polypeptids

Polypeptids are polymers composed of amino acid residues linked by covalent peptide bonds. They form the fundamental building blocks of proteins, and a protein may consist of a single polypeptid chain or multiple polypeptid subunits assembled together. In many contexts the terms polypeptid and polypeptide are used interchangeably, with polypeptid generally referring to a chain-length capable of folding into a functional protein.

The primary structure of a polypeptid is the linear sequence of amino acids, which determines its chemical properties and potential folds. Higher levels of structure include secondary structures such as alpha helices and beta sheets, tertiary structure (the three-dimensional arrangement of a single polypeptid), and quaternary structure when multiple polypeptid chains associate. The amino acid sequence, along with environmental conditions, governs folding and stability.

Biosynthesis occurs mainly through ribosomal translation of messenger RNA, where tRNAs donate amino acids and the ribosome catalyzes peptide bond formation in a condensation reaction. Polypeptids can also be produced synthetically in laboratory settings via solid-phase peptide synthesis. Post-translational modifications, such as phosphorylation or glycosylation, can alter activity, localization, or stability. Some polypeptids remain unstructured under physiological conditions, while others fold into well-defined conformations essential for function.

Applications of polypeptids include studying protein folding, designing therapeutics, and creating materials. Understanding their sequence-structure-function relationships is central to biochemistry, molecular biology, and biotechnology.