MCQs

Protein function depends on proper folding into the native three-dimensional structure.

The heme prosthetic group contains Fe²⁺, which reversibly binds oxygen.

β-sheets are classified according to the orientation of adjacent strands.

Histidine's pKa is close to physiological pH, making it ideal for acid-base catalysis.

Chaperones bind exposed hydrophobic regions of unfolded proteins, preventing aggregation and promoting correct folding.

Heat disrupts weak interactions but usually leaves the covalent peptide backbone intact.

Collagen contains the repeating sequence Gly-X-Y. Glycine occurs every third residue, allowing tight packing of the triple helix.

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.