Heme: the most versatile redox centre in biology?

Iron-porphyrin complexes, known as hemes, form the prosthetic groups of a number of proteins. These hemoproteins exhibit an impressive range of biological functions. These include: Simple electron transfer reactions, oxygen transport and storage, oxygen reduction to the level of hydrogen peroxide or water, oxygenations of organic substrates, and the reduction of peroxides. This diversity of function is often extended further by combining heme groups with other cofactors, e. g. flavins and/or metal ions. Such combinations frequently allow heme cofactors to couple electron transfers with other processes, such as the translocation of protons or the reduction/oxidation of other molecules. This versatility in function is made possible by a combination of differences in both the polypeptide and heme constituents of the various hemoproteins. The aim of this article is to illustrate how nature has used different protein frameworks to exploit the redox properties of heme. This is done by focusing on a carefully chosen selection of hemoproteins which exemplify the numerous redox functions performed by heme in biology.