Methanol Dehydrogenase in Gram-Negative Bacteria

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Abstract
Methanol oxidation in gram-negative bacteria is always catalyzed by the NAD+-independent alcohol dehydrogenase, which was described originally in a pink facultative methylotroph, now called Methylobacterium extorquens [1,2]. Although not specific for methanol, its usual function is to catalyze methanol oxidation, and so it is usually referred to as methanol dehydrogenase (MDH; EC 1.1.99.8). Possession of this enzyme is one feature that appears to be common to all gram-negative methane- and methanol-oxidizing bacteria. Complete descriptions of all MDHs described up to 1985 have been published in an extensive review [3]. The first quinoproteins to be described and shown to have an unusual prosthetic group were this dehydrogenase [2,4,5] and glucose dehydrogenase [6], and the prosthetic group of MDH was the first PQQ to be purified and characterized [5]. This first description showed that the absorption spectrum of MDH was clearly different from that of flavoproteins, having an absorption band due to the prosthetic group with a peak at 345 nm and a shoulder at about 400 nm (Fig. 1); denaturation released a reddish-brown, small, acidic molecule with a characteristic green fluorescence having an excitation maximum at 365 nm and emission maximum at 470 nm. After purification and characterization of this novel prosthetic group, it was concluded that it was either a completely novel compound or an unusual pteridine [5]. The first of these suggestions proved to be correct when it was shown, more than 10 years later, to be PQQ [7–9]. 18 Figure 1 The absorption spectra of MDH. The reduced form (——) corresponds to MDHred in Figure 2 and contains PQQH2 (Fig. 3). The oxidized form (—) corresponds to MDHox in Figure 2 and contains PQQ (Fig. 3); this spectrum is very similar to that of the semiquinone form (MDHsem) in which the enzyme is isolated [4,5]. (From Ref. 38 with permission.)

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