49. The primary biochemical mechanism of action of the commonly used clinical anticancer chemotherapy drug 5-fluorouracil (5-FU) is to bind and inhibit which of the following enzymes, thereby blocking DNA synthesis?
(A) Dihydrofolate reductase
(B) Ribonucleotide reductase
(C) Thymidylate synthase
(D) Topoisomerase

65. After complete degradation in the body, which of the following amino acids is purely ketogenic and prone to cause ketosis when excessively metabolized? (A) Alanine (B) Serine (C) Glutamate (D) Leucine

66. Many proteins must fold into specific three-dimensional structures to function, but there is a class of proteins that lack a fixed 3D structure even under physiological conditions, yet still possess important biological functions. What are these proteins called? (A) Intrinsically disordered proteins (B) Chaperones (C) Fibrous proteins (D) Membrane proteins 

67. After synthesis, several coagulation factors in the human body (such as prothrombin) require the carboxylation of specific glutamate residues in their structure to gain the ability to bind calcium ions. This post-translational modification reaction most critically requires the participation of which vitamin? (A) Vitamin A (B) Vitamin D (C) Vitamin E (D) Vitamin K

68. In the eukaryotic DNA repair mechanism, which of the following enzymes is responsible for catalyzing the first step of base excision repair by removing the damaged base? (A) DNA polymerase (B) AP endonuclease (C) DNA glycosylase (D) DNA ligase

69. Peptidyltransferase catalyzes the formation of the peptide bond. According to the structural analysis mentioned in the text, how does this catalysis occur? (A) A conserved AMP residue in the 23S rRNA abstracts a proton from the amino group of the A-site aminoacyl-tRNA, making it a better nucleophile. (B) The reaction requires the hydrolysis of at least four ATP molecules to activate the carboxyl group of the C-terminal amino acid. (C) A protein subunit within the 50S ribosome acts as a general acid-base catalyst to stabilize the tetrahedral intermediate. (D) The attacking nucleophile in the formation of a peptide bond is a water molecule provided by the surrounding cytoplasm. (E) Peptide bond formation occurs by transferring the growing polypeptide chain from the P-site tRNA to the aminoacyl-tRNA in the A site.

70. Which of the following statements regarding the activation and signal transduction mechanism of protein kinase A (PKA) is correct? (A) PKA must bind cyclic GMP (cGMP) to be activated. (B) PKA must bind cyclic AMP (cAMP) to be activated. (C) Activation of PKA primarily increases intracellular fatty acid biosynthesis. (D) PKA activation is triggered by direct linkage to calcium channels on the cell membrane surface.

71. When a G protein-coupled receptor on the cell membrane activates Phospholipase C (PLC), PLC hydrolyzes PIP2 on the cell membrane to directly produce which two important secondary messengers? (A) cAMP and cGMP (B) Calcium ions (Ca2+) and NO (C) AMP and PPi (D) IP3 (Inositol 1,4,5-trisphosphate) and DAG (Diacylglycerol)

72. In amino acid metabolic pathways, the transamination reaction catalyzed by aminotransferases most heavily requires the participation of which of the following coenzymes? (A) Thiamine pyrophosphate (TPP) (B) Pyridoxal phosphate (PLP) (C) Coenzyme A (CoA) (D) Flavin adenine dinucleotide (FAD) 

73. Regarding energy conversion and the electron transport chain, human brown adipose tissue can utilize the intracellular proton concentration gradient to generate heat instead of synthesizing ATP. This thermogenic mechanism is mainly attributed to which specific protein on the inner mitochondrial membrane? (A)TP synthase (B) Thermogenin/UCP-1 (C) Complex IV (D) Hexokinase

74. Mature red blood cells lack mitochondria. In addition to relying on glycolysis for ATP production, the pentose phosphate pathway (PPP) is also extremely important. What is the most important physiological significance of this pathway for red blood cells? (A) Produces a large amount of ATP for cell membrane pumps. (B) Produces NADPH to maintain glutathione in its reduced state, protecting the cell from oxidative damage. (C) Produces ribose-5-phosphate for de novo nucleic acid synthesis. (D) NADH to be used by lactate dehydrogenase.

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