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

A mutation replaces a residue involved in a salt bridge with a partner carboxylate (–COO$^-$) at pH 7.4. Which new residue is most likely to preserve the electrostatic interaction by remaining positively charged at pH 7.4?

General amino acid: $\mathrm{H_3N^+-CH(R)-COO^-}$

A folded protein is exposed to increasing pH from 7.0 to 11.0, and a surface patch becomes more negatively charged, increasing electrostatic repulsion. Which residue type on the surface is most consistent with gaining negative charge over this pH range due to side-chain deprotonation?

General amino acid: $\mathrm{H_3N^+-CH(R)-COO^-}$

A peptide containing a single histidine is analyzed by isoelectric focusing in a buffer that is stepped from pH 5.5 to pH 8.5. The residue of interest has an imidazole side chain (p$K_a$ near 6). Which conclusion about the histidine side chain is most consistent with its predominant charge state at pH 8.5?

General amino acid: $\mathrm{H_3N^+-CH(R)-COO^-}$

A researcher compares two peptides that differ at one position: one has an R-group of –CH$_2$–OH and the other has –CH$_2$–CH$_2$–S–CH$_3$. In an aqueous buffer at pH 7.4, which conclusion about that position is most consistent with amino acid classification?

General amino acid: $\mathrm{H_3N^+-CH(R)-COO^-}$

A short peptide is placed in a solution at pH 2.0. Consider a residue with a side chain containing a carboxyl group (–CH$_2$–COOH) in addition to the backbone carboxyl. Which statement is most consistent with the side-chain charge of that residue at pH 2.0?

General amino acid: $\mathrm{H_3N^+-CH(R)-COO^-}$

A membrane protein segment is reconstituted into lipid vesicles. A single residue in the middle of a transmembrane helix is mutated, and the mutant shows reduced insertion efficiency into the hydrophobic bilayer core. Which replacement is most consistent with introducing an energetically unfavorable polar/charged group into the bilayer interior?

General amino acid: $\mathrm{H_3N^+-CH(R)-COO^-}$

Which substitution least disrupts a membrane-spanning alpha helix?

Which residue shows the greatest side-chain charge change from pH 6 to 8?

A protein's 280 nm absorbance comes mainly from which side chains?

Raising pH from 7 to 11 disrupts a salt bridge. Which pair is responsible?

Which substitution most increases backbone flexibility in a protein loop?

A soluble cytosolic protein is engineered to increase stability by strengthening its hydrophobic core. One candidate substitution introduces an amino acid with $R = \text{CH}_3$. The backbone is $\mathrm{H_3N^+-CH(R)-COO^-}$ at pH 7.4. Which outcome is most consistent with this residue's classification in a folded, water-soluble protein?

An enzyme active site contains a residue that must act as a nucleophile at pH 7.0 by presenting a deprotonated heteroatom on its side chain. A point mutation replaces this residue with one that is usually protonated (positively charged) at pH 7.0, reducing catalysis. Which substitution is most consistent with this loss of nucleophilicity due to side-chain classification?

An enzyme active site requires a residue that can accept a proton near physiological pH and often participates in acid–base catalysis. A mutation replaces this residue with one whose side chain is $R = \text{CH}_2\text{-CH}_2\text{-COO}^-$. Which substitution is most likely to preserve the original residue's basic classification and catalytic role?

A peptide (10 residues) is analyzed by isoelectric focusing. At pH 3.0 the peptide migrates toward the cathode, but at pH 10.0 it migrates toward the anode. The investigator suspects a single side chain with a p$K_a$ near physiological pH contributes substantially to the charge change. Which residue is most consistent with this behavior based on its side-chain functional group?

A membrane-embedded helix is being designed for stable insertion into a lipid bilayer. A position facing the lipid tails is assigned an amino acid with side chain $R = \text{CH}_2\text{-CH}_2\text{-S-CH}_3$. Which classification best matches this residue and its compatibility with the bilayer core?

A researcher compares two residues for placement on the solvent-exposed surface of a globular protein at pH 7.4: (1) $R = \text{CH}_3$ and (2) $R = \text{CH}_2\text{-COO}^-$. Which choice is most consistent with maximizing surface compatibility with water?

A peptide is synthesized with one residue whose side chain is $R = \text{H}$ (the backbone is $\mathrm{H_3N^+-CH(R)-COO^-}$ at neutral pH). Which classification is most consistent with this residue and its typical role in folded proteins?

A transmembrane helix is predicted to span the lipid bilayer. A single residue in the middle of the helix is mutated, and membrane insertion efficiency drops sharply. Which replacement is most likely to cause this effect by introducing an ionizable side chain into the hydrophobic membrane interior?

A soluble cytosolic protein is engineered to include a new surface-exposed residue to increase aqueous solubility without adding a formal charge at pH 7.4. Which amino acid is most likely to achieve this by providing a polar, uncharged side chain capable of hydrogen bonding?