The prosthetic group of methylamine dehydrogenase from Pseudomonas AM1: evidence for a quinone structure

The effect of EDTA and related chelating agents on the oxidation of methanol by the methylotrophic bacterium, Methylophilus methylotrophus

The effects of EDTA and related metal-chelating agents on the respiratory system of the methylotrophic bacterium Methylophilus methylotrophus have been investigated. EDTA completely inhibited whole cell methanol oxidase activity and concomitantly decreased the aerobic steady-state reduction level of cytochrome c, but only partially inhibited methanol dehydrogenase activity. Analysis of the inhibition kinetics of EDTA and other chelating agents on methanol oxidase activity indicated that they were effective in the order CDTA greater than EDTA greater than HEDTA greater than EGTA greater than EDDA, and that they inhibited in an uncompetitive manner. Inhibition by EDTA varied as a function of the ambient cell mass in the assay, and could be partly reversed by the addition of divalent cations. EDTA had no effect on the activity of solubilised methanol dehydrogenase. These and other results indicate that EDTA binds a divalent metal ion, probably Mg2+, which is involved in the functional association of methanol dehydrogenase with the respiratory membrane. This concept is discussed in terms of the electron transfer properties and spatial organisation of the terminal respiratory chain of M. methylotrophus.

Some electrochemical and chemical properties of methoxatin and analogous quinoquinones

The present study establishes relationships between structure and reactivity for the pyrroloquinoline and phenanthroline quinones. The electrochemical reductions of 1,7- and 1,10-phenanthroline-5,6-quinones, like other quinones, are reversible and occur by 2e- transfer in a single step in aqueous solution and by two 1e(-)-transfer steps in aprotic media. The electron-withdrawing pyridine moieties both increase their potentials and stabilize their aprotic semiquinones. The electrochemistry of the cofactor methoxatin and its trimethylester derivative is similar to the phenanthroline quinones in aqueous solution. However, the electrochemical reductions of methoxatin and its triester in aprotic solutions are characterized by at least three potentials, each accounting for less than 1e-. This has been explained by the proposal of semiquinone complexing with itself and with quinone. Despite an electron-donating pyrrole moiety, methoxatin and its trimethylester have relatively high potentials in aprotic solution. This is presumably due to stabilization of radical anions by the aforementioned complexing or by delocalization with carboxylic acid and ester groups. The reduction potential of methoxatin, in both aqueous and aprotic solvent, suggests that oxidation of methanol should be a thermodynamically favorable process. No evidence for an electrochemically reduced state lower than the quinol was found for any of the compounds. Chemical reactivity is influenced by the orientation of the pyridine nitrogen. The two quinones with a pyridine nitrogen peri to a quinone carbonyl add and oxidize nucleophiles most readily.

Structure and activity of the prosthetic group of methanol dehydrogenase

The reconstructive 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-1H-pyrrolo[2,3-f]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 (18 400 M-1 cm-1 at 249 nm) enables its quantitative anaysis. 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.