25. Certain nucleotide bases in DNA molecules are enzymatically methylated. In all known DNA methylation reactions the methyl group donor is:
(A) S-adenosylmethionine.
(B)  -methylenetetrahydrofolate.
(C) N⁵-methyItetrahydrofolate.
(D)
(E) acetyl-CoA.

21. All are physiological changes initiated by glucagon except: (A) activation of glycogenolysis in liver. (B) stimulation of liver gluconeogenesis. (C) maintenance of steady-state levels of glucose in the blood stream. (D) activation of glycogenolysis in muscles. (E) All are true.

22. Which of the following is not required in the synthesis of fatty acids?(A) Acetyl-CoA (B) Biotin  (C) (D) Malonyl-CoA (E) NADH

23. When a cell with the pentose phosphate pathway has need for more pentose phosphates, but not for additional NADPH: (A) glucose-6-phosphate dehydrogenase is activated. (B) the oxidative and non-oxidative enzymes of the pentose phosphate pathway are active. (C) the non-oxidative enzymes produce pentose phosphates from fructose-6-phosphate- and glyceraldehyde-3-phosphate. (D) all enzymes of glycolysis and pentose phosphate pathway are active. (E) None of the above are true.

24. Which of the following descriptions is correct, regarding to the use of the enzyme glucose oxidase to determine the concentrations of glucose in blood? (A) Glucose is reduced to give glucono-6-lactone. (B) The reaction produces oxygen in company with glucono-6-lactone. (C) The reaction requires hydrogen peroxide as the oxidant. (D) Glucose is oxidized to give glucono-6-lactone. (E) Glucose is oxidized to give glucuronic acid

26. Which amino acid in the above table has a pKa that is very sensitive to the microenvironment and is often involved in enzyme catalysis? (A) I (B) II (C) III (D) IV (E) V

27. Which amino acid in the above table is able to form a salt bridge with a glutamate? (A) I (B) II (C) III (E) V (D) IV

28. Which of the following statements is correct? (A) Nonpolar compounds force surrounding H2O molecules to become disordered. (B) Nonpolar compounds interfere with the hydrogen bonding among H2O molecules. (C) Nonpolar compounds increase entropy (△S) when dissolved in water. (D) Nonpolar compounds decrease enthalpy (△H) when dissolved in water. (E) None of the above

29. 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).

30. Which of the following statements regarding enzyme regulation is true? (A) Addition of an inhibitor to a V system results in kinetics similar to addition of a competitive inhibitor to a typical hyperbolic system. (B) Addition of an allosteric activator to a K system changes the plot of V vs. [S] from a sigmoidal curve to a more hyperbolic curve. (C) Allosteric effectors are always more powerful than covalent modification. (D) The I state of an enzyme generally has more activity than the R state. (E) None of the above is true.

31. In the enzyme catalyzed reaction sequence below, can the intermediate be predicted and why?   (A) Yes, the mechanism is a double-displacement reaction. (B) Yes, the reaction fits the ping-pong model. (C) No, the reaction is random single-displacement. (D) No, the reaction is double-displacement. (E) No, the reaction is 'order single-displacement.