Flavin coenzymes modified

Flavin adenine dinucleotide. See FAD Flavin adenine diphosphate. See FAD Flavin coenzymes 766,780 - 795 modified 788, 789 reduced 794 Flavin radicals 792 color of 794 formation constant 794 Flavocytochrome b2 782, 794, 847 Flavodoxins 793, 799, 800 Flavoprotein(s) 513, 788... 

Due to high biocompability and large surface are of cobalt oxide nanoparticles it can be used for immobilization of other biomolecules. Flavin adenine FAD is a flavoprotein coenzyme that plays an important biological role in many oxidoreductase processes and biochemical reactions. The immobilized FAD onto different electrode surfaces provides a basis for fabrication of sensors, biosensors, enzymatic reactors and biomedical devices. The electrocatalytic oxidation of NADH on the surface of graphite electrode modified with immobilization of FAD was investigated [276], Recently we used cyclic voltammetry as simple technique for cobalt-oxide nanoparticles formation and immobilization flavin adenine dinucleotide (FAD) [277], Repeated cyclic voltammograms of GC/ CoOx nanoparticles modified electrode in buffer solution containing FAD is shown in Fig.37A. 

The biochemical aspects of 8a-modified flavins have been reviewed recently 166), whereas the approaches used to identify the site of attachment of the flavin have been discussed in detail by Singer and Edmondson 164). Hence, the present chapter concentrates on the chemical and spectral properties of biologically relevant modified flavo-coenzymes which have been discovered recently. 

A further typical feature is the high reactivity of 8a-halogenated flavins, which can be compared to that of p-nitrobenzyl-halogenides. The facile replacement of bromine at position 8a by nucleophile functions of amino acids was the key step in the synthesis of the modified coenzymes which will be detailed below. Similarly halogen or -N2 at position 8 can be replaced by a variety of nucleophiles for example by amines, alcohols or mercaptide 76,152,177). 

The second modified flavin of natural origin to be discovered was 8a-S-cysteinyl-FAD, the coenzyme of monoamino oxidase from liver and kidney outer mitochondrial membranes. Taking their departure from investigations of Yasunobu (8J) and Hellerman (SO), which indicated the presence of covalently bound flavin in preparations of this enzyme, Singer and his group (85, 185) isolated the flavinyl peptide by degradation of MAO with trypsin-chymotrypsin and identified cysteine as the amino acid residue bound next to the flavin moiety (184). The absorption spectrum of the flavin peptide from monoamino oxidase is readily differentiated from that of riboflavin by a hypsochromic shift of the second absorption band (360 nm, compare with 372 for riboflavin), in the neutral oxidized state (44, 184). It is similar to that of 8a-histidyl-riboflavin in the cationic state in that the band centered around 400 nm (abs. max. 375 nm, shoulder at 410 nm) is partially resolved. The fluorescence emission (4, 30) is only 10% of that of riboflavin, but oxidation with peracids raises it to 90% of riboflavin emission. 

In the last few years a further type of modified flavocoenzyme has beeen discovered which is structurally related to the coenzymes of succinate dehydrogenase and monoamine oxidase, but differs considerably in its chemical properties. Early literature reports indicated the presence of a flavin in cytochrome C552 from Chromatium which could not be extracted with trichloroacetic acid or acidic ammonium sulfate (7), but could be released, for example by trypsin digestion or by incubation with saturated urea solutions (2). Absorption, fluorescence and ESR behaviour were closely similar to those of 8a-cysteinyl-ribo-flavin (12) and indicated the presence of a covalent link to the protein, through position 8a (185). Strong acid hydrolysis of these peptides liberated the flavin as mixture of riboflavin derivatives oxidation with per-formic acid and acid dephosphorylation yielded a homogeneous riboflavin derivative, which was identical with 8-nor-8-carboxy-riboflavin (185). 

The same type of coenzyme was isolated from pig liver glycolate oxidase, but as a derivative of FMN 126). Also in this case the modified flavocoenzyme appears to be present in the enzyme in varying quantities as a companion of normal FMN. Hence, even more than in the case of the orange flavin (18) the origin of the modification is difficult to explain or to verify experimentally. The enzymes, from which the green chromophores has been isolated, originate from completely different sources, namely aerobic mammalian tissue (pig liver) and a strict anaerobe (Peptostreptococcus elsdenii), and catalyze basically different reactions. 

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