Flavoprotein oxidase properties

LMO from Mycobacterium smegmatis is the best-studied example of this enzyme. The enzyme is an octomer of identical subunits, with one FMN per monomer. Even though no structural information has been obtained for LMO, much can be inferred from the high similarity of LMO to other ct-hydroxy acid oxidizing flavoenzymes. LMO exhibits many common properties of flavoprotein oxidases. It makes an N5 sulfite adduct and stabilizes anionic semiquinone and the benzoquinoid form of 8-mercapto-FMN," all of which are explained by the presence of a positive charge in the protein at the Nl—C2=0 locus. Sequence... 

Flavin-dependent oxynitrilase has many properties common to flavoprotein oxidases. The enzyme binds sulfite in an N5 adduct, and one-electron reduction produces anionic semiquinone.The sulfite adduct and one- or two-electron reduced enzyme are inactive, as is apoenzyme, suggesting that the flavin is involved in catalysis. When the flavin is replaced by 5-carba-5-deaza FAD, a low level of activity is retained. " " However, when the enzyme containing the artificial flavin is exposed to H2O2, the isoalloxazine ring system is partially degraded to a redox-inactive heterocyclic system. " After the formation of this redox-inactive flavin derivative, enzyme activity actually increases dramatically, suggesting that the flavin plays a structural role. It is possible that the redox state of the flavin serves a regulatory role, since enzyme activity decreases when the natural flavin is reduced however, this has yet to be proven. 

The flavin is hicovalendy hound to BBE at the 8-a-position hy a histidine and at the 6-position hy a cysteine. BBE has some common properties of flavoprotein oxidases BBE stabilizes anionic semiquinone and reacts with sulfite, forming an N5 adduct. These properties indicate a positive charge in the active site that can stabilize negative charge at the N1—C2=0 locus of the flavin. 

To explain how H+ transfer occurred across the membrane Mitchell suggested the protons were translocated by redox loops with different reducing equivalents in their two arms. The first loop would be associated with flavoprotein/non-heme iron interaction and the second, more controversially, with CoQ. Redox loops required an ordered arrangement of the components of the electron transport system across the inner mitochondrial membrane, which was substantiated from immunochemical studies with submitochondrial particles. Cytochrome c, for example, was located at the intermembranal face of the inner membrane and cytochrome oxidase was transmembranal. The alternative to redox loops, proton pumping, is now known to be a property of cytochrome oxidase. 

Histamlnase lyiamitte oxidase benzylamine oxj. dase histamine deaminase histamine oxidase E.C- 1.4,3,6. A copper contg enzyme present in tissues esp in kidneys and in the intestinal mucosa. Attacks diamines such as histamine in tbe body by oxidaiive deamination Best. J. Physiol 67, 256 (1929) Zeller Heiv. Chim. Acta 2l, 880 (1938) Advan. Entymol 2, 93 (1942). Extraction from hog kidneys Swedin. Acta Med. Scand. 114, 21 (1943). Identity with diamine oxidase Zeller Fed. Proc. 24, 764 (1965). Appears to have the general properties of a flavoprotein. Review E. A, Zeller "Diamine Oxidases in The Enzymes, vol. 8, P. D. Boyer et al, Eds. (Academic Press, New York 2nd ed., 1963) pp 313-335 Buffoni Pharmacol. Rev. ]8, 1163-1199 (1966) Hansson Scand- J. Clin. Lab, Invest, vol 31, suppl. 129 7 0973). 

A wide variety of different cytochrome-linked electron-transfer systems is encountered in bacteria respiratory chains with oxygen, nitrate or sulphate as electron acceptors, fumarate reductase systems and light-driven cyclic electron-transfer systems (Fig. 3). All these systems are composed of several electron-transfer carriers, the nature of which varies considerably in different organisms. Electron carriers which are most common in bacterial electron-transfer systems are flavoproteins (dehydrogenases), quinones, non-heme iron centres, cytochromes and terminal oxidases and reductases. One common feature of all electron-transfer systems is that they are tightly incorporated in the cytoplasmic membrane. Another important general property of these systems is that electron transfer results in the translocation of protons from the cytoplasm into the external medium. Electron transfer therefore... 

The nature of the cofactor is not yet certain, because an a-oxidation process in cucumber fhiits is not stimulated by flavoproteins and is strongly inhibited by a glucose oxidase system the cucumber system exhibits properties of a metal-requiring enzyme (Galliard and Matthew, 1976). 

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. 

Soluble Cytochrome a. Compounds with the spectral properties of cytochrome a have been obtained in solution by treatment of particles with bile salts. - Such preparations appear to require the presence of bile salt to prevent precipitation, but in the presence of the surface-active substance, fractionation can be carried out. The water insolubility is probably a function of a large lipid content. The preparations can be reduced by reduced cytochrome c, and are autooxidizable. The autooxidation results in the formation of water, not H2O2. It is interesting to note that this solubilized preparation is rather rapidly reduced by the flavoprotein, aldehyde oxidase. ... 

Mayhew, S. G., C. D. Whitfield, S. Ghisla, and M. Schuman-Jorns Identification and Properties of New Flavins in Electron Transferring Flavoprotein from Peptostreptococcus elsdenii and Pig-Liver Glycolate Oxidase. Eur. J. Biochem. 44, 579 (1974). 

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