonuria

ALKAPTONURIA


 Alkaptonuria (Black Urine Disease)

  • It is due to deficiency of homogentisate oxidase.
  • Inheritance- autosomal recessive disorder.
  • It is a first inborn error detected
  • It belongs to Gerrad’s Tetrad.
  • Homogentisate accumulates in tissues and blood and is excreted into urine.

 Biochemical Defect-

  • Homogentisic acid is oxidised by polyphenol oxidase to benzoquinone acetate then polymerized to alkaptone bodies.

Clinical Features-

  • Blackening of Urine.
  • Alkapton deposition occurs in sclera, ear, nose, cheeks and intervertebral disc resulting in a condition called Ochronosis.
  • Leading to pigmentation.
  • Arthritis.
  • *No mental retardation.

Diagnosis-

  • Alkalanization increases darkening of urine.
  • Benedict’s test is positive in urine.
  • Ferric chloride test is positive.
  • Silver nitrate is positive.

 Treatment-

  • Low phenylalanine diet
  • Symptomatic treatment.
  • New drug is Nitisinone.

Exam Important

  • Belongs to Gerrads Terad ( Alkaptonuria, Albinism, Pentosuria, Cystinuria)
  • No mental retardation is seen in alkaptonuria
Don’t Forget to Solve all the previous Year Question asked on ALKAPTONURIA

Module Below Start Quiz

Metabolism of Aromatic Amino Acids

Metabolism of Aromatic Amino Acids

Q. 1

Which of the following is a primary ketone body that is formed from leucine, lysine, phenylalanine and tyrosine?

 A

Acetoacetate

 B

Acetone

 C

Beta hydroxy butyrate

 D

All of the above

Q. 1

Which of the following is a primary ketone body that is formed from leucine, lysine, phenylalanine and tyrosine?

 A

Acetoacetate

 B

Acetone

 C

Beta hydroxy butyrate

 D

All of the above

Ans. A

Explanation:

Acetoacetate is a primary ketone body. Beta hydroxy butyrate and acetone are secondary ketone bodies. Acetoacetate can be formed from acetyl CoA, it can also be formed by the degradation of carbon skeleton of ketogenic amino acids like leucine, lysine, phenylalanine and tyrosine.

Acetone is formed by the degradation of carbon skeleton of acetoacetate. Beta hydroxy butyrate is formed by the reduction of acetoacetate
 
Ref: Textbook of Biochemistry By D M Vasudevan, 3rd Edition, Page 131

 


Q. 2

Which of the following is caused by defective tyrosine metabolism?

 A

Richner-Hanhart syndrome

 B

Neonatal tyrosinemia

 C

Alkaptonuria

 D

All of the above

Q. 2

Which of the following is caused by defective tyrosine metabolism?

 A

Richner-Hanhart syndrome

 B

Neonatal tyrosinemia

 C

Alkaptonuria

 D

All of the above

Ans. D

Explanation:

Metabolic diseases of tyrosine catabolism include tyrosinosis, Richner-Hanhart syndrome, neonatal tyrosinemia, and alkaptonuria.
 
Ref: Harper 28th edition, chapter 29.

 


Q. 3

Which of the following is not synthesised from tyrosine?

 A

Norepinephrine

 B

Melatonin

 C

Thyroxine

 D

Dopamine

Q. 3

Which of the following is not synthesised from tyrosine?

 A

Norepinephrine

 B

Melatonin

 C

Thyroxine

 D

Dopamine

Ans. B

Explanation:

Quiz In Between


Q. 4

Precursor of tyrosine is:  

 A

Cysteine

 B

Histidine

 C

Tryptophan

 D

Phenylalanine

Q. 4

Precursor of tyrosine is:  

 A

Cysteine

 B

Histidine

 C

Tryptophan

 D

Phenylalanine

Ans. D

Explanation:

 

Amino acids and products

  • Cysteine forms glutathione, taurine etc.
  • Histidine forms histamine
  • Tryptophan forms serotonin, melatonin etc.
  • Under normal circumstances, the degradation of phenylalanine mostly occurs through tyrosine. Phenylalanine is hydroxylated at para-position by phenylalanine hydroxylase to produce tyrosine (p-hydroxy phenylalanine)

Q. 5

Melanin is synthesized from:        

 A

Tryptophan

 B

Tyrosine

 C

Methionine

 D

Taurine

Q. 5

Melanin is synthesized from:        

