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

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

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

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

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

50. Which of the following is (are) true of the oxidation of 1 mol of palmitate (a 16-carbon saturated fatty acid； 16:0) by the β-oxidation pathway, beginning with the free fatty acid in the cytoplasm?①Activation of the free fatty acid requires the equivalent of two ATPs.② Inorganic pyrophosphate (PPi) is produced.③Carnitine functions as an electron acceptor.④8 mol of FADH2 are formed.⑤8 mol of acetyl-CoA are formed.⑥There is no direct involvement of NAD+ . (A)①and⑤ (B) ①②and⑤(C) ①② and⑥ (D) ①③ and ⑤ (E)④ and⑤

51. Which of the following peptides is more likely to be nuclear targeting signal? (A) ser-ser-thr-asn-arg-lys-met-thr (B) arg-lys-lys-arg-arg-lys-leu-met (C) met-leu-ile-phe-leu-met-ile-ile (D) ser-ser-thr-thr-thr-thr-ser-thr-ser (E) asn-lys-ser-asn-lys-ser-asn-lys-ser

52. Myoglobin and the subunits of hemoglobin have______ . (A) no obvious structural relationship (B) very different primary and tertiary structures (C) very similar primary and tertiary structures (D) very similar primary structures, but different tertiary structures (E) very similar tertiary structures, but different primary structures