protein structure

1. The three-dimensional structure of a protein that is often key to its enzyme function. The structure of a protein is characterized in four ways: The primary structure is the order of the different amino acids in a protein chain, whereas the secondary structure consists of the geometry of chain segments in forms such as helices or sheets. The tertiary structure describes how a protein folds in on itself; the quaternary structure of a protein describes how different protein chains hook up with each other.