Protéine

Protéine is a macromolecule essential to virtually all cellular processes. In biology, a protéine is a polymer composed of amino acids linked by peptide bonds. The linear sequence of amino acids, encoded by genes, determines the protein’s properties. Proteins are synthesized in cells by ribosomes through transcription and translation, using messenger RNA as a template. Many proteins undergo post-translational modifications that affect activity, localization, or stability.

Proteins fold into specific three-dimensional structures that govern their function. The sequence gives a primary structure; local folding forms secondary structures such as alpha helices and beta sheets; higher-order folding produces tertiary structure, and the assembly of several polypeptides creates quaternary structure. Folding is driven by chemical interactions and sometimes assisted by chaperone proteins; misfolding can contribute to disease.

Functions are diverse: enzymes catalyze biochemical reactions; structural proteins provide support; transport and storage proteins move molecules or ions; signaling and regulatory proteins control cellular processes; immune proteins defend against pathogens. Based on properties, proteins are often classified as fibrous or globular, soluble or membrane-associated, and simple (protein only) or conjugated (bound to a non-protein component such as a metal, carbohydrate, or lipid).

Nutrition and laboratory use: dietary proteins supply essential amino acids not synthesized by the body; protein quality depends on amino acid composition and digestibility. In research and industry, protein concentration is measured by assays such as Bradford or BCA, and quantities are expressed in mass per volume or per unit of activity.