 A

Tryptophan

 B

Tyrosine

 C

Methionine

 D

Taurine

Ans. B

Explanation:

Q. 6

Tyrosine becomes essential in which of the following condition:          

 A

Wilsons disease

 B

Alkaptonuria

 C

Thyrosinosis

 D

Phenylketonuria

Q. 6

Tyrosine becomes essential in which of the following condition:          

 A

Wilsons disease

 B

Alkaptonuria

 C

Thyrosinosis

 D

Phenylketonuria

Ans. D

Explanation:

 

The amino acids arginine, cysteine, glycine, glutamine, histidine, proline, serine and tyrosine are considered conditionally essential, meaning they are not normally required in the diet, but must be supplied exogenously to specific populations that do not synthesize it in adequate amounts.

Individuals living with PKU must keep their intake of phenylalanine extremely low to prevent mental retardation and other metabolic complications.

However, phenylalanine is the precursor for tyrosine synthesis.

Without phenylalanine, tyrosine cannot be made and so tyrosine becomes essential in the diet of PKU patients.

Quiz In Between


Q. 7

Tyrosine enters gluconeogenesis by forming which substrate

 A

Succinyl CoA

 B

oc-ketoglutarate

 C

Fumarate

 D

Citrate

Q. 7

Tyrosine enters gluconeogenesis by forming which substrate

 A

Succinyl CoA

 B

oc-ketoglutarate

 C

Fumarate

 D

Citrate

Ans. C

Explanation:

 

  • TCA cycle intermediates are substrate for gluconeogenesis.
  • Gluconeogenic amino acids enter TCA cycle after their transamination into various intermediates of TCA cycle :‑

a)     Histidine, proline, glutamine and arginine are converted to glutamate which is then transaminated to a­ketoglutarate.

b)       Isoleucine, methionine and valine enter by conversion into succinyl CoA. Propionate (a short chain fatty acid) also enter at this level.

c)     Tyrosine, and phenylalanine enter by conversion into fumarate.

d)     Tryptophan is converted to alanine which is then transaminated to pyruvate.

e)     Hydroxyproline, serine, cysteine, threonine and glycine enter by conversion into pyruvate.


Q. 8

Tyrosine kinase receptor is activated by ‑

 A

Growth hormone

 B

Insulin

 C

TSH

 D

Glucagon

Q. 8

Tyrosine kinase receptor is activated by ‑

 A

Growth hormone

 B

Insulin

 C

TSH

 D

Glucagon

Ans. B

Explanation:

Ans. is ‘b’ i.e., Insulin


Q. 9

Which of the following is derived from tyrosine ‑

 A

Melatonin

 B

Serotonin

 C

Melanin

 D

Niacin

Q. 9

Which of the following is derived from tyrosine ‑

 A

Melatonin

 B

Serotonin

 C

Melanin

 D

Niacin

Ans. C

Explanation:

Ans. is ‘c’ i.e., Melanin

Tyrosine is a precursor of many important compounds such as catecholamines (epinephrine, norepinephrine ), dopamine), thyroxine, triiodothryonine, melanin.

Quiz In Between


Q. 10

Tyrosine utilized in synthesis of all except ‑

 A

Melanin

 B

Melatonin

 C

Dopamine

 D

Thyroxine

Q. 10

Tyrosine utilized in synthesis of all except ‑

 A

Melanin

 B

Melatonin

 C

Dopamine

 D

Thyroxine

Ans. B

Explanation:

Q. 11

Enzyme deficient in tyrosinemia type 1 ‑

 A

Phenylalanine hydroxylase

 B

Tyrosinase

 C

Fumarylacetoacetate hydroxylase

 D

Tyrosine transaminase

Q. 11

Enzyme deficient in tyrosinemia type 1 ‑

 A

Phenylalanine hydroxylase

 B

Tyrosinase

 C

Fumarylacetoacetate hydroxylase

 D

Tyrosine transaminase

Ans. C

Explanation:

 

Tyrosinemia

It is a defect in metabolism of tyrosine. It may be of following types :-

  1. Tyrosinemia type-I (tyrosinosis/hepatorenal syndrome) :- It is due to defect in fumarylacetoacetate hydroxylase deficiency. Patients with chronic tyrosinosis are prone to develop cirrhosis and hepatic carcinoma. There is cabbage like odor in acute tyrosinosis.
  2. Tyrosinemia type – II (Richer-Hanhart syndrome) :- It is due to deficiency of tyrosine transaminase (tyrosine aminotrans-ferase).
  3. Neonatal tyrosinemia : – It is due to deficiency of hydroxyphenyl pyruvate hydroxylase.

