lOMoARcPSD|9336478 Biochem MCQ Enzymes Biomolecules (University College Cork) StuDocu is not sponsored or endorsed by any college or university Downloaded by Tofik Mohammed (mtofik218@gmail.com) lOMoARcPSD|9336478 BC2001 Biomolecules - Enzymes (copy) 1. Enzymes increase reaction rate by: A Altering the change in free energy of the reaction B Inhibiting the rate of reverse reaction C Changing the equilibrium constant of the reaction D Decreasing the energy of activation E Selectively enhancing the rate of the forward reaction 2. Score: Choose the correct description of an enzyme reaction A An enzyme that catalyses A + B to C + D Accelerates the reaction velocity of A + B to C + D, but does not accelerate the reaction velocity of C + D to A + B B Enzymes decrease the activation energy of a reaction, but do not affect the free-energy change. C Enzymes can change the reaction velocity without directly interacting with substrates D Enzymes mainly consist of proteins. Coenzymes are small compounds that support the structure of the enzyme, but are not involved in the reaction E Since enzymes catalyse chemical reactions in much the same manner as inorganic catalysts, the higher the reaction temperature, the higher the reaction velocity becomes 3. ADH requires oxidised nicotinamide adenine dinucleotide (NAD+) for catalytic activity. In the reaction catalysed by ADH, an alcohol is oxidised to an aldehyde as NAD+ is reduced to NADH and dissociates from the enzyme. The NAD+ is functioning as a (an): A Apoenzyme B coenzyme-cosubstrate C coenzyme-prosthetic group D cofactor E heterotrophic effector 4. Which of the following statements is correct? A The change in free energy of a reaction indicates the rate at which it will be catalysed by an enzyme B Specific RNA molecules (ribozymes) cleave phosphodiester bonds in a broad spectrum of substrate, such as RNA, DNA, ATP and NAD+ C Enzymes alter the difference in free energy between reactants and products in the reaction catlaysed D Enzymes usually display a high level of specificity for the reaction they catalyse and for the substrate E None of the above Page 1 of 3 Downloaded by Tofik Mohammed (mtofik218@gmail.com) lOMoARcPSD|9336478 5. In considering the 'active site' of enzymes, a correct generalization is that: A nearly all the amino acid side chains of the protein must directly participate in catalysis B a definite conformation of the active site region does not exist at all until the substrate is bound C all the amino acid residues that are involved in the active site have to be adjacent on the same stretch of the polypeptide backbone D the amino acid residues that form the active site are in close proximity because of the specific three-dimensional conformation the protein has adopted E all of the above 6. The Michaelis constant (Km) is: A Not changed by the presence of a noncompetitive inhibitor B The substrate concentration at which v = 1/2Vmax C The interception of the 1/v axis of a Lineweaver Burk plot D Equal to 1/2Vmax E The equilibrium constant for the dissociation of ES to E + P 7. An example of irreversible regulation of enzyme activities is: A Phosphorylation by protein kinases B Allosteric regulation by substrates C Dissociation of subunit into oligomeric enzymes D Inhibition by competitive inhibitors E Proteolytic conversion of digestive enzymes 8. Which one of the following statements is correct concerning enzyme inhibition? A Competitive inhibition increases the apparent Vmax for an enzyme B In a Lineweaver-Burk plot, competitive inhibition is represented by a family of lines that have a common intercept on the x-axis corresponding to Km C In competitive inhibition an increase of [S] at constant inhibitor concentration results in a decreased inhibition D Noncompetitive inhibition decreases the apparent Km for the substrate E In competitive inhibition the enzyme-substrate-inhibitor (ESI) complex is found 9. The presence of a noncompetitive inhibitor in an enzymatic reaction system A leads to an increase in Km without affecting Vmax B leads to a decrease in both Km and Vmax C leads to both an increase in the Vmax of a reaction and an increase in the Km D leads to a decrease in the observed Vmax E increases the steady-state concentration of ES Page 2 of 3 Downloaded by Tofik Mohammed (mtofik218@gmail.com) lOMoARcPSD|9336478 10. Enzyme X catalyses the reaction A-B. the Km of X is 5mM, the substrate concentration [S] is 2mM, and the maximal velocity is 10mmol/sec. Under these conditions, which one of the following is the correct value for v/Vm? A 0.28 B 0.028 C 2.8 D 0.05 E 0.5 Page 3 of 3 Downloaded by Tofik Mohammed (mtofik218@gmail.com) lOMoARcPSD|9336478 1. D 2. B 3. B 4. D 5. D 6. B 7. E 8. C 9. D 10. A Downloaded by Tofik Mohammed (mtofik218@gmail.com)
