1.6-1.7 Nucleic Acids & Proteins

What chemical feature distinguishes hydrophobic amino acid R groups from hydrophilic ones?

Hydrophobic R groups consist mostly of nonpolar hydrocarbons (C and H), while hydrophilic R groups contain electronegative atoms (O, N) or charges.

Which amino acid has an R group consisting of only a single hydrogen atom?

Glycine (Gly, G).

Which amino acid forms covalent disulfide bridges through its sulfur-containing R group?

Cysteine (Cys, C).

What unique structural property does the special amino acid Proline have?

Its R group forms a ring structure that loops back and connects to its own amino group backbone.

What charge do acidic amino acid R groups carry at biological pH, and which two amino acids belong to this group?

They carry a negative charge (-); examples are Aspartic acid (Asp, D) and Glutamic acid (Glu, E).

What charge do basic amino acid R groups carry at biological pH, and which three amino acids belong to this group?

They carry a positive charge (+); examples are Lysine (Lys, K), Arginine (Arg, R), and Histidine (His, H).

In secondary structure, which specific atoms form the hydrogen bonds that stabilize alpha-helices and beta-pleated sheets?

Hydrogen bonds form between the oxygen of a carbonyl group (C=O) and the hydrogen of an amine group (N-H) along the polypeptide backbone.

What role do R groups play in forming secondary protein structures?

None—secondary structure is stabilized entirely by hydrogen bonding along the peptide backbone, regardless of the R groups.

What four types of interactions between R groups drive tertiary structure folding?

1) Hydrogen bonding between polar R groups, 2) Charge attractions (ionic bonds) between acidic (-) and basic (+) R groups, 3) Hydrophobic interactions/collapse, and 4) Covalent disulfide bridges between cysteines.

Which tertiary structure interaction is the strongest and why?

Disulfide bridges, because they are strong covalent bonds between sulfur atoms in cysteine side chains.