The reconstitutive ability of the isolated prosthetic group of methanol dehydrogenase with the apoenzyme of glucose dehydrogenase and the results of electron spin resonance measurements suggest that the prosthetic group has not been modified during the isolation. This result, and the properties of the directly isolated prosthetic group and derivatives, confirm the suggestion that its structure is 2,7,9-tricarboxy-1 H-pyrrolo[2,3lf]quinoline-4,5-dione.From the activity shown by derivatives of the prosthetic group and of structural analogues in the apoenzyme test it is concluded that the o-quinone structure is essential for activity. Hence the trivial name pyrrolo-quinoline quinone would be appropriate. The testing of the analogues also shows that the pyrrolo ring and the 9-carboxylic acid group are not essential for activity as they can be replaced by a pyridinol ring and a 9-hydroxy group respectively.The determination of the molar absorption coefficient of the prosthetic group (18400 M -l cm-' at 249 nm) enables its quantitative analysis. Thus it could be established that methanol dehydrogenase contains one prosthetic group per enzyme molecule. The consequences of this result in relation to already known properties of this 'quinoprotein' dehydrogenase are discussed.As was previously shown, the prosthetic group of methanol dehydrogenase is a novel, nitrogen-containing o-quinone [I -41. Recently, based on X-ray diffraction analysis of a degradation product, a chemical structure was proposed [5]. However, no evidence was presented for the presence of the o-quinone group in the proposed structure of the prosthetic group. Moreover, the question of whether the proposed structure represents that of the prosthetic group in situ remained unanswered.In this paper, the structure of the directly isolated prosthetic group is described. Furthermore, some chemical and physical properties which are valuable for its characterization and estimation, are reported.The reconstitutive ability of the prosthetic group could be investigated as we found that glucose dehydrogenase from Acinetobacter culcoaceticus contains the same prosthetic group and a suitable apoenzyme could be prepared [6]. Derivatives of the prosthetic group and structural analogues were tested with the apoenzyme system in order to see which structuralAbbreviations. ESR, electron spin resonance; NMR, nuclear magnetic resonance; compound Ia, 2,7,9-tricarboxy-lH-pyrrolo [2, compound IIa, 2,7,fl quinoline-4,5-dihydro-4,5-diol.

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The three‐dimensional structure of quinoprotein methylamine dehydrogenase from Thiobacillus versutus has been determined at 2.25 A resolution by a combination of multiple isomorphous replacement, phase extension by solvent flattening and partial structure phasing using molecular dynamics refinement. In the resulting map, the polypeptide chain for both subunits could be followed and an X‐ray sequence was established. The tetrameric enzyme, made up of two heavy (H) and two light (L) subunits, is a flat parallellepiped with overall dimensions of approximately 76 x 61 x 45 A. The H subunit, comprising 370 residues, is made up of two distinct segments: the first 31 residues form an extension which embraces one of the L subunits; the remaining residues are found in a disc‐shaped domain. This domain is formed by a circular arrangement of seven topologically identical four‐stranded antiparallel beta‐sheets, with approximately 7‐fold symmetry. In spite of distinct differences, this arrangement is reminiscent of the structure found in influenza virus neuraminidase. The L subunit consists of 121 residues, out of which 53 form a beta‐sheet scaffold of a central three‐stranded antiparallel sheet flanked by two shorter two‐stranded antiparallel sheets. The remaining residues are found in segments of irregular structure. This subunit is stabilized by six disulphide bridges, plus two covalent bridges involving the quinone co‐factor and residues 57 and 107 of this subunit. The active site is located in a channel at the interface region between the H and L subunits, and the electron density in this part of the molecule suggests that the co‐factor of this enzyme is not pyrrolo quinoline quinone (PQQ) itself, but might be instead a precursor of PQQ.

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