Many polypeptides are modified after translation, for instance by phosphorylation or glycosylation (addition of one or more phosphates or sugars respectively to specific amino acids in the chain). These modifications account for and enhance the molecular and functional diversity of proteins within and across species.
Hemoglobins exemplify another feature of the structure of many proteins. To be biologically active, globin polypeptides must associate with a prosthetic group, in this case a cyclic organic molecule called heme. At the center of each heme is the iron that reversibly binds oxygen. All kinds of organisms, from bacteria to plants and animals and even in some anaerobic organisms contain some kind of hemoglobin. Other proteins must be bound to different metal ions (magnesium, manganese, cobalt…) to be biologically active.
136 Protein Quaternary Structure & Prosthetic Groups