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MCAT Biochemistry: Amino Acids, Enzymes and Metabolism — quiz

15 multiple-choice questions covering Amino acid polarity, Amino acid charge/pKa, Isoelectric point, Secondary structure/proline, Cysteine disulfide bonds. One answer each; you get a score at the end and the explanation for every question. No account needed.

Updated 2026-11-04. Keys 1–4 pick an answer, Enter moves on. Redo it any time, in order or shuffled.

The questions

Answers and explanations are shown after you finish.

1. Amino acid polarity

   A biochemist is designing the hydrophobic core of a folded protein and must choose residues that avoid water. Which of the following amino acids would be the poorest choice for this core?

   • Phenylalanine
   • Leucine
   • Valine
   • Serine
2. Amino acid charge/pKa

   A researcher measures the protonation state of a side chain at physiological pH and finds it is only about 10% protonated. Which amino acid side chain is this most consistent with?

   • Histidine
   • Aspartate
   • Arginine
   • Lysine
3. Isoelectric point

   Which of the following amino acids would be expected to have the lowest isoelectric point (pI)?

   • Lysine
   • Glycine
   • Glutamate
   • Serine
4. Secondary structure/proline

   A point mutation inserts a proline residue into the middle of an existing alpha helix. What structural consequence is most likely?

   • Enhanced helix stability from added rigidity
   • Disruption of the helix because proline lacks a backbone amide hydrogen for H-bonding
   • Increased hydrophobic packing in the core
   • Formation of a new disulfide bond
5. Cysteine disulfide bonds

   Which amino acid side chain can be oxidized to form a covalent cross-link that stabilizes a protein's tertiary or quaternary structure?

   • Cysteine
   • Methionine
   • Serine
   • Threonine
6. Protein structure levels

   Which level of protein structure is defined specifically by the association of separate, independently folded polypeptide chains into one complex?

   • Secondary
   • Primary
   • Tertiary
   • Quaternary
7. Enzyme kinetics - Km

   Enzyme A has a much lower Km for its substrate than Enzyme B. What does this tell you about Enzyme A?

   • It becomes saturated at a higher substrate concentration
   • It has a lower affinity for its substrate
   • It has a higher affinity for its substrate
   • It has a higher Vmax
8. Enzyme kinetics - Vmax

   Two preparations of the same enzyme show identical Km values but very different Vmax values. What most likely explains this difference?

   • Different inhibitors were present in each sample
   • The preparations contain different amounts of active enzyme
   • Different reaction pH values were used
   • Different substrate concentrations were used
9. Lineweaver-Burk plot

   On a Lineweaver-Burk plot, which feature allows you to directly read off the value of Vmax?

   • The area under the line
   • The y-intercept
   • The slope
   • The x-intercept
10. Enzyme inhibition - competitive

    In an enzyme assay, inhibition is completely reversed when a large excess of substrate is added, and Vmax returns to its uninhibited value. This behavior is characteristic of which type of inhibition?

    • Noncompetitive inhibition
    • Mixed inhibition with equal affinities
    • Competitive inhibition
    • Uncompetitive inhibition
11. Enzyme inhibition - uncompetitive

    An inhibitor binds only to the enzyme-substrate complex, producing parallel lines on a Lineweaver-Burk plot compared to the uninhibited reaction. Which inhibition type does this describe?

    • Competitive
    • Uncompetitive
    • Mixed
    • Noncompetitive
12. Cooperativity

    A binding curve for oxygen to a multi-subunit protein appears S-shaped rather than hyperbolic. What phenomenon best explains this shape?

    • Standard Michaelis-Menten kinetics
    • Positive cooperativity between subunits
    • An unusually low Km for the ligand
    • Competitive inhibition by a second ligand
13. Metabolic regulation - PFK-1

    A cell has abundant ATP and citrate. What effect would this have on the rate-limiting step of glycolysis?

    • It would primarily increase AMP levels
    • It would have no effect on PFK-1 activity
    • It would decrease PFK-1 activity
    • It would increase PFK-1 activity
14. ATP yield - NADH vs FADH2

    Why does oxidation of one molecule of FADH2 generate fewer ATP equivalents than oxidation of one molecule of NADH in the electron transport chain?

    • FADH2 directly inhibits ATP synthase activity
    • FADH2 is chemically less stable than NADH
    • FADH2 requires more oxygen molecules to be fully oxidized
    • FADH2 donates electrons at Complex II, bypassing the proton pumping of Complex I
15. Electron transport chain

    Which complex of the electron transport chain oxidizes FADH2 but does not contribute to proton pumping across the inner mitochondrial membrane?

    • Complex III
    • Complex IV
    • Complex II
    • Complex I