Classic perspectives - electron transfer: Cupredoxins

The cytochromes are ubiquitous heme proteins (>75,000 known)1 that play essential roles in biological electron transfer. They were originally classified on the basis of optical absorbance maxima characteristic of their prosthetic heme groups.2,3 The chemical structures of the most common biological hemes are shown in Fig. 1. Several classes of biological metal-chelating ligands, macrocyclic tetrapyrroles, include the porphyrins (22pe- ligands in hemes a, b, and c), chlorins (20pe- ligands in heme d), isobacteriochlorins (18pe- ligands as in heme d1), and corrins (20pe- ligands). Heme b in b-type cytochromes is an iron protoporphyrin IX complex in which the iron atom is ligated to the four pyrrole nitrogen atoms. Hemes a and c are derivatives of heme b.