LSorbosone Dehydrogenase Membrane Bound

L-Sorbosone ^ L-ascorbate

In current industrial L-ascorbate production processes, 2-keto-L-gulonate is a key intermediate that is chemically converted to L-ascorbate. All processes known to date require a large amount of energy and organic solvent, and thus a cheaper and environmentally conscious substitute process, such as enzymatic conversion, is being looked for. Due to the practical importance of the enzymes leading to L-ascorbate production, there are many papers dealing with the enzymes concerned (see Fig. 1.7). The following outstanding results should be itemized: isolation and characterization of a new PQQ-dependent dehydrogenase, L-sorbose/L-sorbosone dehydrogenase [70], cloning the genes coding for L-sorbose and L-sorbosone dehydrogenases from Gluconobacter oxydans and microbial production of 2-keto-L-gulonate, a precursor of L-ascorbate, in a recombinant G. oxydans [71], cloning and nucleotide sequencing of the membrane-bound L-sorbosone dehydrogenase gene of Acetobacter liquefaciens IFO 12258 and its expression in Gluconobacter oxydans [72], isolation and characterization of a new vitamin C-producing enzyme (L-gulono-y-lactone dehydrogenase of bacterial origin [73], microbial producton of L-ascorbate from D-sorbitol, L-sorbose, L-gulose, and L-sorbosone by Ketogulonicienium vulgare DSM 4025 [74].

Recently, a novel PQQ-dependent enzyme, L-sorbosone dehydrogenase 1 (SNDH1) catalyzing direct conversion of L-sorbosone to L-ascorbate was purified from Ketogluconicigenium vulgare DSM 4025 [75]. SNDH1 is a homo-oligomer of 75 kDa subunits containing PQQ and heme c as the prosthetic group. Two isozymes of SNDH1, SNDH2 consisting of 75 kDa and 55 kDa subunits, and SNDH3 consisting of a 55 kDa subunit, were also purified from the same strain. It was found that the 55 kDa subunit was derived from the 75 kDa subunit after cleavage of the C-terminal domain in the bacterial cells. The three enzymes catalyzed L-ascorbic acid formation as well as 2-keto-L-gulonate from L-sorbose, suggesting that tautomerization of L-sorbosone causes the dual conversion by SNDHs. Industrial L-ascorbic acid production has revealed direct conversion of L-sorbosone to L-ascorbic acid by a membrane-bound quinoprotein L-sorbosone dehydrogenase.

24 | 1 Biooxidation with PQQ- and FAD-Dependent Dehydrogenases 1.3.4

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