Cytochrome c biogenesis: mechanisms for covalent modifications and trafficking of heme and for heme-iron redox control.
Heme is the prosthetic group for cytochromes, which are directly involved in oxidation/reduction reactions inside and outside the cell. Many cytochromes contain heme with covalent additions at one or both vinyl groups. These include farnesylation at one vinyl in hemes o and a and thioether linkages to each vinyl in cytochrome c (at CXXCH of the protein). Here we review the mechanisms for these covalent attachments, with emphasis on the three unique cytochrome c assembly pathways called systems I, II, and III. All proteins in system I (called Ccm proteins) and system II (Ccs proteins) are integral membrane proteins. Recent biochemical analyses suggest mechanisms for heme channeling to the outside, heme-iron redox control, and attachment to the CXXCH. For system II, the CcsB and CcsA proteins form a cytochrome c synthetase complex which specifically channels heme to an external heme binding domain; in this conserved tryptophan-rich "WWD domain" (in CcsA), the heme is maintained in the reduced state by two external histidines and then ligated to the CXXCH motif. In system I, a two-step process is described. Step 1 is the CcmABCD-mediated synthesis and release of oxidized holoCcmE (heme in the Fe(+3) state). We describe how external histidines in CcmC are involved in heme attachment to CcmE, and the chemical mechanism to form oxidized holoCcmE is discussed. Step 2 includes the CcmFH-mediated reduction (to Fe(+2)) of holoCcmE and ligation of the heme to CXXCH. The evolutionary and ecological advantages for each system are discussed with respect to iron limitation and oxidizing environments.
Kranz
Robert G
RG
Department of Biology, Washington University, One Brookings Dr., St. Louis, MO 63130, USA. kranz@biology.wustl.edu
Richard-Fogal
Cynthia
C
Taylor
John-Stephen
JS
Frawley
Elaine R
ER
D000818:Animals; D053898:Biosynthetic Pathways; D003574:Cytochrome c Group / Q000378:metabolism*; D006418:Heme / Q000378:metabolism*; D006801:Humans; D007501:Iron / Q000378:metabolism*; D010084:Oxidation-Reduction; D011499:Protein Processing, Post-Translational; D021381:Protein Transport
D003574:Cytochrome c Group; D006418:Heme; D007501:Iron