Enzyme methanol dehydrogenase

Figure 2. (a) Addition-Elimination (A-E) and (b) Hydride-Transfer (H-T) methanol electro-oxidation mechanisms by Methanol Dehydrogenase Enzyme proposed in the literature. 

The alcohol tolerance of O2 reduction by bilirubin oxidase means that membraneless designs should be possible provided that the enzymes and mediators (if required) are immoblized at the electrodes. Minteer and co-workers have made use of NAD -dependent alcohol dehydrogenase enzymes trapped within a tetraaUcylammonium ion-exchanged Nafion film incorporating NAD+/NADH for oxidation of methanol or ethanol [Akers et al., 2005 Topcagic and Minteer, 2006]. The polymer is coated onto an electrode modified with polymethylene green, which acts as an electrocatalyst... 

NAD(P)+ as Anode Mediator. A majority of redox enzymes require the cation nicotinamide adenine dinucleotide, possibly phosphorylated (NAD(P)+) as a cofactor. Of the oxidoreductases listed in Enzyme Nomenclature, over 60% have NAD(P)+ as a reactant or product.For example, methanol can be oxidized to form formaldehyde by methanol dehydrogenase (MDH, EC 1.1.1.244) according to... 

Pyrroloquinoline quinone (PQQ) (or methoxatin) 6 is a coenzyme, responsible for the oxidation of methanol [7]. It has been found that cyclopropanol 4 inactivates the enzyme from M. methanica [8], the dimeric methanol dehydrogenase and the monomeric enzyme from a Pseudomonas PQQ-dependent methanol dehydrogenase [9] by forming adducts such as 7, through a one-electron oxidation process and the ready ring opening of a cyclopropyloxonium radical, Eq. (3) [8,9]. 

This enzyme [EC 1.1.99.8], also referred to as alcohol dehydrogenase (acceptor) and methanol dehydrogenase, catalyzes the oxidation-reduction reaction of a primary alcohol with an acceptor to generate an aldehyde and the reduced acceptor. The cofactor for this enzyme is pyrroloquinoline qutnone (PQQ). A wide variety of primary alcohols can act as the substrate. See also Alcohol Dehydrogenase... 

More complex reductions of CO2 by enzyme cascades have also been achieved. A combination of an electron mediator and two enzymes, formate dehydrogenase and methanol dehydrogenase, was used to reduce CO2 to methanol. This system operates with current efficiencies as high as 90% and low overpotentials (approximately —0.8 V vs. SCE at pH 7) [125]. The high selectivity and energy efficiency of this system indicate the potential of enzyme cascades. There are also drawbacks to these systems. In general, enzymes are... 

Evidence for a hydride transfer mechanism (Scheme 29) for the PQQ-dependent enzyme methanol dehydrogenase (MDH) was obtained by a theoretical analysis combined with an improved refinement of a 1.9 A resolution crystal structure of MDH from Methylophilus methylotrophus in the presence of CH3OH <2001PNA432>. The alternative mechanism proceeding via a hemiketal intermediate was discounted when the observed tetrahedral configuration of the C-5 atom of PQQ in that crystal structure was shown to be the C-5-reduced form of the cofactor 198, a precursor to the more common reduced form of PQQ 199. 

ZnS colloids were also used by Kuwabata et al. to photoreduce C02 to formate [130]. In this system, the ZnS colloids also reduced pyrroloquinoline quinone which served as an electron mediator to the enzyme, methanol dehydrogenase, which could then reduce formate to methanol. In C02-saturated aqueous solution at pH 7 and under far-UV (280nm) illumination, quantum efficiencies of 7% and 6% were achieved for formate and methanol, respectively. 

Different 2H-, 13C- and/or 15N isotopomers of L-serine, [(S)-2-amino-3-hydroxypro-panoic acid], 95, required for studies of aminoacid metabolism and for studies of peptide and protein structure and dynamics, have been biosynthesized stereoselectively84 using the serine-type methylotrophic bacterium, Methylobacteri extorquens AMI, which contains85 large amounts of the enzymes methanol dehydrogenase and hydroxymethyl-transferase (equation 39). 

Duine, J. A., and Frank, J., 1980b, Studies on methanol dehydrogenase from Hyphomicrobium X. Isolation of an oxidised form of die enzyme. Biochem. J. 187 213n219. 

Goodwin, M. G., Avezoux, A., Dales, S. L., and Anthony, C., 1996, Reconstitution of the quinoprotein methanol dehydrogenase from active Ca -free enzyme with Ca, Sr or Ba t Biochem. J. 319 839A842. 

Harris, T. K., and Davidson, V. L., 1994, Replacement of enzyme-bound calcium with strontium alters the kinetic properties of methanol dehydrogenase. Biochem. J. 300 1759182. 

Methanol dehydrogenase (MDH) enzyme is found in the periplasm of methylotrophic bacteria, and plays a cracial role in the metabolism of these organisms. It catalyzes the oxidation of methanol... 

Figure 1. (a) View of the inside of the Methanol Dehydrogenase (MDH) enzyme with the active site in stick model. The solid surface represents the solvent-accessible MDH external surface showing the binding pocket, (b) View from the binding pocket of the entire MDH active site. Amino acids labels denote their location in the sequence obtained from the entry 1W6S (Methylobacterium Extorquens W3A1 ) of the Protein Data Bank. 

Biocatalytic synthetic reactions also include carbon dioxide fixation with the production of methanol in artificial multi-enzyme systems [188]. Formate dehydrogenase (FDH, EC 1.2.1.2) can catalyze the reduction of carbon dioxide to formate, and methanol dehydrogenase (MDH, EC 1.1.99.8) can catalyze the reduction of formate to methanol. Both of these enzymes require NAD+-NADE1 cofactor, and in the presence of the reduced dimethyl viologen mediator (MV+), they can drive a sequence of enzymatic reactions. The cascade of biocatalytic reactions results in the reduction of CO2 to formate catalyzed by FDEI followed by the reduction of formate to methanol catalyzed by MDH. A more complex system composed of immobilized cells of Parococcus denitrificans has been demonstrated for the reduction of nitrate and nitrite [189]. 

Inhibiting methanol metabolism. Ethanol, which occupies the dehydrogenase enzymes in preference to methanol, competitively prevents metabolism of methanol to its toxic products. A single oral dose of ethanol 1 ml/kg (as a 50% solution or as the equivalent in gin or whisky) is followed by 0.25 ml/kg/h orally or i.v., aiming to maintain the blood ethanol at about... 

---