Gluconeogenesis

• The synthesis of glucose from non-carbohydrate compounds is known as gluconeogenesis.

The major substrates/precursors  for gluconeogenesis:

• Lactate, pyruvate, glucogenic amino acids, propianate and glycerol.
• Tyrosine enters gluconeogenesis by forming Fumarate substrate

IMPORTANCE:

• Brain & CNS, erythrocytes, testes & kidney medulla are dependent on glucose for continuous supply for energy.
• Human brain alone requires about 120 g of glucose per day, out of about 160 g needed by the entire body.
• Glucose is the only source that supplies to the skeletal muscle, anaerobic conditions
• During starvation gluconeogenesis maintains the blood glucose level.
• During prolonged starvation, the rate of gluconeogenesis depends on Increased alanine levels in liver
• Major contribution towards gluconeogenesis is by alanine
• The stored glycogen is depleted within the first 12-18 hours of fasting.
• After 50 gm of glucose is feed orally Decreased gluconeogenesis
• In well fed state gluconeogenesis in liver is inhibited by ADP level
• On prolonged starvation, the gluconeogenesis is speeded up & protein catabolism provides the substrates, namely glucogenic amino acids.

REACTIONS:

• Gluconeogenesis closely resembles  the reversed pathway of glycolysis.
• During gluconeogenesis reducing equivalents from mitochondria to cytosol are transported by Malate
• Lactate and alanine can both serve as substrates causing hyperglycemia in diabetes
• The 3 irreversible steps of glycolysis are catalysed by the 3 enzymes.
  • Hexokinase
  • PFK
  • Pyruvate kinase

• These three stages by passed by alternate enzymes specific to gluconeogenesis.

These are:

• Pyruvate carboxylase:Gluconeogenesis in Fasting state is indicated by Pyruvate Carboxylase activation by Acetyl CoA
• Phosphoenol pyruvate carboxy kinase
• Fructose-1-6-bisphosphatase:Fructose-1,6-bisphosphatase and pyruvate carboxylase  pairs of enzymes is required for the process of gluconeogenesis
• Glucose-6-phosphatase

Gluconeogenesis Enzymes

The enzymes that are same as that of glycolysis are

• Phosphoglucoisomerase
• Enolase
• Phosphoglycerate mutase
• Phosphoglycerate kinase
• G3P dehydrogenase
• Triosephosphate isomerase
• Fructose 1,6 bisphosphate aldolase:Aldolase is the glycolytic enzyme used in gluconeogenesis

The key enzymes of Gluconeogenesis are

• Pyruvate carboxylase:Biotin is the cofactor of pyruvate carboxylase. The carboxylation of pyruvate occurs in the mitochondria whereas the other reactions of gluconeogenesis occur in the cytosol.
• Glucagon stimulates gluconeogenesis. Lactate is not an intermediate in the conversion of pyruvate to glucose. 
• Phosphoenol pyruvate carboxykinase
• Fructose 1,6 bisphosphatase
• Glucose 6 phosphatase:Muscle cannot take part in gluconeogenesis due to absence of Glucose 6 phosphatase

Gluconeogenesis Regulation:

There are 3 types of regulation which takes place at different speed. They are

1. Change in the rate of enzyme synthesis – occurs over several hours

Availability of substrate – 

• Increased availability of Glucogenic amino acid like  Alanine released from muscle stimulates Gluconeogenesis.
•  High glucose increases the synthesis of enzymes of Glycolysis so that the glucose level is brought down.
• The synthesis of gluconeogenic enzymes are decreased so that there is less formation of new glucose.

Through hormones – 

• Gluconeogenesis is increased by the Glucogenic hormones like glucagon, epinephrine and glucocorticoids. 
• Glucagon stimulates phosphoenolpyruvate carboxykinase while insulin reduces the synthesis of these enzymes.

2. Covalent modification by reversible phosphorylation – rapid

• Phosphorylation is the process of addition of a phosphate group to an enzyme. On phosphorylation, some enzymes are activated while some are inactivated. 
• The gluconeogenic hormones like glucagon and epinephrine leads to the phosphorylation of a key enzyme of glucose breakdown, pyruvate kinase which gets inactivated.
• This leads to inhibition of glycolysis and stimulation of Gluconeogenesis. This type of regulation acts as a rapid response to the presence of low glucose

3. Allosteric modification – instantaneous

• In addition to the enzymes that speeds up a reaction, other substances called allosteric activators further increase the rate of reaction. Acetyl coA is one such allosteric activator.