Q. 12

Catecholamines are synthesized from

 A

Tryptophan

 B

Tyrosine

 C

Methionine

 D

Histidine

Q. 12

Catecholamines are synthesized from

 A

Tryptophan

 B

Tyrosine

 C

Methionine

 D

Histidine

Ans. B

Explanation:

Ans. is ‘b’ i.e., Tyrosine 

  • Catecholamines (epinephrine, norepinephrine and dopamine) are synthesized from tyrosin.
  • Has been explained in previous seesions.

Quiz In Between



Metabolism of Aromatic Amino Acids

Metabolism of Aromatic Amino Acids


METABOLISM OF AROMATIC AMINO ACID

  • Phenylalanines, Tyrosine, Tryptophan are aromatic amino acids.
  • Phenylalanine is an essential amino acid.
  • Tyrosine is a non- essential amino acid.
  • Phenylalanineconversion to tyrosine can reduce the dietary requirement of phenylalanine and the phenomenon is called as sparing action of tyrosine on phenylalanine.
  • It is partly glucogenic and partly ketogenic.
  • It is hydrophobic amino acid.
  • Tyrosine- is synthesized from phenylalanine.
  • It is partly glucogenic and partly ketogenic.
  • Tyrosine is involved in the synthesis of-
  1. epinephrine
  2. norepinephrine
  3. Dopamine
  4. Thyroid hormones (thyroxine and triidothryonine)
  5. Melanin pigment. 
  • Conversion of phenylalanine to tyrosine– Phenylalanine is hydroxylated by phenylalanine hydroxylase (in liver) to produce tyrosine.
  • It needs NADPH, NADH and tetrahydrobiopterine as conenzymes.
  • It is an irreversible reaction.
  • Catabolism of Tyrosine-
  • Phenylalanine is converted to tyrosine.
  • Enzymes are-
  1. Tyrosine transaminase- PLP is the coenzyme for this reaction.
  2. Para hydroxyl phenylpyruvate Hydroxylase- this enzyme belongs to Dioxygenase. Cofactor for this reaction is Copper. Ascorbic acid is also present.
  3. Homogentisate oxidase- belongs to dioxygenase. Contains Iron also.
  4. Maleyl acetoacetate cis-trans isomerase- belongs to isomerase. Need glutathione as cofactor.
  • Specialized Products from Tyrosine- Melanin, Catecholamines, Thyroxine.

Synthesis of Melanin

  • Melanin is the pigment of skin, hair and eye.
  • It takes place in the melanosome of melanocyte present in the deeper layers of epidermis.
  • Tyrosine is the precursor for melanin and only one enzyme, tyrosinase.
  • It is a rate limiting step and Copper is the cofactor.
  • DOPA (Dihydroxyphenylalanine) can act as a cofactor for tyrosinase.
  • Tyrosinase present in melaonocytes converts tyrosine to DOPA.
  • The presence of moles on the body represents a localised severe hyperpigmentation due to hyperactivity of melonocytes.

Defects of Melanin-

  • Localized absence or degeneration of melanocytes results in white patch on the skin is known as leucoderma.
  • Albinism is an inborn error with generalised lack of melanin synthesis. (tyrosine is absent)
  • Malignant Malenoma- malenoblasts increased resulting in malignant malenoma
  • Graying of hair – due to absence of malenocytes in the roots of hair.

Synthesis of Catecholamines-

  • The amine derivative of catechol is called catecholamines.
  • Catecholamines are derived from tyrosine.
  • Catecholamines includes-
  1. Epinephrine
  2. Norepinephrine
  3. Dopamine
  • Conversion of tyrosine to catecholamines takes place in adrenal medulla (epinephrine) and sympathetic ganglia (norepinephrine).
  • Conversion of tyrosine to catecholamines takes place in 4 steps-
  1. Ring hydroxylation
  2. Decarboxylation
  3. Side chain hydroxylation
  4. Methylation
  • Tyrosine hydroxylase – hydroxylates tyrosine to 3,4 dihydroxyphenylalanine.
  • It catalyzes rate limiting reactions and requires tetrahydrobioprotein as coenzyme. 
  • DOPA is decarboxylated to form Dopamine by DOPA decarboxylase.
  • PLP is the coenzyme for this reaction. 
  • Dopamine is an inhibitor of prolactin secretion and important neurotransmitter in extrapyramidal tract, substantia nigra and striatal tract. 
  • Dopamine is hydroxylated to form norepinephrine. 
  • Methylation of norepinephrine by S- adenosylmethionine gives epinephrine. Adrenaline and epinephrine same name for hormone. 

