Proteins do the work of the cell. They catalyze reactions, transport molecules, provide structure, transmit signals, and defend against infection. They are large molecules made of amino acids linked in a chain. The sequence of amino acids determines how the chain folds, and the folded shape determines the function. There are about 20,000 protein-coding genes in the human genome, but many more proteins, because of alternative splicing and post-translational modifications.

Proteins have four levels of structure. The primary structure is the amino acid sequence. The secondary structure is local folding into alpha helices and beta sheets. The tertiary structure is the overall three-dimensional shape of a single polypeptide. The quaternary structure is the arrangement of multiple polypeptides into a complex. The shape is stabilized by hydrogen bonds, disulfide bridges, hydrophobic interactions, and ionic bonds.

Proteins are studied in proteomics, the large-scale analysis of protein expression, modification, and interaction. They are produced industrially for therapeutic use, like insulin and monoclonal antibodies. They are engineered for improved stability and activity. They are targeted by drugs that block or enhance their function.

Protein misfolding causes diseases like Alzheimer's, Parkinson's, and cystic fibrosis. Understanding how proteins fold and function is one of the great challenges of biology.