46. Which of the following is the mechanism of action of 5-fluorouracil?
(A) competitive inhibitor of dihydrofolate reductase,
(B) suicide substrate for thymidylate synthase,
(C) competitive inhibitor of ribonucleotide reductase,
(D) non-competitive inhibitor of thioredoxin reductase,
(E) none of the above.

1. The α-helix and β-strand are: (A) primary structure, (B) secondary structure, (C) tertiary structure, (D) quaternary structure, (E) regular structure.

2. What is the active form of methionine and acts as an intracellular methyl group donor? (A) S-adenosylhomocysteine, (B) homocysteine, (C) S-adenosylmethionine, (D) 5-methylthioadenosine, (E) None of the above.

3. What is the overall net charge on the peptide Arg-Gly-Ser-Lys-Glu-Asp at pH 7.0? (A) +2, (B) +1, (C) -2, (D) -1, (E) 0.

4. A Ramachandran plot shows (A) the amino acid residues which have the greatest degree of rotational freedom, (B) the sterically allowed rotational angles between R groups and α-carbons in a peptide, (C) the sterically allowed rotational angles between Cα and the amide nitrogen (Cα-N) as well as between Cα and the amide carbonyl carbon (Cα-CO), (D) the sterically allowed rotational angles about the amide nitrogen (NH) and CO, (E) the amino acid residues that form α-helix, β-sheet, etc.

5. The two enzymes that catalyze the bulk of the nitrogen flow from amino acids to ammonium ion are? (A) transaminases and dehydratases, (B) transaminases and glutamate dehydrogenase, (C) transaminases and amino acid oxidase, (D) glutamate dehydrogenase and dehydratases, (E) amino acid oxidase and dehydratases.

6. Molecules of a given protein have all EXCEPT: (A) a fixed amino acid composition, (B) a defined amino acid sequence, (C) an invariant molecular weight, (D) a nucleotide sequence from which they are encoded, (E) a sequence read from C-terminal end to N-terminal end.

7. The peptide bond has partial ________________ character. (A) hydrogen bond, (B) double bond, (C) triple bond, (D) van der Waals bond, (E) all of the above.

8. A polypeptide "ala-ser-val-asp-leu-gly-glu" is folded into an α-helix. Which amino acid is the carbonyl group of alanine hydrogen bonded? (A) serine, (B) aspartic acid, (C) valine, (D) leucine, (E) glutamic acid.

9. Which amino acid acts as a helix breaker due to its unique structure? (A) histidine, (B) tyrosine, (C) arginine, (D) serine, (E) proline.

10. Biological molecules denaturation will occur in all EXCEPT: (A) dramatic increase in temperature, (B) change in ionic strength, (C) refrigeration, (D) addition of strong acid or base, (E) none, all will denature biological macromolecules.

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