Functions of  catecholamines-

  • Epinephrine and norepinephrine increases the blood pressure.
  • Adrenaline increases the rate and force of myocardial contraction.
  • Dopamine and norepinephrine acts as neurotransmitters in the brain and ANS.
  • Adrenalin is anti-insulin in nature. 

Degradation of Catecholamines-

Catecholamines Enzyme Degradation
Epinephrine and Norepinephrine Catechol O methyltransferase & mono amino oxidase Vanillyl Mandelic acid
Dopamine   Homovanillic acid

Exam Important

  • Specialized Products from Tyrosine-
    • Melanin
    • Catecholamines
    • Thyroxine
  • Epinephrine and norepinephrine is catabolized by Catechol O Methyl then by Monoamino Oxidase.
  • The major end product of epinephrine and norepinephrine is Vanillyl Mandelic Acid.
  • Diagnosis of Phenylketonuria is by Guthries test.
Don’t Forget to Solve all the previous Year Question asked on Metabolism of Aromatic Amino Acids

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Metabolism of Amino Acids

Metabolism of Amino Acids

Q. 1

A vitamin B6 deficiency reduces the effectiveness of transaminase enzymes. Which amino acid is formed from transamination of a-ketoglutarate?

 A

Glycine

 B

Glutamine

 C

Asparagine

 D

Glutamate

Q. 1

A vitamin B6 deficiency reduces the effectiveness of transaminase enzymes. Which amino acid is formed from transamination of a-ketoglutarate?

 A

Glycine

 B

Glutamine

 C

Asparagine

 D

Glutamate

Ans. D

Explanation:

Glutamate is formed from a-ketoglutarate by the enzymes aspartate aminotransferase and alanine aminotransferase.


Q. 2

Transamination reaction is :

 A

Net deamination with splitting of NI-b

 B

c and d both

 C

Transaminase enzyme & pyridoxial P01 binding is covalent

 D

Glutamate is formed

Q. 2

Transamination reaction is :

 A

Net deamination with splitting of NI-b

 B

c and d both

 C

Transaminase enzyme & pyridoxial P01 binding is covalent

 D

Glutamate is formed

Ans. D

Explanation:

C i.e. Transaminase enzyme & pyridoxial PO4 binding is covalent; D i.e. Glutamate is formed


Q. 3

Co-enzyme used in transamination

 A

NAD

 B

Biotin

 C

Pyridoxal phosphate

 D

Riboflavin

Q. 3

Co-enzyme used in transamination

 A

NAD

 B

Biotin

 C

Pyridoxal phosphate

 D

Riboflavin

Ans. C

Explanation:

C i.e. Pyridoxal phosphate

–                        Pyridoxal phosphate (active B6) dependent conditions (in which it is used in treatment) are Homocystinuria, Oxaluria, Cystathioninuria and Xanthurenic acid uriaQ, Mn – “HOCX or Homo Ox Siton Zen”

– Homocystinuria is vitamin B6, B12 and folate. dependentQ

Maple syrup urine disease is d/t defective branched chain a-ketoacid dehydrogenase enzyme; and it may be a/ w thiamin (vitamin B1) deficiencyQ

Methylmalonyl acidttria is seen in vitamin B12 deficiencyQ.

Figlu (N- formimimino-glutamate) uria following a dose of histidine occurs in folate (folic acid) deficiency (Histidine load test). AICAR (amino imidazole carboxamide ribosyl 51-P) uria also occurs in folate deficiency.

Quiz In Between


Q. 4

Transamination of pyruvate and glutamic acid leads to the formation of

 A

Oxaloacetate

 B

a-ketoglutarate

 C

Aspartate

 D

Malate

Q. 4

Transamination of pyruvate and glutamic acid leads to the formation of

 A

Oxaloacetate

 B

a-ketoglutarate

 C

Aspartate

 D

Malate

Ans. B

Explanation:

 

Transamination of pyruvate and glutamic acid (or glutamate) 1/t formation of alanine and a-keto (oxo) glutarate.


