A protein complex is a group of two or more associated proteins formed by protein-protein interaction that is stable over time. Protein complexes are a form of quaternary structure. Understanding the functional interactions of proteins is an important research focus in biochemistry and cell biology. Similarly to phosphorylation, complex formation often serves to activate or inhibit one or more of the associated proteins. Many protein complexes are established, particularly in the model organism Saccharomyces cerevisiae, a yeast. The discovery of protein complexes is now performed genome wide; the elucidation of most protein complexes of the yeast is undergoing. The molecular structure of protein complexes can be determined by experimental techniques such as X-ray crystallography or nuclear magnetic resonance. Increasingly the theoretical option of protein-protein docking is also becoming available.
