43. If the substrate concentration in an enzyme catalyzed reaction is equal to 0.5 Km, the initial reaction velocity will be _______.
(A) 0.25 Vmax
(B) 0.33 Vmax
(C) 0.50 Vmax
(D) 0.67 Vmax
(E) 0.75 Vmax

39. Correct statements regarding arachidonic acid include all of the following except_______ . (A) it is a precursor of thromboxane A2 (B) it activates lipoxygenase (C) it is a 20-carbon fatty acid with three double bonds (D) it can be derived from linoleic acid (E) it is esterified to the sn-2 position of glycerophospholipids

40. The cost in high-energy phosphate bonds for the formation of 1 mol of glucose from lactate is_______ . (A) 8 mol of ATP (B) 6 mol of ATP (C) 4 mol of ATP (D) 2 mol of ATP (E) none of the above

41. In the sequence of reactions that convert lactate to glucose in cells capable of gluconeogenesis, three reactions are used to bypass enzymes of the glycolytic pathway in which glucose is converted to lactate. All of the following enzymes fall into this category except_______ . (A) pyruvate carboxylase (B) pyruvate dehydrogenase (C) phosohoenolpyruvate carboxykinase (D) fructose 1,6-diphosphatase (E) glucose 6-phosphatase

42. Carbon monoxide inhibits mitochondria electron transport by_______ . (A) binding to hemoglobin in the erythrocytes and so blocking the transport of oxygen to tissues (B) binding to the oxygen-binding site of cytochrome oxidase (C) blocking electron transport at the level of the cytochrome b-cytochrome c1 complex (complex III) (D) combining with coenzyme Q and preventing its interaction with nonheme iron-surfur-protein center of complex II (E) inhibiting the electron transfer of complex I

44. An enzyme that catalyzes the conversion of an aldose sugar to a ketose sugar would be classified as one of the_______. (A) oxidoreductases (B) transferases (C) hydrolases (D) isomerases (E) lyases

45. An analysis of chromosomal DNA, using the Southern blot technique, involves the following five major steps: ①Autoradiography； ②Blotting； ③ Cleavage；④Electrophoresis； ⑤Hybridization；Which of the following sequences of steps best illustrates this technique? (A)①②③④⑤ (B) ①③②④⑤ (C) ③⑤②④①(D) ③②⑤④①(E) ③④②⑤①

46. The two nitrogen atoms in urea arise from______. (A) ammonia and glutamine (B) glutamine and aspartic acid (C) glutamine and glutamic acid (D) glutamic acid and alanine (E) ammonia and aspartic acid

47. Which of the following statements about antibiotic inhibitors of protein synthesis is correct? (A) Chloamphenicol inhibits the peptidyl transferase activity of the large ribosomal subunit in eukaryotes. (B) Cycloheximide inhibits the peptidyl transferase activity of the large ribosomal subunit in eukaryotes. (C) Puromycin blocks protein synthesis in prokaryotes only. (D) Streptomycin inhibits protein synthesis by binding to 30S ribosomal subunits. (E) Erythromycin inhibits translocation in eukaryotes only. 

48. A biochemist is attempting to separate a protein X from protein C in a solution. The proteins have the following properties: What type of protein separation techniques might be the best one to separate protein X from protein C? (A) Gel filtration (B) SDS-polyacrylamide gel electrophoresis (C) Affinity chromatography (D) Ion-exchanger chromatography (E) Agarose gel electrophoresis

49. A sequence of amino acids in a certain protein is found to be –Ser–Gly–Pro–Gly–. The sequence is most probably part of a(n)______. (A) β-turn (B) parallel β-sheet (C) α-helix (D) α-sheet (E) antiparallel β-sheet