6533b870fe1ef96bd12cf036

RESEARCH PRODUCT

Assembly of Transmembrane b-Type Cytochromes and Cytochrome Complexes

subject

description

Cytochromes are involved in charge-transfer reactions, and many cytochromes contain a transmembrane domain and are part of membrane-localized electron transfer chains. Protoporphyrin IX (heme b) is the first heme product in the tetrapyrrole/heme biosynthesis pathway. In contrast to c-type cytochromes, there is no need for a specialized machinery catalyzing covalent attachment of the heme molecule to a b-type apo-cytochrome, nor is the cofactor further modified, as in a-, d- and o-type cytochromes. Thus, formation of a holo-cytochrome is relatively simple for b-type cytochromes, and this class of proteins probably represents the most ancient members of transmembrane cytochromes. However, assembly of individual transmembrane b-type cytochromes as well as of larger cytochrome complexes involves multiple steps, which have to be tightly controlled and aligned: the apo-protein as well as the heme cofactor needs to be synthesized, targeted to, and integrated into a membrane prior to holo-cytochrome formation. Spontaneous folding and assembly of individual transmembrane b-type cytochromes involves folding of the polypeptide chain and formation of a heme-binding cavity, which allows specific and tight binding of the cofactor. Additional biogenesis steps are eventually required for maturation of transmembrane b-type cytochrome complexes.