Tetraheme CytochromesC₃

Abstract

Tetraheme cytochromes c 3 constitute the best‐studied group among the multiheme cytochromes belonging to the class III of Ambler's classification. Since 1954, when cytochrome c 3 was isolated from Desulfovibrio vulgaris for the first time by Postgate, these proteins have been widely characterized, especially those from the Desulfovibrio ( D .) genus. All these cytochromes have their heme group attached to the polypeptide chain by cysteine residues and show bis‐histidinyl coordination of the heme iron and a highly conserved heme‐binding motif (CXXCH or CXXXXCH). Cytochromes c 3 show a wide variety of redox potentials, ranging from 0 to −400 mV, are usually periplasmatic proteins, and are believed to play key roles in the electron transfer chains. Recently, the cytochrome c 3 group was divided into two main types on the basis of structural, functional, and genetic differences: type I cytochrome c 3 (TpI‐ c 3 ) and type II cytochrome c 3 (TpII‐ c 3 ). This paper reviews their structural, spectroscopic, and functional properties, based on the wealth of information currently available. These include physicochemical properties, redox mechanisms, coupling between electron and proton transfer, and data concerning the interaction with their redox partners obtained by experimental and molecular modeling methods.