FMN/FMNH

The UV-VIS spectrum of a biomolecule reveals much about its molecular structure. Therefore, a spectral analysis is one of the first experimental measurements made on an unknown biomolecule. Natural molecules often contain chromophoric (color-producing) functional groups that have characteristic spectra. Figure 5.8 displays spectra of the well-characterized biomolecules DNA, FMN, FMNH, NAD, NADH, and nucleotides. Spectral analysis in the visible region is used in Experiment 8 to identify pigments isolated from plants. 

Following this mechanism, luciferase-bound reduced FMN (FMNH intermediate I) reacts with oxygen to form the 4a-hydroperoxy-4a,5-dihydroFMN intermediate II (HF-OCT). The addition of a long-chain aliphatic aldehyde generates the 4a-peroxyhemiacetal-4a,5-dihydroFMN intermediate III (HF-OOCH(OH)R), which is subsequently converted to a radical pair of 4a-hydroxy-4a,5-dihydroFMN radical cation (IV+ HF-OH+ ) and a carboxylic acid radical anion RC(0H)0" A 1-e transfer from RC(0H)0- to IV+ produces the excited 4a-hydoxy-4a,5-dihydroFMN intermediate IV. Relaxation of IV to the ground state produces bio luminescence ( max 490 nm) with a quantum yield of about 0.16. Finally, IV decays to form FMN and water. The present report addresses several key issues of this mechanism with respect to the identity of the proposed excited emitter HF-OH, the energetics of its formation, and the requirement of a hydrophobic luciferase active site for a high quantum yield of the emitter. 

Summarizing the results obtained by controlled potential electrolysis and polarography, the reaction process for the electrolytic evolution of CO2 was estimated to be as follows the first step was one electron transfer from DMFC in NB to FMN in W as in Eq. (7). The second step was the catalytic reduction of O2 by FMNH as in Eq. (8). The final step was the oxidation of pyruvic acid by the reduction product of O2, H2O2, in W as in Eq. (9), well-known as an oxidative decarboxylation of a-keto acids [43] ... 

Two main Oy generating reactions have been described for the mitochondrial respiratory chain. In both cases, the intermediate semiquinone of two redox pairs of components of the respiratory chain, ubiquinol/ubiquinone and the FMNH2/FMN coenzyme of NADH dehydrogenase, UQH and FMNH , respectively, are collision and non-enzymatically auto-oxidized by molecular O2 to yield Oy [3] ... 

The bacterial bioluminescent reaction requires the contribution of the flavine mononucleotide (FMN) reduced form (FMNH), a long-chain aldehyde (tet-radecanal), and oxygen. All these molecules produce an oxidative reaction in the presence of bacterial luciferase (EC 1.14.14.3), giving tetradecanoic acid, FMN, wateq and hght emission under these conditions. 

For the opposite mode (oxidation of alcohol to ketone) the system needs to be coupled to a flavin cofactor (FMNH 2 -> FMN) where oxygen is reduced to H2O2. The practicability of applying HLADH as a chiral oxidoreduction catalyst was demonstrated in many instances. Two examples are as follows ... 

Sensitive methods for SBA based on bioluminescence measurement have recently been deve p using hydroxysteroid dehydrogenase (3o( or 12d or 7d -HSD). The HSD catalyzes the conversion of the bile acid hydroxy g up (3o( or 7d or 12 c. ) to a keto-group in the presence of NAD(P). The resulting NAD(P)H, in presence of NAD(P)H FMN oxydoreductase (OXRED), converts FMN to its reduced form (FMNH ). This in turn reacts with decanal and oxygen in the presence dr bacterial luciferase to produce light. The intensity of the light emitted is proportional to the amount of bile acids in the initial raction. The overall scheme,for 7 -HSD for instance, is this ... 

FMNH-, flavin mononucleotide, neutral semiquinone radical FMN- , flavin mononucleotide, anion semiquinone radical hfc, hyperfine coupling ... 

The foundation for the chemical mechanism of bacterial luciferase is primarily laid by the work of Hastings and colleagues. " These studies and the work of others were the subjects of a number of reviews. Main features of the luciferase chemical mechanism are summarized in Scheme 136.2. The luciferase-bound Nl-deprotonated FMNH" (Intermediate I) reacts with oxygen to generate the 4a-hydroperoxy-FMNH (HF-4a-OOH Intermediate II), which undergoes a dark decay to form FMN and HjOj in the absence of aldehyde or reacts with aldehyde to form the 4a-peroxyhemiacetalFMNH (Intermediate III). 

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