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Proteins are large, complex biomolecules essential to virtually all biological processes. They are polymers of amino acids linked by peptide bonds, and their function is determined by the sequence of amino acids and the three-dimensional shape that results from folding. In Spanish and Italian, proteina is the word used for protein.

Protein structure is commonly described in four levels. The primary structure is the linear amino acid sequence. Secondary structures such as alpha helices and beta sheets arise from hydrogen bonding. Tertiary structure refers to the overall three-dimensional fold of a single polypeptide, while quaternary structure describes how multiple polypeptide chains assemble into a functional unit. Folding is driven by hydrophobic effects, electrostatic interactions, and sometimes disulfide bonds; it may be assisted by molecular chaperones and can be disrupted by heat or pH changes, a process called denaturation.

Proteins carry out essential roles. Enzymes act as catalysts for chemical reactions; structural proteins provide support (for example, collagen, keratin); transport proteins move molecules (such as hemoglobin); cell signaling and receptors rely on protein interactions; antibodies defend against pathogens; and motor proteins enable movement. Some proteins are conjugated with carbohydrates, lipids, or metal cofactors, forming glycoproteins, lipoproteins, or metalloproteins.

Synthesis and nutrition: In cells, proteins are produced by ribosomes through transcription and translation, guided by genetic information. The body can synthesize most amino acids, but nine are typically essential and must be obtained from the diet. Dietary protein sources are categorized as complete (containing all essential amino acids) or incomplete. After ingestion, proteins are digested by digestive enzymes; amino acids and small peptides are absorbed in the small intestine and transported to the liver and other tissues.