Ž . Journal of Molecular Catalysis B: Enzymatic 7 1999 241–249 www.elsevier.comrlocatermolcatb Cytochrome c as a biocatalyst Rafael Vazquez-Duhalt ) Instituto de Biotecnologıa UNAM, Apartado Postal 510-3, CuernaÕaca, Mor. 62250, Mexico ´ Abstract Type c cytochromes, which are involved in the electron transport system, are also able to catalyze peroxidase-like reactions in the presence of an electron acceptor, such as hydrogen peroxide or an organic hydroperoxide. This work reviews the catalytic activity of cytochrome c, and the potential design by site-directed mutagenesis and chemical modification of new biocatalysts for environmental purposes. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Cytochrome; Biocatalyst; Hemoprotein; Peroxidase; PAH; Thermostability; Stability 1. Introduction The c type cytochromes are probably among the most popular proteins for biochemical stud- ies and have been the subject of several review w x books 1–3 . In in vivo conditions, cytochromes c are part of the energy-conserving electron transport system. They have a protoheme pros- thetic group covalently attached by two thioether bridges between the cysteine residues of the protein and the vinyl side chains of the heme. wx Mammalian peroxidases 4 and type c cy- tochromes are the only hemoproteins with a covalently bound heme group. The location and role of mitochondrial cytochrome c are well known but the means by which cytochrome c conducts electrons between its membrane reduc- tase and oxidase remains controversial. How- ever, arguments have been put forward for the diffusion of cytochrome c across the membrane ) Tel.: q52-5-6227600; Fax: q52-73-172388; E-mail: vazqduh@ibt.unam.mx surface to interact separately with its reductase and oxidase. Cytochrome c is a protein ubiquitous to all eukaryotic organisms and the sequence of many such proteins have been determined. This inten- sive inventory of sequences has been used to wx trace the phylogeny of the eukaryotes 5 . Com- parison of these sequences shows that cy- tochromes c in eukaryotic organisms are highly conserved. High-resolution structures of five eu- karyotic and six bacterial cytochromes c have been completed to date. The high degree of sequence homology expresses itself in a high degree of structural conservation among the these proteins. 2. Biocatalysis In living systems no catalytic activity of cy- tochrome c has been described. However, forty years ago the ability of cytochrome c to induce lipid peroxidation as well as its involvement in 1381-1177r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S1381-1177 99 00033-8