43. The attached figure shows the catalytic mechanism of a protease. According to this figure, what is the catalytic function of Asp X as shown in step 1?
(A)Acid
(B) Base
(C) Oxidant
(D) Reductant
(E) Proton donor

1. Tfunction of the eukaryotic DNA replication factor PCNA (proliferating cell nuclear antigen) is similar to that of the β-subunit of bacterial DNA polymerase III in that it: (A)Participates in DNA repair (B) Facilitates replication of telomeres (C) Has a 3'→5' proofreading activity (D) Increases the speed but not the processivity of the replication complex (E) Forms a circular sliding clamp to increase the processivity of replication

2. The 5' → 3' exonuclease activity of E. coli DNA polymerase I is involved in: (A)Sealing of nicks by ligase action (B) Formation of Okazaki fragments (C) Proofreading of the replication process (D) Removal of RNA primers by nick translation (E) Formation of a nick at the DNA replication origin

3. The role of the Dam methylase is to: (A)Remove a methyl group from thymine (B) Replace a mismatched nucleotide with the correct one (C) Add a methyl group to uracil, converting it to thymine (D) Remove a mismatched nucleotide from the template strand (E) Modify the template strand for recognition by repair systems

4. In homologous recombination in E. coli, the protein that assembles into long, helical filaments that coat a single-strand region of DNA is: (A)Histone (B) RecA protein (C) DNA methylase (D) RecBCD enzyme (E) DNA polymerase

5. Which amino acid has an unusual side-chain that includes the nitrogen of the amino group attached to the α-carbon? (A)Asparagine (B) Proline (C) Tryptophan (D) Tyrosine (E) Histidine

6. Human blood plasma contains about 7% protein. These plasma proteins have pK values close to 4 or 5. In the test tube, these proteins will form an insoluble precipitate after all of the following treatments except (A)Boiling the serum for 5 minutes (B) Adding sodium chloride to a concentration of 35% (C) Adjusting the pH to 4.5 (D) Boiling the serum with 6 N hydrochloric acid for 10 hours (E) Mixing one volume of plasma with two volumes of pure alcohol

7. The molecule 2,3- bisphosphoglycerate (BPG) is present in red blood cells, in which it binds noncovalently to hemoglobin. Which functional groups in hemoglobin can make the strongest noncovalent interactions with BPG at a pH value of 7.0? (A)Sulfhydryl groups (B) Alcoholic hydroxyl groups (C) Hydrocarbon groups (D) Amino groups (E) Carboxyl groups

8. In sickle cell hemoglobin, a Glu is mutated to a _____ . This causes the formation of _____ between hemoglobin molecules, ultimately forming large aggregates. (A)Arg； salt bridges (B) Cys； disulfide bonds (C) Val； hydrophobic interactions (D) Pro； disrupted o-helix, resulting in several H-bonds and salt bridges (E) None of the above

9. Which of the following statements about the reactions of glycolysis is correct? (A)Glucose -1:6-bisphosphate is split into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (B) Fructose-1:6-bisphosphate is split into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (C) Fructose-6-phosphate is split into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (D) Glucose-6-phosphate is isomerized to fructose-1:6-bisphosphate (E) None of the above

10. Which one of the following molecules donates a phosphate group to ADP to produce ATP in the reactions of glycolysis? (A)Glucose-6-phosphate (B) Phosphoenolpyruvate (C) Fructose-6-phosphate (D)Fructose-1,6-bisphosphate (E) None of the above

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