Metallo-flavoproteins

Aleman, V., S. T. Smith, K. V. Rajagopalan and P. Handler Soluble metallo-flavoproteins. In Non-Heme Iron Proteins Role in Energy Conversion, A San Pietro, ed. Antioch Press, Yellow Springs, Ohio, pp. 327—348 (1965). 

Metallo-Flavoproteins. As was mentioned in the case of cytochrome reductase, enzymes are known that contain metal cofactors in addition to flavin. These are called metallo-flavoproteins. The presence of metals introduces complexity into the reaction, since the metals involved, iron, molybdenum, copper, and manganese, all exist in at least two valence states and can participate in oxidation-reduction reactions. The enzymes known to be metallo-flavoproteins include xanthine oxidase, aldehyde oxidase, nitrate reductase, succinic dehydrogenase, fatty acyl CoA dehydrogenases, hydrogenase, and cytochrome reductases. Before these are discussed in detail some physical properties of flavin will be presented. 

A simplified version of the complex chain of electron transport from the Krebs cycle reactions to oxygen is shown below (Fig. 4.12). This also indicates the sites of oxidative phosphorylation. CoQ in this diagram represents a quinone termed co-enzyme Q which is now thought to be interposed in the chain between the metallo-flavoproteins (FP) and the cytochrome complex. 

In summary, organisms can use biological reductants such as NAD(P)H, capable of hydride transfer (two electron transfer), and reduced flavoproteins and metallo-proteins, capable of single electron donation. Although not necessarily intended to interact with xenobiotic organic compounds, when such organic chemicals come in contact with suitably reactive bioreductants in vivo, reductions can occur. 

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