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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Heal Yourself With Qi Gong

Qigong also spelled Ch'i Kung is a potent system of healing and energy medicine from China. It's the art and science of utilizing breathing methods, gentle movement, and meditation to clean, fortify, and circulate the life energy qi.

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