Free MCAT sample questions: Amino Acids, Proteins and Enzymes

Q1 What is the approximate net charge of the tripeptide Lys-Asp-Gly at pH 7.4?

• A. -1
• B. 0
• C. +1
• D. +2

Answer: B. At pH 7.4 the free N-terminal amine is +1, the Lys side chain is +1, the Asp side chain is -1, and the free C-terminal carboxylate is -1, for a net charge of 0. Answering +1 or +2 forgets one or both negative groups, and -1 forgets the N-terminus.

Q2 Why does the peptide bond not rotate freely?

• A. It is a hydrogen bond between backbone groups
• B. It is an ionic bond between charged ends
• C. It is a disulfide bond between two cysteines
• D. It has partial double bond character from resonance

Answer: D. Delocalization of the nitrogen lone pair onto oxygen makes the amide plane rigid. A hydrogen bond, ionic bond, or disulfide would be a different type of interaction and would not describe the amide linkage.

Q3 An enzyme transfers an amino group from one molecule to another. Which EC class?

• A. Transferase
• B. Ligase
• C. Lyase
• D. Isomerase

Answer: A. Group transfer defines EC 2 (transferases), as with aminotransferases. Ligases join molecules using ATP, lyases cleave without water or redox, and isomerases rearrange one molecule.

Q4 Aspirin inhibits cyclooxygenase by:

• A. Covalently acetylating a serine in the active site
• B. Competing with arachidonic acid reversibly
• C. Binding the enzyme-substrate complex
• D. Removing the heme

Answer: A. Aspirin transfers an acetyl group to a Ser, permanently blocking the enzyme. Reversible competitors (such as ibuprofen) behave differently, and aspirin does not target ES or heme.

Q5 Iron in the heme of hemoglobin must be in which state to bind oxygen?

• A. Fe2+
• B. Fe3+
• C. Fe0
• D. Fe4+

Answer: A. Fe2+ (ferrous) binds O2 reversibly. Fe3+ (methemoglobin) does not bind oxygen, and Fe0 and Fe4+ are not the physiological states in heme.