Biochemistry Questions and Answers Part-15

1. Removal of phosphoryl groups is catalyzed by ___________
a) Diphteria toxin and cholera toxin
b) Dinitrogenase reductase
c) Protein phosphatases
d) Protein kinases

Answer: c
Explanation: The enzymes that catalyze the ADP-ribosylation and inactivation of key cellular enzymes or proteins are diphteria toxin and cholera toxin. Dinitrogenase reductase is responsible for the regulation of biological nitrogen fixation. The attachment of phosphoryl groups to specific amino acid residues is catalyzed by protein kinases.

2. The attachment of phosphoryl groups to specific amino acid residues is catalyzed by ___________
a) Diphteria toxin and cholera toxin
b) Dinitrogenase reductase
c) Protein phosphatases
d) Protein kinases

Answer: d
Explanation: The enzymes that catalyse the ADP-ribosylation and inactivation of key cellular enzymes or proteins are diphteria toxin and cholera toxin. Dinitrogenase reductase is responsible for the regulation of biological nitrogen fixation. Removal of phosphoryl groups is catalyzed by Protein phosphatases.

3. Which of the following is false about the following reaction?
Phosphorylase a + 2H2O → phosphorylase b + 2Pi
a) Phosphorylase a is less active and phosphorylase b is more active
b) Glycogen phosphorylase catalyzes this reaction
c) Phosphorylase b can be transformed back into phosphorylase a by phosphorylase kinase
d) Phosphorylase kinase catalyzes the transfer of phosphoryl groups from ATP to Phosphorylase b

Answer: a
Explanation: Phosphorylase is more active and phosphorylase b is less active.

4. Which of the following takes place due to phosphorylation of isocitrate dehydrogenase?
a) Inhibits the binding of citrate at active site
b) Degrades the enzyme
c) Enhances the substrate-binding affinity
d) No reaction

Answer: a
Explanation: The electrostatic repulsion by the phosphoryl group inhibits the binding of citrate at active site.

5. Which of the following enzyme catalyzes a reaction that introduces reduced nitrogen ito cellular metabolism?
a) Bacterial glutamine synthase
b) Bacterial dinitrogenase reductase
c) Bacterial dinitrogenase oxidase
d) Phosphatase

Answer: a
Explanation: Bacterial glutamine synthase is one among the most complex regulatory enzymes known. It catalyzes a reaction that introduces reduced nitrogen into cellular metabolism.

6. Which of the following is false about feedback regulation?
a) It is a type of enzymatic activity regulation
b) The synthesis of concerned enzyme is inhibited by interfering with the gene of that enzyme
c) Concentration of enzyme is reduced by the end product
d) The best example is the biosynthesis of L-isoleucine from L-threonine in bacteria

Answer: d
Explanation: The best example is reduction of HMG CoA reductase enzyme by dietary cholesterol.

7. Which of the following is false?
a) Allosteric modulators may be inhibitory or stimulatory
b) Based on the nature of modulator, allosteric enzymes are of two types
c) Homotropic allosteric enzymes have the substrate and modulator same
d) Heterotropic allosteric enzymes have the substrate and modulator same

Answer: d
Explanation: In case of heterotropic enzymes both substrate and modulator are different.

8. In the case of allosteric enzymes what is the graphical representation when initial velocity is plotted against substrate concentration?
a) Straight line with negative slope
b) Hyperbola
c) Sigmoid curve
d) Parabola

Answer: c
Explanation: Allosteric enzymes do not obey Michaelis-Menten kinetics. So the curve is not hyperbolic.

9. How many types of enzymatic regulation mechanism occurs in the cells?
a) 2
b) 3
c) 4
d) 5

Answer: d
Explanation: Feedback inhibition, reversible covalent modification of enzymes, proteolytic activation of the enzyme, feedback regulation and regulation of isozymes.

10. Which is the enzyme that acts as a regulatory enzyme in a four-step metabolic pathway?
a) First enzyme
b) Fourth enzyme
c) Second enzyme
d) Third enzyme

Answer: a
Explanation: First reaction is the best place to regulate a metabolic pathway. So, the first enzyme acts as the regulatory enzyme.