27. Which statement about the transition state of an enzyme-catalyzed reaction is true?
(A) The transition state is identical to the enzyme-substrate complex.
(B) The transition state always has a lower free energy than the substrate.
(C) Enzymes form a lower-energy transition state complex, reducing activation energy.
(D) The transition state cannot be bound by an enzyme due to its instability.
(E) Enzymes stabilize the substrate more than the transition state.

23. The NADH produced by glycolysis produces different amounts of ATP in different tissues. This is because: (A) different tissues have different shuttle system to transport cytosolic NADH into mitochondria. (B) the averaged amounts of mitochondria in the cells of different tissues are different. (C) the efficiency of the respiratory chain in different tissues are different. (D) some tissues can utilize cytosolic NADH to produce ATP; however, some tissues cannot. (E) different tissues have different energy need.

24. Energy balance in the cell (ATP/ADP/AMP levels) is an important factor in the metabolic pathways we have studied. How does this regulation take place? (A) ATP, ADP and/or AMP bind to DNA to modulate transcription of genes for enzymes involved in metabolism. (B) ATP, ADP and/or AMP bind to enzymes involved in metabolism, allosterically changing the enzyme activity. (C) High levels of ATP result in nonenzymatic phosphorylation of the enzymes involved in metabolism, changing their activity. (D) High levels of ATP result in excessive production of cyclic AMP, causing activation of pathways such as glycogenolysis and gluconeogenesis. (E) None of the above.

25. When a muscle is stimulated to contract aerobically, less lactic acid is formed than when it contracts anaerobically because: (A) glycolysis does not occur to significant extent under aerobic conditions. (B) muscle is metabolically less active under aerobic than anaerobic conditions. (C) the lactic acid generated is rapidly incorporated into lipids under aerobic conditions. (D) under aerobic conditions in muscle, the major energy-yielding pathway is the pentose phosphate pathway, which does not produce lactate. (E) under aerobic conditions most of the pyruvate generated as a result of glycolysis is oxidized by the citric acid cycle rather than reduced to lactate.

複選題26. Glutamine provides the critical entry point for the amine resource. Which statement is correct? (A) Glutamine synthetase is active when adenylation. (B) Glutamine synthetase is modulated by PIII protein. (C) Adenylyl transferase promotes uridylylation/deuridylylation of glutamine synthetase. (D) The active/inactive glutamine synthetase is also modulated by uridylyl transferase. (E) Glutamine is the primary molecule carrying CO2.

28. The tautomeric forms of bases can lead to: (A) Base-pair mismatches and spontaneous mutations. (B) Higher melting temperature (Tm). (C) Increased stability of the DNA helix. (D) Covalent bonds between bases across strands. (E) Accelerated replication speed in eukaryotes.

29. When a ligand binds a receptor tyrosine kinase, the first step in the signaling cascade is usually: (A) Conversion of GTP to cGMP. (B) GTP hydrolysis by the α-subunit of the G protein. (C) CAMP formation by adenylyl cyclase. (D) Internalization of the receptor into lysosomes. (E) Auto-phosphorylation of specific tyrosine residues on the receptor.

30. What role does phospholipase C play in signal transduction? (A) Activates adenylate cyclase. (B) Produces IP3 and DAG from phosphatidylinositol 4,5-bisphosphate (PIP2). (C) Converts cAMP to AMP. (D) Activates protein kinase C directly. (E) Converts GTP to GDP.

31. Which of the following equations represents the Michaelis-Menten equation? (A) (B)(C)(D)(E) ν = E-[ES]

32. What is the effect of a competitive inhibitor on an enzyme-catalyzed reaction? (A) Has no effect on either (B) Increases without affecting . (C) Increases without affecting (D) Decreases without affecting (E) Decreases

33. Which of the following statements is   incorrect   regarding the cholesterol metabolism? (A) Chylomicrons contain a high portion of cholesterol, which are synthesized from dietary fats in the endoplasmic reticulum of enterocytes. (B) Cholesteryl esters are formed in the liver, converting the cholesterol to a more hydrophobic form that can be carried in lipoproteins to tissues that need cholesterol. (C) Bile acids, one of the forms of cholesterol exported from the liver, are the principal components of bile, a fluid stored in the gallbladder and excreted into the small intestine. (D) The uptake of cholesterol from the bloodstream is regulated by receptor-mediated endocytosis. (E) Excess cholesterol in extrahepatic tissues, including macrophages and foam cells, is transported back to the liver in reverse cholesterol transport.