Flavin mononucleotide reduction

It should be emphasized that increasing molarity of the buffer inhibits the light emission reaction, probably, due to decrease of flavin mononucleotide reduction, and in 100 mM phosphate buffer the reaction is completely inhibited. 

All bacteria where nitrate ester degradation has been characterized have very similar enzymes. The enzymes eatalyze the nicotinamide cofactor-dependent reductive eleavage of nitrate esters that produces alcohol and nitrite. Purification of the PETN reduetase from Enterobacter cloacae yielded a monomerie protein of around 40 kilo Daltons, which required NADPH as a co-faetor for aetivity. Similar enzymes were responsible for the nitrate ester-degrading activity in Agrobacterium radiobacter (Snape et al. 1997) - nitrate ester reductase - and in the strains of Pseudomonas fluorescens and Pseudomonas putida (Blehert et al. 1999) - xenobiotic reduetases . All utilize a non-covalently bound flavine mononucleotide as a redox eofactor. 

Physiologic electron acceptors flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) produced similar effects on cathodic hydrogen evolution from mild steel as achieved with methyl viologen (Bryant and Laishley 1990). These experimental results showed that the mild steel rods reacting with phosphate can preferential act as electron donors for the reduction of low-potential electron carriers. All hydrogenases catalyze a reversible reaction for the formation and oxidation of hydrogen, which requires low-potential electron carriers for the enzyme activity (Church et al. 1988 Fauque et al. 1988). 

Riboflavin (vitamin B2) is a component of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), coenzymes that play a major role in oxidation-reduction reactions (see Section 15.1.1). Many key enzymes involved in metabolic pathways are actually covalently bound to riboflavin, and are thus termed flavoproteins. 

Riboflavin (vitamin Bj) is chemically specified as a 7,8-dimethyl-10-(T-D-ribityl) isoalloxazine (Eignre 19.22). It is a precnrsor of certain essential coenzymes, such as flavin mononucleotide (FMN) and flavin-adenine dinucleotide (FAD) in these forms vitamin Bj is involved in redox reactions, such as hydroxylations, oxidative carboxylations, dioxygenations, and the reduction of oxygen to hydrogen peroxide. It is also involved in the biosynthesis of niacin-containing coenzymes from tryptophan. 

Knowledge of the enzymes used by microorganisms in the transformation of nitroaromatic compounds is limited. Blasco Castillo (1993) characterized an inducible nitrophenol reductase from Rhodobacter capsulatus that catalyzed the reduction of 2,4-dinitrophenol (DNP) to 2-amino-4-nitrophenol. This enzyme was a dimer that contained flavin mononucleotide and possibly nonheme iron as... 

Semisynthetic enzymatic oxidation of peptide alcohols employs equine liver alcohol dehydrogenase. Amino alcohols with nonpolar side chains and Z-Om[CH2OH] worked as effective substrates while polar amino alcohols such as H-Arg[CH2OH] and H-Lys[CH2OH] failed as substrates. To attain complete oxidation, semicarbazide was present in the reaction mixture to immediately trap the aldehyde, and flavin mononucleotide was used to oxidize the NADH to NAD+, which serves to oxidize the alcohol 41] Configurational stability was confirmed by NMR spectroscopy as in the case of Ac-Phe[CH2OH], which was prepared by sodium borohydride reduction of Ac-Phe-H 4 1... 

Nitrobenzene reductase activity has been detected in the fat body, gut, and Malpighian tubules of the Madagascar cockroach, G. portentosa (Rose and Young, 1973). Anaerobic conditions are essential for activity. The enzymes in the microsomes are strongly NADH dependent, whereas those in the soluble fraction are strongly NADPH dependent. Activity is enhanced by the addition of flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN) or riboflavin. It appears that the true substrate for the nitroreductase is FMN and that the reduction of the nitro compounds occurs nonenzymatically (Figure 8.15). Similar results are obtained using azofuchsin as substrate. 

Flavodi-iron proteins containing a distinctive nonheme diiron/ flavin mononucleotide active site, catalyze reductive scavenging of dioxygen and NO in air sensitive microorganisms. Anaerobic addition of NO up to one NO per diferrous rmit results in formation of a diiron mononitrosyl complex, whereas further addition of NO results in two reaction pathways, one of which produces N2O and the diferric site and the other which produces a stable diiron-dinitrosyl complex. The production of N2O upon addition of NO to the mononitrosyl deflavo-protein was interpreted in terms of the li5rponitrite mechanism (71). 

Vitamin B2 (riboflavin) acts mainly as the coenzyme FAD (flavin adenine nucleotide) rmd FMN (flavin mononucleotide), which are used in many oxidation-reduction reactions in which hydrogen atoms are received or donated. Particularly noteworthy examples are their uses in... 

Figure 18.12. Oxidation States of Flavins. The reduction of flavin mononucleotide (FMN) to FMNH2 proceeds through a semiquinone intermediate.

Faster rates of release have usually been achieved when the iron has been reduced to and when a small ligand for it is present for example (52), at pH 7, thioglycollic acid removed —5% of the core iron after a 10-min incubation, and reduced flavin mononucleotide (FMNH2) removed —25% after only a 2.5-min incubation. At lower pH values the rates are even faster for example, after 10 min, thioglycollic acid at pH 4 released —50% of the core iron. Other reductants, such as O2 (S, 16) diphenols (22), dithionite (52), bipyridyls (68), and violo-genes (68, 135), also release iron rapidly. 

Reactions with HLADH typically occur at temperatures between 4°C and 25°C and in the pH range of 5 to 10. For catalysis of a reduction the optimum pH is 7 while for the reverse oxidation it is 8. Reaction times vary from a few hours in the most favourable substrates and 2-3 weeks for the slowest. The disadvantage of HLADH has been the high cost of coenzymes. Fortunately, several recycling methods are available that allow reduction of substrates at the research scale (up to 1 kg of substrate).27-30 Por example, the ethanol-coupled method has been used for reduction and flavin mononucleotide (FMN) recycling for oxidation. 

In higher mammals, riboflavin is absorbed readily from the intestines and distributed to all tis.sues. It is the precursor in the biosynthesis of the cocnzyme.s flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). The metabolic functions of this vitamin involve these Iwocoenzymes. which participate in numerous vital oxidation-reduction proces.ses. FMN (riboflavin 5 -phosphate) is produced from the vitamin and ATP by flavokinasc catalysis. This step con be inhibited by phcnothiazincs and the tricyclic antidepressants. FAD originates from an FMN and ATP reaction that involves reversible dinucicotide formation catalyzed by flavin nucleotide pyrophosphorylase. The.se coenzymes function in combination with several enzymes as coenzyme-en-zyme complexes, often characterized as, flavoproteins. 

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