Isoenzymes

• • Isoenzymes are the multiple forms of the same enzyme in a single species that catalyze the same chemical reaction or reactions, but differ from each other structurally, electrophoretically and immunologically.
  • Though the same chemical reaction is catalyzed, the different isoenzymes may catalyze the same reaction at different rates.
  • Isoenzymes have different pH optimes, Km and V max values.
  • Isoenzymes may differ in their amino acid sequence and their quarternary structures.
  • The isoenzymes may have different properties also for e.g. LDH-4 and LDH-5 are easily destroyed by heat, whereas LDH-1 and LDH-2 are not, if heated upto 60°C. (Heat resistant).
  • Individual isoenzymes (isozymes) are distinguished and numbered on the basis of electrophoretic mobility, with the number 1 being assigned to that form having the highest mobility toward the anode, for e.g. LDH-1 has the highest mobility towards the anode and LDH-5 is the slowest.
  • Isoenzymes have different tissue distributions. Therefore the pattern of isoenzymes found in the plasma may serve as a means of identifying the site of tissue damage. Example of the diagnostic use of isoenzymes are the study of Lactate Dehydrogenase and Creatine Kinase.

• Isozymes are the physically distinct forms of the same enzymes that catalyze the same reaction, and differ from each other structurally, electrophoretically and immunologically.
• They differ in their physical properties because of genetically determined difference in amino acid sequence.
• They are separated by electrophoresis as they have different electrophoretic mobility.
• They have different Km value.
• Isoenzyme of an oligomeric enzyme process differ in combination of its peptide protomer.

Ans. is ‘a’ i.e., LDH1

Ans. is ‘c’ i.e., Myoglobin 

Ans. is ‘a’ i.e., K. 

• The K_m of an enzyme is the concentration of the substrate that enables the enzyme to function at half maximum activity and is therefore a measure of the specificity of a substrate_.for the enzyme” Clinical biochemistry
• Actually enzyme specificity is not measured by K_m alone.
• It is measured by the ratio K_cat/K_m which is a second order rate constant for the reaction between substrate and free enzyme.
• This ratio is important, for it provides a direct measure of enzyme efficiency and specificity.
• Note : ^Kcat is turnover number and measures the rate of the catalytic process.

Ans. is ‘a’ i.e., 2

• Most enzyme show a 50-300% (average 200%) increase in reaction rate when the temperature is increased by 10°, and the ratio of rate constant at two temperatures 10° apart is usually between 1.5 to 4 (average 2) for most enzymes.
• This value is termed as Q10.
• “The rate of enzymatic reaction doubles with every 10° rise in temperature. “

Ans. is ‘c’ i.e., Catalase 

Measurement of enzyme activity

• The activity of enzyme is measured in terms of the following :
• Unit of enzyme activity : – By international agreement, one unit enzyme activity is defined as the amount causing transformation of 1.0 micro mole of substrate per minute at 25° C. It is usually expressed as mole of substrate disappeared or mole of product formed per minute.
• Specific activity : – It refers to the number of enzyme units per milligram of protein. It is a measure of enzyme purity; higher the enzyme purity, more is the specific activity.
• Turn over number : – This refers to the number of substrate molecules transformed per unit time by a single enzyme molecule (or by a single catalytic site), when the enzyme concentration alone is rate-limiting factor. Catalase has the highest turnover number and hence is the fastest active enzyme. Carbonic anydrase has the 2″ fastest turnover number; therefore, it is 2^nd fastest active enzyme (after catalase). Lysozyme has the lowest turnover number and therefore is slowest acting.