Biochemistry Practice Problems V with Answers


1. Consider substrates “A” and “B,” each tested using a liver homogenate containing an
enzyme following M-M kinetics. Two things are noted: (1) the Rate vs. [S] curve for A is
shifted well to the right of the curve for substrate B, and (2) the same Vmax as that achieved with A also can be reached with B. What may we conclude about substrate B?
a. [S] at Vmax is higher
b. Km is higher
c. affinity is greater
d. allosterism is present

2. The area of the enzyme that is optimized to conduct the actual desired reaction, i.e. to bind the substrate, create a short-lived structural form required to facilitate the change and then release a product to the immediate environment, is referred to as the:
a. analogue state
b. active site
c. transition site
d. intermediate site

3. Enzymes that catalyze the same reaction, but structurally are somewhat different from each
other and are each the product of a different gene are called:
a. isozymes
b. ribozymes
c. nucleosomes
d. histones

4. In order for an enzymatic reaction obeying M-M kinetics to reach 25% of its maximum
initial velocity:
a. [S] is 1/3 of Km
b. Km is 1.33 times [S]
c. Km is 1/3 [S]
d. none of the above are correct

5. How is the rate expressed, for the enzyme reaction below that produces [P]?

a. k4[E] [P] E + S ↔ ES ↔
E + P
b. k3 [E] [P] / [E] [S] k2 k4 (“k” of reverse reactions)
c. k3 [ES]
d. k1 [S] / [P]

6. In the Lineweaver-Burk plot of enzyme activity, the slope of the line is equal to:
a. Km / Vmax
b. 1 / [S]
c. Vmax / [V]
d. [S] / Km

7. How is the M-M equation altered when [S] is very high versus Km?
a. Vmax becomes negligible and velocity approaches 50% which is equal to Km
b. velocity falls, since high [S] inhibits the reaction at saturating levels
c. affinity becomes equal to Km as [S] passes through the . maximal point
d. the term [S] divided by [S] + Km approaches a value of “1”

8. Find the initial velocity of an M-M enzymatic reaction when Vmax is 3.95 mol/sec, [S] is 0.45 M and Km is 0.76 M.
a. 0.058 mol / sec
b. 1.40 mol / sec
c. 0.280 mol / sec
d. 1.47 mol / sec

9. A research scientist conducted an in vitro enzyme inhibition study using an investigational new drug, and found that the drug bound to an unknown site that lowered the catalytic activity of an enzyme regulating cholesterol synthesis. A Michaelis-Menten kinetic analysis performed in his laboratory showed a linear inhibitory “curve” using the Lineweaver-Burk type graph. As drug concentrations were increased, the apparent Km remained relatively constant. The scientist noted that increased [S] failed to overcome the observed inhibitory effect on turnover number. What type of inhibition does this drug show?
a. noncompetitive
b. competitive
c. uncompetitive
d. mixed allosterism

10. The anticancer drug 5-fluorouracil acts as a ________ of the enzyme ________, which is a key enzyme involved in ___________ synthesis.
a. suicide inhibitor / thymidylate synthetase / DNA
b. allosteric inhibitor / aspartate transcarbamoylase / pyrimidine
c. noncompetitive inhibitor / uracil transcriptase / PALA
d. irreversible inhibitor / proline racemase / RNA

11. If an enzyme manipulates a functional group of a substrate molecule so that it is pointed toward an activated amino acid residue that composes the active site, this event would be considered as which of the following?
a. Proximity coupling
b. Coupling to protein motion
c. Orbital steering
d. Transition State Stabilization

12. Which of the following “structures” of Chymotrypsin contains the chemical function that makes a “nucleophilic attack” on the amide carbon near the bond being broken?
a. oxyanion hole
b. specificity pocket
c. catalytic triad
d. hydrophobic pocket

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