Practice Questions

Secondary structures are stabilized by hydrogen bonding between peptide backbone atoms rather than side chains.

Basic amino acids have side chains that accept protons. Lysine contains an ε-amino group that is positively charged at physiological pH.

Peptide bond formation is a dehydration synthesis. The carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH₂) of another, releasing a water molecule (H₂O) and forming a covalent amide linkage (-CO-NH-).

Primary structure is the linear, genetically determined sequence of amino acids in a polypeptide chain, held together by covalent peptide bonds. This sequence dictates all higher levels of protein structure.

Complete acid hydrolysis (e.g., 6M HCl at 110°C for 24 hours) cleaves all peptide bonds in a protein, releasing the constituent free amino acids. Partial hydrolysis yields smaller peptides (di-, tri-, and oligopeptides).

In proline, the R-group forms a pyrrolidine ring by bonding back to the amide nitrogen. This cyclization eliminates the amide hydrogen needed for H-bonding in an α-helix and imposes a rigid, fixed kink in the polypeptide backbone, disrupting the regular helical conformation.

The thiol (-SH) group of cysteine's side chain can be oxidized to form a covalent disulfide bond (-S-S-) with another cysteine residue. This bond is critical for stabilizing the tertiary structure of secreted proteins like insulin and immunoglobulins. Methionine contains sulfur but cannot form disulfide bridges.

A carbon atom must be bonded to four different groups to be chiral. The α-carbon of glycine is bonded to an amino group, a carboxyl group, and two hydrogen atoms. Since two substituents are identical, it is not a chiral center, and glycine is optically inactive.

The pI is the pH where the net charge on the amino acid is zero. At this pH, the molecule is a zwitterion and will not move towards either the anode or cathode during electrophoresis. For neutral amino acids, pI is the average of pKₐ₁ and pKₐ₂.

At the isoelectric point (pI), the amino group is protonated (-NH₃⁺) and the carboxyl group is deprotonated (-COO⁻). The molecule carries equal positive and negative charges, making it electrically neutral overall, termed a zwitterion.