Substrate Oxidation Sites

Complex II The Succinate Dehydrogenase Complex. Succinate dehydrogenase is the only enzyme of the TCA cycle that is embedded in the inner membrane. Its four subunits include two iron-sulfur proteins, one of which also has a covalently attached FAD. As in NADH dehydrogenase, the substrate-oxidation site is on the matrix side of the membrane (fig. 14.10). 

Ruiz-Duenas FJ, Morales M, Garcia E et al (2009) Substrate oxidation sites in versatile peroxidase and other basidiomycete peroxidases. J Exp Bot 60 441 152... 

Fig. 3.5 Two substrate oxidation sites in VP left, Oxidation site for high redox-potential substrates (such as VA, RB5, and lignin) by a LRET pathway (dotted arrow) from a tryptophan residue forming a catalytic neutral radical (W164-) to a heme methyl group via a leucine residue [57-59] right, Oxidation site for Mn2+, at the internal propionate of heme, involving three acidic amino acid residues [10]. Axial view of the heme region (a water molecule, represented as van der Waals spheres, is seen at the top position on the heme iron)...

The type 1 copper site, which is the substrate oxidation site, is maintained in the C-terminal BCB domain (domain 3 in AO and LC and... 

The high-resolution structure of the blue copper enzyme ascorbate oxidase (AO) prompted several laboratories to examine the internal electron flow from the substrate oxidation site [type 1 Cu(II)] to that of dioxygen reduction. Again, both flash photolysis and pulse radiolysis were employed, providing a comparison of their respective features. 

A Mossbauer study of the protein reacted with benzaldehyde (in parallel with EPR detection of Mo(V) signals) shows partial reduction of the iron—sulfur centers, indicating the involvement of the clusters in the process of substrate oxidation and rapid intramolecular electron transfer from the molybdenum to the iron—sulfur sites. 

Figure 17.3 Anatomy of a redox enzyme representation of the X-ray crystallographic structure of Trametes versicolor laccase III (PDB file IKYA) [Bertrand et al., 2002]. The protein is represented in green lines and the Cu atoms are shown as gold spheres. Sugar moieties attached to the surface of the protein are shown in red. A molecule of 2,5-xyhdine that co-crystallized with the protein (shown in stick form in elemental colors) is thought to occupy the broad-specificity hydrophobic binding pocket where organic substrates ate oxidized by the enzyme. Electrons from substrate oxidation are passed to the mononuclear blue Cu center and then to the trinuclear Cu active site where O2 is reduced to H2O. (See color insert.)...

The activity of the FePeCli6-S/tert-butyl hydroperoxide (TBHP) catalytic system was studied under mild reaction conditions for the synthesis of three a,p-unsaturated ketones 2-cyclohexen-l-one, carvone and veibenone by allylic oxidation of cyclohexene, hmonene, and a-pinene, respectively. Substrate conversions were higher than 80% and ketone yields decreased in the following order cyclohexen-1-one (47%), verbenone (22%), and carvone (12%). The large amount of oxidized sites of monoterpenes, especially limonene, may be the reason for the lower ketone yield obtained with this substrate. Additional tests snggested that molecular oxygen can act as co-oxidant and alcohol oxidation is an intermediate step in ketone formation. 

Domanski, T. L., He, Y. A., Khan, K. K., Roussel, F., Wang, Q., and Halpert, J. R. (2001) Phenylalanine and tryptophan scanning mutagenesis of CYP3A4 substrate recognition site residues and effect on substrate oxidation and coop-erativity. Biochemistry 40, 10,150-10,160. 

So-called blue multinuclear copper oxidase enzymes, such as laccase and ascorbate oxidase, catalyze the stepwise oxidation of organic substrates (most likely in successive one-electron steps) in tandem with the four-electron reduction of O2 to water, i.e. no oxygen atom(s) from O2 are incorporated into the substrate (Eq. 4) [15]. Catechol oxidase, containing a type 3 center, mediates a two-electron substrate oxidation (o-diphenols to o-chinones), and turnover of two substrate molecules is coupled to the reduction of O2 to water [34,35]. The non-blue copper oxidases, e.g. galactose oxidase and amine oxidases [27,56-59], perform similar oxidation catalysis at a mononuclear type 2 Cu site, but H2O2 is produced from O2 instead of H2O, in a two-electron reduction. 

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