Practice Test 2 - Biochemistry

20 questions. Answer them in any order, then submit to see your score and every explanation.

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1. A newly discovered protein is found to contain a zinc-finger motif. This structural motif is characterized by a precise coordination of a zinc ion by cysteine and/or histidine residues, creating a compact finger-like fold. Based on the known functions of many other zinc-finger proteins, what is the most probable biochemical function of this new protein?
2. A researcher compares ATP yield from glucose under three conditions: (1) aerobic respiration, (2) alcoholic fermentation, and (3) lactic acid fermentation. If the efficiency of aerobic respiration is defined as 100%, what is the approximate efficiency of the fermentation processes?
3. A mutant protein has a cysteine residue replaced with serine at a critical position. Based on the chemical properties of these amino acids, which functional consequence is most likely?
4. Based on the description of these binding curves, what can be concluded about the interaction of these ligands with the receptor?
5. A researcher identifies a single nucleotide deletion in the second exon of a gene that normally produces a 300-amino acid protein. If the deletion occurs at nucleotide position 150 (where position 1 is the start codon), what is the most likely consequence for the resulting protein?
6. An experimenter synthesizes three different RNA molecules with identical nucleotide composition but different sequences. Molecule A forms extensive secondary structure with multiple hairpins, Molecule B remains largely single-stranded, and Molecule C forms a compact globular structure. When tested for stability against RNase digestion, thermal denaturation, and chemical modification, which molecule would most likely show the highest resistance to RNase but the greatest sensitivity to thermal denaturation, and what does this suggest about its structural organization?
7. Two muscle fiber types show different metabolic responses to the same exercise intensity. Type I fibers maintain steady ATP levels while Type II fibers show 25% ATP depletion. If both fiber types have similar mitochondrial density, what best explains the difference in metabolic efficiency?
8. Which of the following best describes fermentation's role in cells when oxygen is unavailable for aerobic respiration?
9. Soluble dietary fiber is effective at lowering serum cholesterol levels. The primary mechanism for this effect involves the interruption of the enterohepatic circulation of which class of molecules?
10. The high energy density of fatty acids is fundamentally due to the reduced state of their carbon atoms. Consider palmitic acid (C₁₆H₃₂O₂) and a hexose like glucose (C₆H₁₂O₆). How does the average oxidation state of carbon in palmitic acid compare to that in glucose?
11. A patient with type 2 diabetes shows elevated fasting glucose despite taking metformin. Laboratory analysis reveals normal muscle glucose uptake but excessive hepatic glucose production. The physician considers adding a medication that specifically targets liver metabolism. Based on the tissue-specific metabolic defect, which enzymatic target would be MOST appropriate?
12. A cell responds to an external signal through a pathway where a receptor tyrosine kinase (RTK) activates a downstream kinase cascade. The signal is terminated in part by a protein tyrosine phosphatase (PTP) that dephosphorylates and inactivates the RTK. A mutation that significantly increases the affinity of the PTP for the activated receptor would have what effect on the signaling process?
13. An experiment measures the incorporation of $^{14}C$ from [1-$^{14}C$]pyruvate (carbon-1 labeled) into acetyl-CoA by the pyruvate dehydrogenase complex. If the reaction proceeds normally, where will the radioactive carbon appear in the acetyl-CoA product?
14. During prolonged fasting (24-48 hours), a patient shows elevated blood ketone bodies and decreased muscle protein synthesis. If this patient were given a glucose infusion sufficient to raise blood glucose to normal fed levels, which metabolic change would occur FIRST in skeletal muscle tissue?
15. Acetylcholinesterase rapidly hydrolyzes acetylcholine at nerve synapses, proceeding through a covalent acyl-enzyme intermediate with Ser203. The active site contains an 'anionic subsite' that binds the positively charged quaternary ammonium group of acetylcholine, and an 'esteratic subsite' where hydrolysis occurs. Mutation of Trp86 in the anionic subsite to alanine reduces kcat/Km by 3000-fold, despite this residue being 20 Å away from the site of bond cleavage. Which explanation BEST accounts for this dramatic effect?
16. A researcher uses size exclusion chromatography to separate a mixture of proteins using a column with an exclusion limit of 150 kDa and a fractionation range of 10-150 kDa. The mixture contains: Protein A (200 kDa), Protein B (75 kDa), Protein C (25 kDa), and Protein D (5 kDa). If Protein D unexpectedly elutes much later than predicted based on its molecular weight, what is the most likely explanation?
17. The folding of small, single-domain proteins is often a highly cooperative, 'all-or-none' process. Which experimental observation provides the strongest evidence for this cooperativity?
18. Which enzyme produces NADH during $α$-ketoglutarate → succinyl-CoA?
19. Context: Primary structure determines where side chains appear along the backbone. How do peptide bonds influence the function of a protein?
20. Which of the following describes a β-sheet configuration where adjacent strands run in the same direction?

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