Practice Questions

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.

The α-carbon of 19 of the 20 standard amino acids is attached to four different groups, making it a chiral center. With very rare exceptions, ribosomes exclusively incorporate amino acids with the L-configuration into proteins. Glycine has two hydrogens and is thus achiral.

The peptide bond exhibits resonance between the carbonyl oxygen and the amide nitrogen. This resonance gives the C-N bond approximately 40% double-bond character, restricting rotation and making the six atoms of the peptide group lie in a single plane.

Essential amino acids lack the necessary biosynthetic pathways in the organism. For humans, there are nine essential amino acids (e.g., lysine, valine, phenylalanine). Non-essential amino acids can be synthesized from common metabolic intermediates.

All 20 standard amino acids (except proline, which is an imino acid) are α-amino acids. They contain a central α-carbon to which an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable R-group are attached.

Acylglycerols are esters formed by the reaction of glycerol with one or more fatty acids.