Role of quinoprotein glucose-dehydrogenase in gluconic acid production by Acinetobacter calcoaceticus

Methanopterin is probably the first coenzyme involved in the reduction of CO 2 to CH 4 by Methanobacterium thermoautotrophicum. It is reduced and labelled with 14C02 in whole cells (Daniels and Zeikus, 1978; Keltjens et al., 1982) and it stimulates the formation of CH 4 in resolved cell-free extracts (J. Leigh, personal communication). We developed a quantitative, anaerobic and non-destructive method to measure the conversion of methanopterin in cell-free extracts with reversed-phase HPLC and a gradient of 0-25% methanol in 25 mM acetate buffer (pH 6.0). The conversion of methanopterin and the production of methane proceeded under identical conditions and required the presence of Mg 2+ plus ATP, hydrogen and methylcoenzyme M. The main products formed during the enzymatic conversion of methanopterin were two reduced pterins, which had lost the major part of the side chain of methanopterin, the side chain itself and a fourth unidentified reaction product. Chemical reduction of methanopterin with H 2 and a Pd/C catalyst yielded the same products as found with the enzymatic conversion, except for the fourth reaction product. Methanopterin and a number of degradation products were purified and analysed. Detailed NM R studies showed that methanopterin from M. thermoautotrophicum contains ribitol, ribose-5-phosphate, a glutarate derivative, a pterin substituted at the C 6and C7-positions and a second chromophore, which is probably an aniline derivative.