MCQs

Quaternary structure exists only in proteins composed of more than one polypeptide chain, such as hemoglobin.

The hydrophobic effect causes non-polar side chains to cluster in the interior, minimizing contact with water and stabilizing the folded structure.

In an α-helix, the carbonyl oxygen of residue i hydrogen bonds with the amide hydrogen of residue i+4, producing the stable helical conformation.

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.

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.

A monoglyceride consists of glycerol esterified with a single fatty acid.

The phospholipid bilayer provides stability while remaining fluid.