MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 1a Amino Acid Structure Classification

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This deck focuses on 1a Amino Acid Structure Classification, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: Which amino acid is sulfur-containing but is not capable of forming disulfide bonds?

Answer: Methionine. Its thioether side chain contains sulfur but lacks the reactive thiol needed for disulfide formation.

Flashcard 2: Which amino acid has a guanidinium side chain and is strongly basic at physiological pH?

Answer: Arginine. The guanidinium group has a very high pKa, ensuring it stays protonated and basic in physiological conditions.

Flashcard 3: At pH $7.4$, which side chain is most likely to act as a general acid/base in enzyme active sites?

Answer: Histidine. Its imidazole side chain has a pKa near 6.5, enabling proton donation or acceptance at physiological pH in catalysis.

Flashcard 4: Which amino acid has an imidazole side chain and can be neutral or positive near physiological pH?

Answer: Histidine. The imidazole ring has a pKa around 6, allowing it to toggle between protonated and deprotonated states near pH 7.

Flashcard 5: Which amino acids have amide side chains (carboxamide functional group)?

Answer: Asparagine (Asn) and glutamine (Gln). Amide groups in their side chains are derived from aspartate and glutamate but are neutral and polar.

Flashcard 6: Which amino acids have hydroxyl-containing side chains (alcohol functional group)?

Answer: Serine and threonine. Their side chains contain alcohol groups, which are polar and can form hydrogen bonds or be phosphorylated.

Flashcard 7: Which amino acids have carboxylate side chains (acidic side chains)?

Answer: Aspartate (Asp) and glutamate (Glu). Their additional carboxylic acid groups deprotonate at physiological pH, conferring acidity and negative charge.

Flashcard 8: Which amino acids are classified as branched-chain amino acids (BCAAs)?

Answer: Valine, leucine, isoleucine. These amino acids have branched aliphatic side chains, which are metabolized via specific pathways in muscle tissue.

Flashcard 9: Identify the amino acid that is both polar and aromatic in standard MCAT classifications.

Answer: Tyrosine. Its phenolic side chain combines an aromatic ring with a polar hydroxyl group, fitting both categories.

Flashcard 10: Which amino acid has a side chain that is a primary amine and is typically positively charged?

Answer: Lysine. The ε-amino group has a high pKa, remaining protonated and positively charged at physiological pH.

Flashcard 11: What is the definition of an amino acid side-chain $pK_a$ in acidbase terms?

Answer: $pK_a$ is the pH where the group is $50\%$ protonated. It represents the equilibrium point where half of the side-chain groups are protonated and half are deprotonated.

Flashcard 12: Which amino acids are classified as nonpolar (hydrophobic) aliphatic side chains on the MCAT?

Answer: Gly, Ala, Val, Leu, Ile, Met, Pro. These amino acids have hydrocarbon side chains that lack polarity, promoting hydrophobic interactions in proteins.

Flashcard 13: Which amino acids are aromatic on the MCAT classification scheme?

Answer: Phe, Tyr, Trp. Their side chains contain aromatic rings, which can participate in π-π stacking and hydrophobic interactions.

Flashcard 14: Which amino acids are polar uncharged at physiological pH on the MCAT?

Answer: Ser, Thr, Asn, Gln, Cys, Tyr. These side chains have polar functional groups like hydroxyls, amides, or thiols but remain neutral at pH 7.4.

Flashcard 15: Which amino acids have side chains that are negatively charged at physiological pH?

Answer: Aspartate (Asp), Glutamate (Glu). Their carboxylic acid side chains have low pKa values, leading to deprotonation and negative charge at pH 7.4.

Flashcard 16: Which amino acids have side chains that are positively charged at physiological pH?

Answer: Lysine (Lys), Arginine (Arg), Histidine (His). Their basic side chains have high pKa values, remaining protonated and positively charged at physiological pH.

Flashcard 17: Which amino acid side chain commonly forms disulfide bonds in oxidizing environments?

Answer: Cysteine. Its thiol group can oxidize to form covalent disulfide bridges, stabilizing protein tertiary and quaternary structures.

Flashcard 18: Which amino acid has a phenolic side chain and is often a phosphorylation site?

Answer: Tyrosine. The phenolic hydroxyl group can be phosphorylated by kinases, regulating protein function in signaling pathways.

Flashcard 19: Which three amino acids are the most common targets for phosphorylation in proteins?

Answer: Serine, threonine, tyrosine. Their side-chain hydroxyl groups serve as nucleophiles for phosphate attachment in post-translational modifications.

Flashcard 20: What is the zwitterion form of a typical amino acid at physiological pH?

Answer: $\mathrm{NH_3^+}$ and $\mathrm{COO^-}$ on the same molecule. At physiological pH, the amino group is protonated and the carboxyl group is deprotonated, forming a dipolar ion.

Flashcard 21: Which amino acid is not chiral among the 20 standard amino acids?

Answer: Glycine. Its side chain is a hydrogen atom, making the α-carbon non-asymmetric and thus achiral.