peptidyl

Peptidyl is a biochemical term used to describe a fragment of a peptide that is attached to another molecule or to the growing peptide chain in a biological system. The word combines peptide with the suffix -yl, a conventional way to denote a substituent or radical derived from a larger molecule. The term emphasizes that the moiety in question is peptide-derived and not a free amino acid or complete protein.

In biology, peptidyl often appears in the context of protein synthesis and enzymatic processes. In translation, the peptidyl-tRNA carries the nascent peptide chain, and the peptidyl group is transferred to an incoming aminoacyl-tRNA in the ribosome’s peptidyl transferase step, extending the growing polypeptide. The term also appears in naming peptide fragments or derivatives that remain attached to another molecule, such as peptidyl-tRNA, peptidyl-lysine, or peptidyl-resin used in solid-phase peptide synthesis. In these contexts, peptidyl denotes the peptide portion that is bound to, or part of, a larger structure.

Peptidyl is distinct from the full peptide or protein itself; it denotes a peptide fragment acting as a substituent or intermediary in a chemical or biological process. Related concepts include peptide, polypeptide, peptidyl transferase, and peptide synthesis, which describe the origin, transfer, and assembly of peptide fragments in cells and laboratory settings.