Many proteins with one or more haem groups bound per polypeptide chain are called cytochromes. They function in electron transfer reactions and some are involved directly in the catalysis of chemical reactions, most prominently the reduction of oxygen to water in the terminal step of cell respiration. When unmodified haem is present the cytochromes are referred to as b-type, but if the haem is covalently attached to thiol groups of a Cys-Xaa-Xaa-Cys-His motif then the cytochrome is a c-type. Neither the purpose of this post-translational modification, nor the mechanisms of the machineries that are necessary for formation of the thioether bonds between protein and haem, are fully understood. In bacteria the c-type cytochromes function in the periplasm where they are involved in a range of electron transport activities, including the reactions of denitrification, in which nitrate is reduced sequentially via nitrite, nitric oxide and nitrous oxide to nitrogen gas. Other types of cytochromes have haem molecules with modifications to their porphyrin ring. These include the a-, d-, d1- and o-types. Although Keilin first described the a-, b- and c- types of cytochrome more than 60 years ago, we still do not have clear explanations as to why one type of haem moiety does not suffice for the requirements of mitochondrial, thylakoid and bacterial electron transport.

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