1 Introduction to Biochemistry (CHEM 501) Exam II 10/29/08 1. One of the enzymes involved in glycolysis, aldolase, requires Zn2+ for catalysis. Under conditions of zinc deficiency, when the enzyme may lack zinc, it would be referred to as the: A) apoenzyme. B) coenzyme. C) holoenzyme. D) prosthetic group. E) substrate. 2. Enzymes are potent catalysts because they: A) are consumed in the reactions they catalyze. B) are very specific and can prevent the conversion of products back to substrates. C) drive reactions to completion while other catalysts drive reactions to equilibrium. D) increase the equilibrium constants for the reactions they catalyze. E) lower the activation energy for the reactions they catalyze. 3. Which of the following is true about zymogens? A) Proproteins are one type of zymogen. B) Zymogens are inactivated by inhibitor proteins. C) Zymogens are enzymatically inactive. D) Zymogens cleave proteases. E) Zymogens catalize the formation of Zymogenase 4. To calculate the turnover number of an enzyme, you need to know: A) the enzyme concentration. B) the initial velocity of the catalyzed reaction at [S] >> Km. C) the initial velocity of the catalyzed reaction at low [S]. D) the Km for the substrate. E) both A and B. 5. In competitive inhibition, an inhibitor: A) binds at several different sites on an enzyme. B) binds covalently to the enzyme. C) binds only to the ES complex. D) binds reversibly at the active site. E) lowers the characteristic Vmax of the enzyme.2 6. A metabolic pathway proceeds according to the scheme, R → S → T → U → V → W. A regulatory enzyme, X, catalyzes the first reaction in the pathway. Which of the following is most likely correct for this pathway? A) Either metabolite U or V is likely to be a positive modulator, increasing the activity of X. B) The first product S, is probably the primary negative modulator of X, leading to feedback inhibition. C) The last product, W, is likely to be a negative modulator of X, leading to feedback inhibition. D) The last product, W, is likely to be a positive modulator, increasing the activity of X. E) The last reaction will be catalyzed by a second regulatory enzyme. 7. Compare the two reaction coordinate diagrams below and select the answer that correctly describes their relationship. In each case, the single intermediate is the ES complex. A) (a) describes a strict “lock and key” model, whereas (b) describes a transition-state complementarity model. B) The activation energy for the catalyzed reaction is #5 in (a) and is #7 in (b). C) The activation energy for the uncatalyzed reaction is given by #5 + #6 in (a) and by #7 + #4 in (b). D) The contribution of binding energy is given by #5 in (a) and by #7 in (b). E) The ES complex is given by #2 in (a) and #3 in (b).3 8. Michaelis and Menten assumed that the overall reaction for an enzyme-catalyzed reaction could be written as k1 k2 E + S ES → E + P k-1 Using this reaction, the rate of breakdown of the enzyme-substrate complex can be described by the expression: A) k1 ([Et] − [ES]). B) k1 ([Et] − [ES])[S]. C) k2 [ES]. D) k-1 [ES] + k2 [ES]. E) k-1 [ES]. 9. In the following diagram of the first step in the reaction catalyzed by the protease chymotrypsin, the process of general base catalysis is illustrated by the number ________, and the process of covalent catalysis is illustrated by the number _________. A) 1; 2 B) 1; 3 C) 2; 3 D) 2; 3 E) 3; 24 10. Which of these statements about enzyme-catalyzed reactions is false? A) At saturating levels of substrate, the rate of an enzyme-catalyzed reaction is proportional to the enzyme concentration. B) If enough substrate is added, the normal Vmax of a reaction can be attained even in the presence of a competitive inhibitor. C) The rate of a reaction decreases steadily with time as substrate is depleted. D) The activation energy for the catalyzed reaction is the same as for the uncatalyzed reaction, but the equilibrium constant is more favorable in the enzyme-catalyzed reaction. E) The Michaelis-Menten constant Km equals the [S] at which V = 1/2 Vmax. 11. The double-reciprocal transformation of the Michaelis-Menten equation, also called the Lineweaver-Burk plot, is given by 1/V0 = Km /(Vmax[S]) + 1/Vmax. To determine Km from a double-reciprocal plot, you would: A) multiply the reciprocal of the x-axis intercept by −1. B) multiply the reciprocal of the y-axis intercept by −1. C) take the reciprocal of the x-axis intercept. D) take the reciprocal of the y-axis intercept. E) take the x-axis intercept where V0 = 1/2 Vmax. 12. The basic structure of a proteoglycan consists of a core protein and a: A) glycolipid. B) glycosaminoglycan. C) lectin. D) lipopolysaccharide. E) peptidoglycan. 13. In starch and glycogen, the glucose monomers are joined by (α1  4) linkages, whereas in cellulose, the glucose monomers are joined by (β 1  4) linkages. What is a biological consequence of this difference in sugar linkage? A) Cellulose is generally not digestible by animals, whereas starch is easily digestible. B) Cellulose takes on a helical structure and starch forms fibers. C) Starch has more tensile strength than cellulose. D) Glycogen is unbranched, while cellulose is highly branched. E) None of the above 14. Lectins are A) carbohydrates that can bind a protein. B) proteins linked to carbohydrates. C) proteins that bind carbohydrates. D) the oligosaccharide moieties on glycoproteins. E) None of the above.5 15. Which of the following is true about reducing ends? A) All polysaccharides have an equal number of reducing ends as nonreducing ends. B) Polysaccharides grow in the direction of the nonreducing end. C) Disaccharides are named starting with the reducing end. D) All disaccharides have one reducing end. E) None of the above. 16. When two carbohydrates are epimers: A) one is a pyranose, the other a furanose. B) one is an aldose, the other a ketose. C) they differ in length by one carbon. D) they differ only in the configuration around one carbon atom. E) they rotate plane-polarized light in the same direction. 17. Which of the following pairs is interconverted in the process of mutarotation? A) D-glucose and D-fructose B) D-glucose and D-galactose C) D-glucose and D-glucosamine D) D-glucose