Protein structure

Proteins do almost everything in a cell: as enzymes, antibodies, transporters, hormones and muscle fibres. Their structure is described at four levels.

Primary structure: the sequence of amino acids, joined by peptide bonds, each formed with the loss of a water molecule. A protein is written from its N-terminal end (left) to its C-terminal end (right): Tyr-Ala-Gly-Ser-Leu is read tyrosyl-alanyl-glycyl-seryl-leucine. Most protein chains have 50 to 2,000 amino acids, each weighing about 110 daltons.

The peptide bond is flat and cannot rotate, because it has partial double-bond character (about 40%). So the chain can bend only at two angles on either side of each alpha-carbon, called phi and psi. G.N. Ramachandran (1963) worked out which phi-psi combinations are possible, in the Ramachandran plot.

Secondary structure, described by Pauling and Corey (1951): parts of the chain fold into regular shapes.

• The alpha-helix: a right-handed coil, held by hydrogen bonds between amino acids four places apart. Found in ferritin, keratin (hair) and myosin.
• The beta-pleated sheet: stretched strands lying side by side, running the same way (parallel) or opposite ways (antiparallel), held by hydrogen bonds between strands.

Tertiary structure: the whole chain folds in three dimensions, held by disulfide bonds (between two cysteine residues), hydrogen bonds, hydrophobic interactions and ionic bonds. Water-loving groups face outward; water-fearing ones hide inside. Quaternary structure: some proteins are made of several folded chains (subunits) packed together, alike (homodimers) or different (heterodimers) and more. Protein structures are worked out by X-ray crystallography and NMR. The rule of thumb: sequence decides shape, and shape decides job.