Flavins thiols

Enzymes in cell-free systems from carrot Daucus carota, Apiaceae) and peppermint Mentha piperita Lamiaceae) catalyze the reversible formation of nerol and neryl phosphate from geraniol and geranyl phosphate, respectively. Geranyl phosphate appears to have been isomerized directly to the neryl derivative without the intervention of an aldehyde intermediate. The presence of a flavin, thiol, and light was required for the reaction (Croteau, 1984). 

This thiol-disulfide interconversion is a key part of numerous biological processes. WeTJ see in Chapter 26, for instance, that disulfide formation is involved in defining the structure and three-dimensional conformations of proteins, where disulfide "bridges" often form cross-links between q steine amino acid units in the protein chains. Disulfide formation is also involved in the process by which cells protect themselves from oxidative degradation. A cellular component called glutathione removes potentially harmful oxidants and is itself oxidized to glutathione disulfide in the process. Reduction back to the thiol requires the coenzyme flavin adenine dinucleotide (reduced), abbreviated FADH2. 

A rather different route, involving formation of an organic nitrite by a process requiring reduced flavin, has been proposed [67]. The organic nitrite then nitrosates a cellular thiol and NO is formed from the nitrosothiol (Eqs. (14) and (15)). 

Flavins and their analogues (vitamin B2 family) bound to a cationic hydrophobic aggregate serve as efficient oxidizing agents for carbanions and thiols (see Section 7). 

The conversion (19) of thiols to disulphides coupled with reduction of flavin (vitamin B2 family) is a topic of import in connection with coenzyme reactivity in flavoenzymes. Since flavin oxidation of thiols involves nucleophilic attack of thiolate ion in the rate-determining step (Loechler and Hollocher, 1975 Yokoe and Bruice, 1975), this biologically important reaction would be markedly affected by hydrophobic environments. 

N-,0-,S-dealkylation N-, 0- or S-alkyl derivatives Flavine monooxygenases thiol compounds N-oxides... 

Monoamine oxidase (MAO) (E.C. 1.4.3.4) is an enzyme found in all tissues and almost all cells, bound to the outer mitochondrial membrane. Its active site contains flavine adenine dinucleotide (FAD), which is bound to the cysteine of a -Ser-Gly-Gly-Cys-Tyr sequence. Ser and Tyr in this sequence suggest a nucleophilic environment, and histidine is necessary for the activity of the enzyme. Thiol reagents inhibit MAO. There are at least two classes of MAO binding sites, either on the same molecule or on different isozymes. They are designated as MAO-A, which is specific for 5-HT (serotonin) as a substrate, and MAO-B, which prefers phenylethylamine. Similarly, MAO inhibitors show a preference for one or the other active site, as discussed below. 

These reactions, which have provided a means of inhibiting the flavin-linked monoamine oxidases, enable us to end on a clinical note. The monoamine oxidases are responsible for the deamination of monoamines such as adrenaline, noradrenaline, dopamine, and serotonin, which act as neurotransmitters. Imbalances in the levels of monoamines cause various psychiatric and neurological disorders Parkinson s disease is associated with lowered levels of dopamine, and low levels of other monoamines are associated with depression. Inhibitors of monoamine oxidases may consequently be used to treat Parkinson s disease and depression. The flavin moiety is covalently bound to the enzyme by the thiol group of a cysteine residue (equation 9.17). The acetylenic suicide inhibitor N,N-dimethyl-propargylamine inactivates monoamine oxidases by alkylating the flavin on N-5.25 A likely mechanism for the reaction is the Michael addition of the N-5 of the reduced flavin to the acetylenic carbon 2... 

The conversion of NO to HNO can proceed by several mechanisms, including formal reduction by metalloenzymes such as superoxide dismutase (SOD) (83-85) and xanthine oxidase (XO) (86) or reductants such as flavins (87) and ubiquinol (88). The reaction of 5-nitrosthiols, which would be formed initially upon NO biosynthesis, with excess thiols also releases HNO (89-92). 

The lipase-solubilized reductase is inhibited by p-mercuribenzoate, is protected from this inhibition by NADPH, and the inhibition is relieved by thiols (10). Careful titration of this enzyme with p-mercuribenzoate at pH 6.5 results in an almost 3-fold stimulation upon addition of 2 moles of mercurial per flavin the control activity is again observed when 7 equivalents have been added. At pH 7.7, a stimulation of 70% is seen with 1 equivalent and loss of activity is complete (extrapolated) with 6 equivalents (245). The protection of the enzyme by NADPH against mercurial inhibition is reminiscent of the effects with NADH cytochrome 63 reductase (360). 

Nitrite reduction in assimilatory nitrate-reducing Neurospora crassa, Torulopsis nitratophila, Azotobacter vinelandii, and Azotobacter chro-ococcum appears to be catalyzed by enzyme systems which require flavin and metals. The enzyme from N. crassa has been partially purified, and its molecular weight has been estimated to be 300,000 (344, 346, 351, 367). The enzyme reduces both nitrite and hydroxylamine to ammonia and utilizes NADH or NADPH as electron donor. It is reported to be a FAD-dependent enzyme and to contain iron, copper, and active thiol (346, 367). Three moles of NADH are oxidized per mole of nitrite reduced to ammonia. It has been suggested that the reduction of nitrite occurs in three steps, each involving two electrons. Thus, hyponitrite and hydroxylamine have been proposed as successive intermediates in the re-... 

Flavins (8), 8-azaflavin (9) and their analogs are characteristic oxidants toward thiols under anaerobic and other specific conditions (equation 12). S-Arylidene-l,3-dimethylbarbituric acid derivatives (10) also work as the oxidant in dioxane at 12(X-1S0 C, and the method was applied to synthesis of unsymmetrical disulfides. Electrochemical oxidation and photolysis of thiols to disulfidqs are also known. 

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