Proteine

Proteine, or proteins, are large, complex molecules that perform most biological activities in living organisms. They are polymers built from amino acids linked by peptide bonds, and their amino acid sequence is encoded by genes. Proteins contain common elements such as carbon, hydrogen, oxygen, and nitrogen, and some sulfur atoms in certain amino acids. The vast diversity of protein sequences and shapes underlies their wide range of functions, from catalysis to structural support.

Protein structure is organized in four levels. The primary structure is the linear amino acid sequence. The secondary structure refers to local arrangements such as alpha helices and beta sheets. The tertiary structure is the overall three-dimensional fold of a single polypeptide, while quaternary structure describes assemblies of multiple polypeptide chains. Folding is guided by intramolecular interactions and often assisted by chaperone proteins; denaturation by heat, extremes of pH, or chemicals can disrupt function.

Proteins fulfill numerous roles: enzymes accelerate chemical reactions; structural proteins provide support in tissues; transport or storage proteins carry molecules; signaling and regulatory proteins coordinate cellular responses; antibodies defend against pathogens; motor proteins drive movement; channels and receptors mediate material exchange and communication across membranes. Many proteins require post-translational modifications to become fully active or targeted to specific cellular locations.

Proteins are produced in cells by transcription and translation, using mRNA templates and ribosomes. After synthesis, polypeptides may be folded, assembled, and modified. Digestion of dietary proteins in the digestive system releases amino acids and small peptides that are reused for new protein synthesis. Essential amino acids cannot be synthesized by humans and must be supplied through the diet. Protein quality depends on amino acid composition and digestibility.