82. A researcher wants to isotope label purines with ¹⁵N for future spectroscopic studies. Which substrates when labeled with the heavy nitrogen would result in the most1 ¹⁵N-enriched purines through de novo synthesis?
(A) Glutamine, aspartate, glycine
(B) Glycine, glutamine, N¹⁰-fbrmyl-THF
(C) Aspartate, glutamine, asparagine
(D) Glycine, carbamoyl aspartate, glutamine
(E) Aspartate, glycine, 5-phosphoribosyl»a-pyrophosphate

22. There is no codon for the amino acid hydroxyproline, but this amino acid is a prominent feature ol collagen structure. Which of the following is a likely explanation? (A) Hydroxyproline is substituted for proline after translation by a cut and patch mechanism. (B) Proline is covalently modified to give hydroxyproline after translation. (C) There is an alternative mechanism for synthesis of proteins that contain hydroxyproline. (D) All of the above are likely explanations. (E) It is not possible to fbnn a hypothesis from tiie information given.

23. Which metabolite levels determine whether acetyl-CoA derived from fetty acids can enter the citric acid cycle? (A) Citrate (B) Succinyl CoA (C) Oxaloacetate (D) a-Ketoglutarate (E) None of the above

24. In one turn of the citric acid cycle, how many steps in the metabolism of oxaloacetate and acetyl-CoA involve net carboxylic acid oxidation? (A) 0 (B) 1 (C) 2 (D) 3 (E) 4

25. Atherosclerosis is a condition characterized by poor metabolism of cholesterol, which is conveyed by lipoproteins. Please indicate the lipoproteins causing the development of this disease: (A) LDL (B) HDL (C) VLDL (D) Chylomicrons (E) Triglycerides

26. Gluconeogenesis is induced in die liver alter severe physical activity. Which chemical is largely used in gluconeogenesis in this case? (A) alanine (B) glucose (C) glutamate (D) lactate (E) pyruvate

27. An enzyme that follows standard Michaelis-Menten kinetics was assayed for reaction velocity at various substrate concentrations. The data are shown below: Based on these kinetic data for the enzyme, what would be the closest estimate of the enzyme's Km value? (A) 2.0mM (B) 1.0mM (C) 250mM (D) 0.5 mM (E) 3.0mM

28. Orotate is converted to what two nucleotides which are used in nucleic acid synthesis? (A) AMP andGMP (B) TMP andCMP (C) UMPandAMP (D) GMP andCMP (E) CMP and AMP

29. How many di施rent reactions involve substrate-level phosphorylation during glycolysis? (A) 2 (B) 3 (C) 4 (D) 5 (E) 0

30. Uncoupling in mitochondria refers to: (A) Stopping electron flow but not stopping ATP synthesis. (B) Stopping ATP synthesis but not stopping electron flow. (C) Blocking the electrons ftom NADH from entering the electron transport system. (D) Interruption of electron flow. (E) None of the above.

31. Which of the following description of membrane potential is false? (A) Resting potential in neurons means when membrane potential became 0 mV. (B) Postsynaptic potentials are graded, and action potentials are all-or-none. (C) Hypetpolarization means membrane potential became more negative. (D) Under physiological condition, Na+ ions entering cells will cause depolarization. (E) Membrane potential is the result of differential ion concentration across plasma membrane of die cell.

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