Q. 5

Co-enzyme used in transamination ‑

 A

NAD

 B

Biotin

 C

Pyridoxal phosphate

 D

Riboflavin

Q. 5

Co-enzyme used in transamination ‑

 A

NAD

 B

Biotin

 C

Pyridoxal phosphate

 D

Riboflavin

Ans. C

Explanation:

Ans. is ‘c’ i.e., Pyridoxal phosphate

Pyridoxal phosphate (active form of vitamin B6) is the coenzyme for transamination reactions.


Q. 6

Transamination of alanine results in the formation of- 

 A

 Pyruvate

 B

Oxaloacetate

 C

 Aspartate

 D

 Arginine

Q. 6

Transamination of alanine results in the formation of- 

 A

 Pyruvate

 B

Oxaloacetate

 C

 Aspartate

 D

 Arginine

Ans. A

Explanation:

Quiz In Between


Q. 7

Coenzyme not required in formation of glutamate-

 A

 Thiamine pyrophosphate

 B

Pyridoxial phosphate

 C

Niacin

 D

None of the above

Q. 7

Coenzyme not required in formation of glutamate-

 A

 Thiamine pyrophosphate

 B

Pyridoxial phosphate

 C

Niacin

 D

None of the above

Ans. A

Explanation:

During transamination reaction glutamate is formed. Pyridoxial Phosphate acts as coenzyme.


Q. 8

True about transamination reaction are all except-

 A

Transfer of alpha amino group from alpha amino acid to keto acid

 B

Alpha ketoglutarate is the most common receptor

 C

Threonine does not undergo transamination

 D

Biotin is required as a coenzyme.

Q. 8

True about transamination reaction are all except-

 A

Transfer of alpha amino group from alpha amino acid to keto acid

 B

Alpha ketoglutarate is the most common receptor

 C

Threonine does not undergo transamination

 D

Biotin is required as a coenzyme.

Ans. D

Explanation:

Pyridoxial phosphate is a coenzyme in transamination reaction.

 


Q. 9

Glutamine in blood acts as-

 A

NH3 transporter

 B

 Toxic element

 C

Store energy

 D

Abnormal metabolite

Q. 9

Glutamine in blood acts as-

 A

NH3 transporter

 B

 Toxic element

 C

Store energy

 D

Abnormal metabolite

Ans. A

Explanation:

Glutamine is the major form of transport of ammonia

Quiz In Between


Q. 10

Ammonia is detoxified in brain to-

 A

Uric acid

 B

GABA

 C

Urea

 D

 Glutamine

Q. 10

Ammonia is detoxified in brain to-

 A

Uric acid

 B

GABA

 C

Urea

 D

 Glutamine

Ans. D

Explanation:

The brain is a rich source of glutamine synthase and predominantly detoxifies ammonia by synthesis of glutamate.


Q. 11

Which amino acid binds with NH4+ covalently and makes it non-toxic for transportation-

 A

Serine

 B

Aspartate

 C

Glutamate

 D

Histidine

Q. 11

Which amino acid binds with NH4+ covalently and makes it non-toxic for transportation-

 A

Serine

 B

Aspartate

 C

Glutamate

 D

Histidine

Ans. C

Explanation:

Q. 12

Co factors for glutamate dehydrogenase-

 A

NAD

 B

FADH2

 C

FMN

 D

 FAD

Q. 12

Co factors for glutamate dehydrogenase-

 A

NAD

 B

FADH2

 C

FMN

 D

 FAD

Ans. A

Explanation:

Hepatic L- glutamate dehydrogenase can use either NAD+ or NADP+.

Quiz In Between


Q. 13

Increased alanine during prolonged fasting represents-

 A

Increased breakdown of muscle proteins

 B

Impaired renal function

 C

Decreased utilization of amino acid from Glucogenesis

 D

Leakage of amino acids from cells due to plasma membrane leakage

Q. 13

Increased alanine during prolonged fasting represents-

 A

Increased breakdown of muscle proteins

 B

Impaired renal function

 C

Decreased utilization of amino acid from Glucogenesis

 D

Leakage of amino acids from cells due to plasma membrane leakage

Ans. A

Explanation:

During prolonged fasting there is increased gluconeogenesis. Alanine is provided by the muscle is one of the substrates for gluconeogenesis  and is called Glucose Alanine cycle.

So plasma level of alanine increases in prolonged starvation.


