Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Coenzymes – special natural specialized reagents
Flavin coenzymes
Covalently bound and other modified flavin coenzymes

An exceptionally active flavin-containing enzyme of animal Cell/35.html">Mitochondria is succinate dehydrogenase [reaction (8-49)]. This enzyme is not only firmly embedded in the mitochondrial cristae membranes, but also contains a flavin attached to the protein via a covalent bond. The Chemical Nature of this bond has recently been elucidated: a modified FAD containing 8α-(N-3-histidyl)-riboflavin has been isolated [103–105]:

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The exact same prosthetic group has been found in 6-hydroxynicotinate oxidase from Arthrobacter oxidans [106] and is probably present in Sarcosine (N-methylglycine) dehydrogenase [107]. In Liver monoamine oxidase, FAD is attached via the same 8α-position to a sulfur atom of a Cysteine residue [108, 109]. In the "flavocytochrome" (cytochrome $c_{552}$ of Chromatium), however, a different type of covalent bond, a thiohemiacetal bond, has been discovered [110]. Cytochrome $b$ reductase also contains a covalently bound flavin [111]. It will be interesting to learn how many variations on this theme nature has created.

The NADH dehydrogenase of Peptostreptococcus elsdenii contains, alongside FAD, a large amount of 6-hydroxy-FAD and 8-hydroxy-FAD (in which the 8-methyl group is replaced by an OH group) [112], although the Biological Significance of this phenomenon remains unclear. Another modified flavin, roseoflavin, acts not as a coenzyme, but as an antibiotic isolated from Streptomyces davawensis. Its probable Structure has been determined by X-Ray Diffraction Analysis [113].



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