Peptide bonds form through a condensation reaction between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of another, releasing H2O. They are covalent and stable.

ER chaperones like BiP (heat shock protein 70) assist in protein folding, prevent aggregation, and aid in proper disulfide bond formation during synthesis.

LDH catalyzes the interconversion of lactate and pyruvate using NAD+ as an electron acceptor and NADH as an electron donor in the coupled redox reaction.

Collagen triple helix is stabilized by hydrogen bonds between the three polypeptide chains and covalent cross-links (lysine and hydroxylysine residues) between molecules, providing mechanical strength.

Leucine is a nonpolar, hydrophobic amino acid with an isobutyl side chain. Serine and threonine are polar uncharged, while aspartate is acidic and polar.

Denaturation is disruption of non-covalent interactions (hydrogen bonds, hydrophobic interactions, ionic bonds) that maintain higher-order structures. Peptide bonds (primary structure) remain intact. Some proteins can refold (renature) if conditions permit, as demonstrated by Anfinsen's ribonuclease experiments.

The isoelectric point is the pH at which the net charge on the protein is zero, resulting in minimum solubility and maximum precipitation.

Proteases are endopeptidases and exopeptidases that hydrolyze peptide bonds in proteins. Amylase acts on carbohydrates, lipase on fats, and nuclease on nucleic acids.

Hemoglobin has quaternary structure consisting of 2 α-globin and 2 β-globin subunits held together by non-covalent interactions.

Leucine has a nonpolar, hydrophobic isopropyl side chain. Serine and asparagine are polar, while lysine is positively charged.