Q. 14

Amino acid absorption is by-

 A

Facilitated transport

 B

Passive transport

 C

Pinocytosis

 D

Active transport

Q. 14

Amino acid absorption is by-

 A

Facilitated transport

 B

Passive transport

 C

Pinocytosis

 D

Active transport

Ans. D

Explanation:

 Free amino acids are absorbed across the intestinal mucosa by sodium-dependent active transport. There are several different amino acid transporters, with specificity for the nature of the amino acid side-chain. Transporters of Amino Acids.

  • For Neutral Amino Acids
  • For Basic Amino acids and Cysteine
  • For Imino Acids and Glycine
  • For Acidic Amino Acids
  • For Beta Amino Acids (Beta Alanine)

Meisters Cycle

  • For absorption of Neutral Amino acids from Intestines, Kidney tubules and brain.
  • The main role is played by Glutathione. (GSH)
  • For transport of 1 amino acid and regeneration of GSH 3 ATPs are required.

Quiz In Between



Metabolism of Amino Acid

Metabolism of Amino Acid


METABOLISM OF AMINO ACID

  • The amino acids undergoes 4 steps-
  1. Transamination
  2. Oxidative deamination
  3. Ammonia transport
  4. Reactions of urea cycle

1. Transamination-

  • Transfer of alpha amino group from one amino acid to a keto acid to form another pair of amino acid and keto acid.
  • It is catalyzed by amino transferase (transaminase).
  • All transaminases require pyridoxal phosphate (PLP), a coenzyme derived from Vitamin B6.
  • In almost all cases, the amino group is accepted by alpha ketoglutaric acid so that glutamic acid is formed.
  • Transamination is a reversible process.
  • Transamination occurs via “ping-pong mechanism”.
  • Most transaminases use α- ketoglutarate (α- keto acid) as a common acceptor of α- amino group.
  • Most important transaminases-
  • L- alanine + α- ketoglutarate —à Pyruvate + L- glutamate
  • This reaction catalyzed by Alanine transaminases (ALT) or Serum glutamate pyruvate transaminase (SGPT).
  • L- aspartate + α- ketoglutarate –à Oxaloacetate + L- glutamate
  • Catalyzed by Aspartate Amino transferase (AST) or Serum glutamate oxaloacetate transaminase (SGOT)
  • Amino acid which do not undergo Transamination-
  • Proline
  • Hydroxyproline
  • Threonine
  • Lysine
  • Amino group from amino acids are concentrated in the form of Glutamate.

2. Trans-deamination-

  • Conversion of α amino nitrogen to ammonia is by concerted action of amino transferase and Glutamate Dehydrogenase is termed as Transdeamination.
  • Transamination + Oxidative Deamination = Transdeamination
  • Oxidative deamination– The liberation of free ammonia from amino group of amino acid coupled with oxidation is called deamination.
  • Oxidative deamination primarily occurs in Liver (major) and kidney.
  • Glutamate undergoes oxidative deamination by the action L- glutamate dehydrogenase (GDH)
  • GDH requires NAD+ or NADP.
  • Glutamate dehydrogenase activated by GDP & ADP.
  • Glutamate dehydrogenase inhibited by GTP & ATP.

Nonoxidative Deamination-

  • Amino dehydrases for amino acids with hydroxyl group- Serine, Threonine, Homoserine.
  • Amino acid desulfhydrases – Cysteine, Homocystein

3. *Transport of Ammonia-

  • Metabolism of Ammonia– ammonia exists in ammonium ion form.
  • Ammonia is formed in almost all tissues.
  • The intracellular ammonia is trapped by glutamic acid to form glutamine (brain) by enzyme glutamine synthetase.
  • This is called first line trapping of ammonia.(ATP required)
  • NH3 transport from muscle to liver by glucose- alanine Cycle.
  • The brain is a rich source of glutamine synthase & detoxifies ammonia by this route.
  • In the liver, glutaminase removes the ammonia from Glutamine.
  • Ammonia enters into urea cycle in the liver.

