Enzyme Kinetics and Regulation of Activity MCQs

Explanation: Phosphofructokinase-1 (PFK-1) is a classical allosteric enzyme and a key regulatory step of glycolysis, whose activity is modulated by allosteric effectors like ATP and citrate (inhibitors) and AMP, ADP, and fructose-2,6-bisphosphate (activators). These regulators bind to sites other than the active site (allosteric sites), causing conformational changes and leading to a sigmoidal reaction curve , unlike Michelis-Menten enzymes. Hexokinase shows feedback inhibition by glucose-6-phosphate but is not a classical allosteric enzyme. Chymotrypsin and trypsin are digestive proteases regulated by zymogen activation (proteolytic cleavage) and not by allosteric modulation.

Ans. is 'a' i.e., Peptidyl transferase [Ref Harper 29thIe p. 405] Some RNA molecules have intrinsic catalytic activity. The activity of these ribozymes often involves the cleavage of nucleic acid. Two important RNA enzymes or ribozymes are : ? The peptidyl transferase that catalyzes peptide bond formation on the ribosome and Ribozymes involved in the RNA splicing.

Ans. 'D' Nitric acid Xanthoproteic test: The ring systems in phenylalanine, tyrosine and tryptophan undergo nitration on treatment with concentrated nitric acid when heated. The end product is yellow in color which is intensified in strong alkaline medium. This reaction causes the yellow stain in the skin by nitric acid. 319. Taurine is biosynthesized by ? a) Arginine b) Leucine c) Valine d) Cysteine Correct Answer - D Ans. is 'd' i.e., Cysteine [Ref Harper 250/e p. 298-99] Taurine is synthesized from cysteine by 3 enzyme-catalyzed reactions :

Glycine is the only optically inactive amino acid. Glycine is the simplest amino acid and has no asymmetric carbon atom and therefore, shows no optical activity. All others are optically active. Option b - Sulfur containing amino acids are Cysteine and Methionine . Option c- Guanidinium group is present in Arginine. Option a - Glycine is a non-essential amino acid. Reference: Harper's 32nd edition, page 19

Noncompetitive Inhibition A type of inhibition in which the inhibitor bind to a site distinct from the substrate binding site. Two different types are: Reversible Noncompetitive Inhibition (Only few Noncompetitive inhibition are reversible) Irreversible Noncompetitive Inhibition (Most of Noncompetitive are irreversible). Features of noncompetitive inhibition Inhibitor have no structural resemblance to substrate Mostly Irreversible (Except a few reversible non- competitive inhibition) Excess substrate do not abolish the inhibition Km remains the same Vmax decreases. Reference: Harper's 32nd edition, page 82,83.

Ans. is 'b' i.e., Enzyme units per mg of protein [Ref Lippincott's 3"1/e p.

Ans. is 'b' i.e., N-acetyl glutamate Carbamoyl phosphate synthase-I (CPS-I), a mitochondrial enzyme, catalyzes the formation of carbamoyl phosphate° by condensation of CO2 and ammonia. Two molecules of ATP are required for the reaction. CPS-I is the rate limiting enzyme of urea cycleQ. It is an allosteric enzyme and allosterically activated by N-acetyl glutamate

Competitive Inhibition A type of inhibition in which the inhibitor competes directly with a normal substrate for an enzyme’s substrate binding site. Features of competitive inhibition Inhibitor will be structural analogue of substrate Reversible Excess substrate abolishes inhibition Vmax remains the same Km increases. Reference: Harper's 32nd edition, page 82.