Proline

Proline is a protein-building amino acid whose side chain bonds to its backbone nitrogen, forming a five-membered ring. This structure constrains its shape and gives its peptide bonds distinctive behavior.

Amino acid: An organic molecule with amino and carboxyl groups that serves as a protein building block. Proline belongs to this family but bonds its side chain to its amino nitrogen.
Ramachandran plot: A graph showing allowed protein-backbone torsion angles for amino-acid residues. Proline occupies a restricted region because its ring limits backbone rotation.
Collagen: A structural protein that forms fibers and provides strength to connective tissues. Collagen’s repeating proline and hydroxyproline residues help stabilize its triple helix.
Glycine: The smallest protein-building amino acid, with hydrogen as its side chain. Glycine increases backbone flexibility, opposite proline’s strong conformational constraint.
Peptide bond: A covalent bond linking the carboxyl group of one amino acid to the amino group of another. Proline’s backbone nitrogen participates in peptide bonds despite being part of its ring.
Cis-trans isomerism: A form of geometric isomerism caused by restricted rotation around a bond. Proline-containing peptide bonds switch between cis and trans forms more often than most peptide bonds.
Hydroxyproline: A hydroxylated derivative of proline found abundantly in collagen. It is formed by post-translational modification of collagen proline residues.
N-methyl amino acid: An amino acid whose backbone nitrogen bears a methyl group. Like proline, it modifies backbone nitrogen chemistry, but without proline’s ring closure.
Protein structure: The organization of a protein’s amino-acid chain into local and three-dimensional forms. Proline’s constrained backbone influences how protein chains fold.
Peptidyl-prolyl isomerase: An enzyme that accelerates cis-trans isomerization of peptide bonds preceding proline. These enzymes help proteins overcome slow proline-bond rearrangements during folding.

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