4. Urea is the major end product of protein catabolism in the body.

Exam Important

  • GDH is a mitochondrial matrix enzyme.
  • GDH catalyzes reversible oxidative deamination.
  • α-ketoglutarate is the keto acid which accept the amino group of α-amino acids and produces other α amino acid ie. Glutamate.
  • Deamination (removal of –NH2 group)
  • Glutamine is a nontoxic major transport form of ammonia
  • Transport of ammonia from skeletal muscle as Alanine.
  • Sources of Urea-

NH2 → from ammonia

CO → CO2

NH2 → from aspartate

 

Don’t Forget to Solve all the previous Year Question asked on Metabolism of Amino Acid

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Citric Acid Cycle

citric acid cycle

Q. 1

All of the following vitamins are required in citric acid cycle, EXCEPT:

 A

Riboflavin

 B

Niacin

 C

Thiamin

 D

Ascorbic acid

Q. 1

All of the following vitamins are required in citric acid cycle, EXCEPT:

 A

Riboflavin

 B

Niacin

 C

Thiamin

 D

Ascorbic acid

Ans. D

Explanation:

Four of the B vitamins are essential in the citric acid cycle and hence energy-yielding metabolism: (1) riboflavin, in the form of flavin adenine dinucleotide (FAD), a cofactor for succinate dehydrogenase; (2) niacin, in the form of nicotinamide adenine dinucleotide (NAD), the electron acceptor for isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase; (3) thiamin (vitamin B1), as thiamin diphosphate, the coenzyme for decarboxylation in the α-ketoglutarate dehydrogenase reaction; and (4) pantothenic acid, as part of coenzyme A, the cofactor attached to “active” carboxylic acid residues such as acetyl-CoA and succinyl-CoA
 
Ref: Harper 28th edition, chapter 17

Q. 2

The citric acid cycle is inhibited by which of the following?

 A

Fluoroacetate

 B

Fluorouracil

 C

Arsenic

 D

Aerobic conditions

Q. 2

The citric acid cycle is inhibited by which of the following?

 A

Fluoroacetate

 B

Fluorouracil

 C

Arsenic

 D

Aerobic conditions

Ans. A

Explanation:

Fluoroacetate can be converted to fluorocitrate, which is an inhibitor of aconitase. Arsenic is not a direct inhibitor, but arsenite is an inhibitor of lipoic acid–containing enzymes such as α-ketoglutarate dehydrogenase. Malonate, not malic acid, is an inhibitor of succinate dehydrogenase. The citric acid cycle requires oxygen and would be inhibited by anaerobic, not aerobic, conditions. Fluorouracil is a suicide inhibitor of thymidylate synthase and blocks deoxythymidylate synthesis. 

Ref: Bender D.A., Mayes P.A. (2011). Chapter 17. The Citric Acid Cycle: The Catabolism of Acetyl-CoA. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper’s Illustrated Biochemistry, 29e.

Q. 3

In vivo control of citric acid cycle is effected by:

 A

Acetyl CoA

 B

Coenzyme A

 C

ATP

 D

Citrate

Q. 3

In vivo control of citric acid cycle is effected by:

 A

Acetyl CoA

 B

Coenzyme A

 C

ATP

 D

Citrate

Ans. C

Explanation:

C i.e. ATP, E i.e. NADH

Quiz In Between


Q. 4

Which end product of citric acid cycle is used in detoxification of ammonia in brain?

 A

Oxaloacetate

 B

Alphaketoglutarate

 C

Succinate

 D

Citrate

Q. 4

Which end product of citric acid cycle is used in detoxification of ammonia in brain?

 A

Oxaloacetate

 B

Alphaketoglutarate

 C

Succinate

 D

Citrate

Ans. B

Explanation:

Q. 5

All are components of Citric acid cycle EXCEPT:

 A

Fumarase

 B

Malonate

 C

Succinate dehydrogenase

 D

Alpha-ketoglutarate dehydrogenase

Q. 5

All are components of Citric acid cycle EXCEPT:

 A

Fumarase

 B

Malonate

 C

Succinate dehydrogenase

 D

Alpha-ketoglutarate dehydrogenase

Ans. B

Explanation:

 

Malonate competitively inhibits succinate dehydrogenase, which converts succinate to fumarate


Q. 6

Which of the following glucogenic amino acid enters the citric acid cycle at succinyl CoA

 A

Phenylalanine

 B

Tyrosine

 C

Isoleucine

 D

Tryptophan

Q. 6

Which of the following glucogenic amino acid enters the citric acid cycle at succinyl CoA

 A

Phenylalanine

 B

Tyrosine

 C

Isoleucine

 D

Tryptophan

Ans. C

Explanation:

Ans. is ‘c’ i.e., isoleucine

Quiz In Between


Q. 7

Not an intermediate product of citric acid cycle is:

 A

Acyl Co-A

 B

Succinyl Co-A

 C

Citrate

 D

a-ketoglutarate

Q. 7

Not an intermediate product of citric acid cycle is:

