23. The isoelectric point of tyrosine is_____________ .(PK₁ = 2.2； pK₂= 9.11； pKa = 10.07 of the side chain)
(A) 5.66
(B)6.135
(C) 7.126
(D) 9.11
(E) 9.59

19. Which of the following molecule can cross cell membranes freely? (A) Carbon dioxide (B) Na+ (C) K+ (D) All of the above (E) None of the above

20. The allosteric enzyme PFK-1 (Phosphofructokinase-1) catalyzes the committed step of glycolysis. Which of the following statements about PFK-1 is correct? (A) It is inhibited by ADP and stimulated by AMP. (B) It is inhibited by high pH. (C) It is stimulated by citrate. (D) It is stimulated by insulin but is inhibited by epinephrine in muscle cells. (E) It is stimulated by insulin and inhibited by glucagon in liver cells.

21. Which of the following pairs of amino acids could form a charge-charge interaction through their R- groups (side chains)? (A) serine and glutamic acid (B) glutamine and lysine (C) methionine and histidine (D) lysine and arginine (E) aspartic acid and lysine

22. NADPH is synthesized mainly in which of the following pathways? (A) Glycolysis (B) Pentose Phosphate Pathway (C) TCA cycle (D) Urea cycle (E) Gluconeogenesis

24. The disease gout is a disease of the joints, usually in males, caused by an elevated concentration of _________in the blood and tissues. (A) heme (B) xanthine (C) hypoxanthine (D) uric acid (E) inosine

25. Which of the following amino acids does not belong to glucogenic amino acid? (A) Ala(B) Arg (C) Cys (D) Glu (E) Leu

26. Which is not an enzyme required for glycolysis? (A) Hexokinase (B) Phosphofructokinase (C) Triosephosphate isomerase (D) Cyclin-dependent kinase (E) Pyruvate kinase

27, Which of the following is not a post-translational modification of a protein? (A) Methylation (B) Phosphorylation (C) Alternative splicing (D) Disulfide bond (E) Acetylation

28. Lecithin is composed of _________.(A) Glycerol + Fatty acid + Phosphoric acid + Serine (B) Glycerol + Fatty acid + Phosphoric acid + Ethanolamine (C) Glycerol + Fatty acid + Phosphoric acid + Betaine (D) Glycerol + Fatty acid + Phosphoric acid + Choline (E) None of the above

29. Amino acid residues which are predominantly involved in the protein-DNA interaction are___________. (A) positively charged (B) negatively charged (C) proline (D) alanine (E) glycine