Polipeptid

Polipeptid, often called a polypeptide in English, is a polymer of amino acids linked by peptide bonds. Each amino acid joins to the next through a condensation reaction between the amino group of one residue and the carboxyl group of the preceding one, releasing water. The chain has an amino terminus (N-terminus) and a carboxyl terminus (C-terminus). Short chains may be referred to as peptides; longer chains are usually called polypeptides or, when functional, proteins.

The primary structure of a polipeptid is the linear sequence of amino acids. This sequence determines how the molecule folds into higher levels of structure: secondary structures such as alpha helices and beta sheets, tertiary structure as overall three-dimensional shape, and, for multi-chain assemblies, quaternary structure. A single polipeptid can function on its own or be a subunit of a larger protein composed of multiple polypeptide chains.

Biosynthesis occurs mainly on ribosomes through translation of messenger RNA. Polipeptids may undergo post-translational processing, including cleavage, chemical modification, or folding aided by chaperone proteins, to become mature proteins or active peptides. Some polipeptids act as hormones, signaling molecules, or neurotransmitters; others are enzymes or structural components of cells and tissues. In many cases, a functional protein consists of one or more polipeptids.

Chemical properties include the planarity of the peptide bond due to resonance and resistance to rotation around the bond, which influences folding. Peptide bonds can be hydrolyzed by acids or proteolytic enzymes. The biological activity of polipeptids depends on their sequence, length, and three-dimensional conformation.