 A

Acyl Co-A

 B

Succinyl Co-A

 C

Citrate

 D

a-ketoglutarate

Ans. A

Explanation:

Ans. a. Acetyl Co-A


Q. 8

Substrate level phosphorylation is seen in reaction catalyzed by which enzyme of citric acid cycle‑

 A

Pyruvate kinase

 B

Succinate thiokinase

 C

Phosphoglycerate kinase

 D

All of the above

Q. 8

Substrate level phosphorylation is seen in reaction catalyzed by which enzyme of citric acid cycle‑

 A

Pyruvate kinase

 B

Succinate thiokinase

 C

Phosphoglycerate kinase

 D

All of the above

Ans. B

Explanation:

Ans. is ‘b’ i.e., Succinate thiokinase 

Quiz In Between



Citric Acid Cycle

Citric Acid Cycle


CITRIC ACID CYCLE/KREBS CYCLE/TRICARBOXYLIC ACID (TCA Cycle)

  • The Citric acid cycle is the most important metabolic pathway for the energy supply to the body in which 65-70% of ATP is synthesized.
  • Citric acid cycle involves final common pathway of the oxidation of acetyl CoA to CO2 & H2O from carbohydrates, fatty acids and amino acids.
  • The site of occurance of TCA cycle enzymes is mitochondrial matrix except for succinate dehydrogenase found in inner mitochondrial membrane.
  • The cycle operates in aerobic conditions. 

Reactions of Critric acid cycle-

  • Step 1- The 4 carbon, oxaloacetate condenses with 2 carbon, acetyl CoA to form 6 carbon compound, a tricarboxylic acid by enzyme citrate synthase. (irreversible step) 
  • Step 2-  Citrate is isomerized to isocitrate by the enzyme aconitase. 
  • Step 3- The enzyme isocitrate dehydrogenase catalyses the conversion of isocitrate to oxalosuccinate and then to α-ketoglutarate. (irreversible step & NADH is formed) 
  • Step 4- Conversion of α-ketoglutarate to succinyl CoA catalyzed by α-ketoglutarate dehydrogenase. (NADH formed) 
  • Step 5- Succinyl CoA is converted to succinate by enzyme Succinate thiokinase. This is substrate level phosphorylation
  • GTP is converted to ATP by enzyme nucleoside diphosphate kinase. 
  • Step 6- Succinate is oxidized by succinate dehydrogenase to fumarate.(FADH2 formed) 
  • Step 7- Fumate is converted to malate by enzyme fumarase. 
  • Step 8- Malate is oxidised to oxaloacetate by malate dehydrogenase. (NADH formed) 

Inhibitors of Krebs Cycle-

Enzyme Inhibitor
Aconitase Fluoroacetate
α-ketoglutarate dehydrogenase Arsenite
Succinate dehydrogenase Malonate
  •  12 ATPs are produced from one acetyl CoA. 
  •  Krebs cycle is both catabolic and anabolic in nature so called amphibolic.
  • Anaplerotic reactions are “filling up” reactions or “influx” reactions or replenishing reactions which supply 4 carbon units to the TCA cycle. 
  • ATP, NADH, acetyl CoA & succinyl CoA acts as an allosteric inhibitor to citrate synthase.
  • ATP and NADH inhibit isocitrate dehydrogenase and activated by ADP. 
  • Alpha ketoglutarate dehydrogenase is inhibited by succinyl CoA and NADH. 
  • TCA cycle is involved in gluconeogenesis, transamination, deamination. 
  • Vitamins play a key role in TCA cycle-
  1. Riboflavin
  2. Niacin
  3. Thiamine
  4. Pantothenic acid  

Exam Important

  • There is no net generation oxaloacetate or cycle intermittents.
  • First product of the cycle is citrate.
  • Two irreversible steps are- 
    • oxaloacetate to citrate catalyzed by citrate (synthase)
    • alpha ketoglutarate to succinyl CoA.
  • NADH is produced and CO2 is liberated.
  • GTP is produced by substrate level phosphorylation by succinate thiokinase.
  • 4 dehydrogenases are used in Kreb’s cycle.
  • Isolucine, Methionine, Valine enter by conversion into succinyl CoA.
  • Tyrosine, phenylalanine enter conversion into fumarate.
  • Pyruvate Carboxylase reaction is the classical example of the anaplerotic reaction.
Don’t Forget to Solve all the previous Year Question asked on Citric Acid Cycle

Module Below Start Quiz

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