BIOGENESIS OF RESPIRATORY CYTOCHROMES IN BACTERIA

Biogenesis of respiratory cytochromes is defined as consisting of the posttranslational processes that are necessary to assemble apoprotein, heme, and sometimes additional cofactors into mature enzyme complexes with electron transfer functions. Different biochemical reactions tak e place during maturation: (i) targeting of the apoprotein to or throu gh the cytoplasmic membrane to its subcellular destination; (ii) prote olytic processing of precursor forms; (iii) assembly of subunits in th e membrane and oligomerization; (iv) translocation and/or modification of heme and covalent or noncovalent binding to the protein moiety; (v ) transport, processing, and incorporation of other cofactors; and (vi ) folding and stabilization of the protein. These steps are discussed for the maturation of different oxidoreductase complexes, and they are arranged in a linear pathway to best account for experimental finding s from studies concerning cytochrome biogenesis. The example of the be st-studied case, Le., maturation of cytochrome c, appears to consist o f a pathway that requires at least nine specific genes and more genera l cellular functions such as protein secretion or the control of the r edox state in the periplasm. Covalent attachment of heme appears to be enzyme catalyzed and takes place in the periplasm after translocation of the precursor through the membrane. The genetic characterization a nd the putative biochemical functions of cytochrome c-specific maturat ion proteins suggest that they may be organized in a membrane-bound ma turase complex. Formation of the multisubunit cytochrome be, complex a nd several terminal oxidases of the bo(3), bd aa(3), and cbb(3) types is discussed in;detail, and models for linear maturation pathways are proposed wherever possible.