External acceptor

It appears that Cluster C catalyzes the chemistry of CO oxidation and transfers electrons to Cluster B, which donates electrons to external acceptors such as ferredoxin. Since a crystal structure of this protein does not exist, the proposed structure of Cluster C is based on spectroscopic measurements. In some cases, the EPR spectrum of a metal center is diagnostic of the type of center. However, the EPR spectra of Cluster C are unusual. The paramagnetic states of Cluster C (Credi and Cred2) have g-values that are atypical of standard [4Fe-4S] clusters (Table III) and are similar to those in a variety of structurally unrelated systems including a t-oxo bridged ion dimer), a [Fe4S4] ... 

These two isomeric forms of R2 are physically different because of the pairing of the nuclear spins on the hydrogen nnclei in the molecule. They can be physically separated by chromatography at very low temperatures. Generally to convert one isomer to the other requires a cleavage of the R-R bond. As with the hydrogen-isotope-exchange reactions, no external acceptor or donor is required. 

The proximal cluster donates an electron to the external acceptor. Two electrons are transferred from the hydride, at first reducing the nickel from Ni(III) to Ni(I). An electron is transferred from the nickel to the proximal cluster. 

In contrast to the flavin oxidases, flavin dehydrogenases pass electrons to carriers within electron transport chains and the flavin does not react with 02. Examples include a bacterial trimethylamine dehydrogenase (Fig. 15-9) which contains an iron-sulfur duster that serves as the immediate electron acceptor167 169 and yeast flavocytochrome b2, a lactate dehydrogenase that passes electrons to a built-in heme group which can then pass the electrons to an external acceptor, another heme in cytochrome c.170-173 Like glycolate oxidase, these enzymes bind their flavin coenzyme at the ends of 8-stranded a(i barrels similar... 

Photo-induced demethylation of 4-nitro-iV,A-dimethylaniline,to give 4-nitro-N-methylaniline, has been observed upon excitation in the presence of an external acceptor in benzene or the methoxide ion in methanol. A quantum chemical study of the photodissociation of the alkyl C-N bond in AT-substituted anilines, PhNH-R (R = Me, PhCH2, Ph2CH, Ph3C), has also been carried out. " ... 

Yeast mitochondrial flavocytochrome 2 (lactate cytochrome c oxido-reductase) catalyzes the transfer of electrons from L-lactate to various acceptors, cytochrome c being the physiological acceptor. The protoheme and flavin mononucleotide, because of their higher redox potential, can both be reduced completely by L-lactate the enzyme accepts a total of three electrons per protomer, which amounts to twelve electrons for the stable active tetramer. Both types of prosthetic group are quantitatively reoxidized by external acceptors. 

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