Cytochrome cofactor regenerating enzymes

Moreover, an electron transfer chain could be reconstituted in vitro that is able to oxidize aldehydes to carboxylic acids with concomitant reduction of protons and net production of dihydrogen (213, 243). The first enzyme in this chain is an aldehyde oxidoreductase (AOR), a homodimer (100 kDa) containing one Mo cofactor (MOD) and two [2Fe—2S] centers per subunit (199). The enzyme catalytic cycle can be regenerated by transferring electrons to flavodoxin, an FMN-con-taining protein of 16 kDa (and afterwards to a multiheme cytochrome and then to hydrogenase) ... 

The Krebs cycle is a series of reactions catalyzed by sev en enzymes in mitochondria. Its function is to catalyze removal of electrons from nutrients and to transfer them to NAD and FAD, producing NADH plus H, and FADHj, respectively. These reduced cofactors exist only momentarily in their reduced (err oxidized) forms as they continually accept and then donate electrons to the respiratory chain. The respiratory chain, composed of a number of cytochromes, uses electrons for reduction of to water, This reduction process is accompanied by or coupled with the regeneration of ATP, that is, conversion of A DP back to ATE The overall effect may be summarized thus The Krebs cycle and respiratory chain arc used for oxidizing nutrients io COy and for the production of energy. 

Previous studies had shown that tetrahydrobiopterin could be a cofactor in mitochondrial electron transfer. The tetrahydropterin enters the chain at the level of cytochrome c and is able to catalyze oxygen reduction, but without the generation of ATP. The pool of tetrahydropterins can be continuously regenerated through the action of an NADPH dependent dihydropterin reductase, which catalyzes the reduction of the quinoid dihydropterin. It is not known if this enzyme is also present in the mitochondria. However, it has been shown that tetrahydropterin can easily cross the mitochondrial membrane. The physiological function of this system, which apparently couples NADPH/NADH pools directly through tetrahydropterin, cytochrome c, cytochrome oxidase to oxygen, but without simultaneous oxidative phosphorylation, is still not known. 

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