[17 Redox Properties of Vanillyl Alcohol Oxidase

By Robert H. H. van den Heuvel, Marco W. Fraaije, and Willem J. H. van Berkel

Redox Properties of Flavoproteins

Flavins are a ubiquitous class of redox-active coenzymes that are able to catalyze a number of different chemical reactions when bound to apoproteins. They play an important role in (de)hydrogenation and hydroxylation reactions, in oxygen activation, and in one- and two-electron transfer processes from and to redox centers.1'2 Because of their chemical versatility, flavins are involved in a wide range of biological processes. They have been shown to be involved in programmed cell death by signal transduction3 and in detoxification of a wide variety of aromatic compounds.4 They also have a function in regulating biological clocks,5 in DNA damage repair,6 and plant phototropism.7 These unique properties of flavins are always controlled by specific noncovalent or covalent interactions with the apoproteins to which they are bound.

The first flavin-containing protein was isolated in 1933 by Warburg and Christian.8 Since then, a large number of flavoproteins have been purified and characterized. Most of these proteins contain either a tightly but noncovalently bound flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) prosthetic group as redox-active center. However, a significant number of flavoproteins contain a covalently linked flavin.9 Mammalian succinate dehydrogenase was the first protein recognized to have such a covalent protein-flavin linkage.10 At present, about 30 covalent flavoproteins have been reported. The precise function of the covalent linkage in most flavoproteins is unknown, but it has been suggested that the anchoring prevents flavin dissociation, increases protein stability, and improves

3 S. A. Susin, H. K. Lorenzo, N. Zamzami, I. Marzo, B. E. Snow, G. M. Brothers, J. Mangion, E. Jacotot, P. Costantini, M. Loeffler, N. Larochette, D. R. Goodlett, R. Aebersold, D. P. Siderovski, J. M. Penninger, and G. Kroemer, Nature (London) 397,441 (1999).

4 B. Entsch and W. j. H. van Berkel, FASEB J. 9,476 (1995).

5 A. R. Cashmore, J. A. Jarillo, Y. J. Wu, and D. Liu, Science 284,760 (1999).

6 M. S. J├Ârns, B. Wang, and S. P. Jordan, Biochemistry 26, 6810 (1987).

7 M. Salomon, J. M. Christie, E. Knieb, U. Lempert, and W. R. Briggs, Biochemistry 39, 9401 (2000).

8 o. Warburg and W. Christian, Biochem. Z. 266, 377 (1933).

9 M. Mewies, W. S. Mclntire, and N. S. Scrutton, Protein Sei. 1,1 (1998).

10 E. B. Kearney and T. P. Singer, Biochim. Biophys. Acta 17,596 (1955).

Flavoquinone

H3C.

H3C.

p K = 10 H3C, H,C

Flavosemiquirione

Flavohydroquinone h,c pk = 8.3 h3c " h3c fih2 h 0

0 0

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