Heme cofactors of transmembrane cytochromes are crucial for mediating charge transfer across membranes during aerobic and anaerobic electron transport reactions. In addition, several in vivo observations indicate that heme cofactors directly or indirectly impact the folding, assembly, and stability of individual transmembrane cytochromes and also of oligomeric cytochrome complexes. In this article, we review the function of heme molecules for the formation of transmembrane cytochromes in vivo and in vitro, and discuss distinct steps during the assembly of cytochromes and cytochrome complexes. We furthermore highlight the need of in vitro studies using isolated apo-cytochromes for analyzing the role of the heme cofactor for folding and stability of single proteins or larger cytochrome complexes. In combination with in vivo studies, this approach holds the potential to obtain a comprehensive picture on how binding of the heme cofactor to its apoprotein determines not only the biological function of a heme protein but also its three-dimensional structure.
Keywords: Co-factor, cytochromes, cytochrome oxidase, electron transfer, membrane protein assembly, Two-Stage-Model
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