Molybdenum enzymes

TorD, an Essential Chaperone for TorA Molybdoenzyme Maturation at High Temperature

TorD has been recognized as an accessory protein that improves maturation of TorA, the molybdenum cofactor-containing trimethylamine oxide reductase of Escherichia coli. In this study, we show that at 42 degrees C and in the absence of TorD TorA is poorly matured and almost completely degraded. Strikingly, TorD restores TorA maturation to the same level whatever the growth temperature. In vitro experiments in which apoTorA was incubated with or without TorD at various temperatures confirm that TorD is an essential chaperone for TorA at elevated temperatures preventing apoTorA mis-folding before cofactor insertion.

Role of conserved tyrosine 343 in intramolecular electron transfer in human sulfite oxidase

Tyrosine 343 in human sulfite oxidase (SO) is conserved in all SOs sequenced to date. Intramolecular electron transfer (IET) rates between reduced heme (Fe(II)) and oxidized molybdenum (Mo(VI)) in the recombinant wild-type and Y343F human SO were measured for the first time by flash photolysis. The IET rate in wild-type human SO at pH 7.4 is about 37% of that in chicken SO with a similar decrease in k(cat). Steady-state kinetic analysis of the Y343F mutant showed an increase in K(m)(sulfite) and a decrease in k(cat) resulting in a 23-fold attenuation in the specificity constant k(cat)/K(m)(sulfite) at the optimum pH value of 8.25. This indicates that Tyr-343 is involved in the binding of the substrate and catalysis within the molybdenum active site. Furthermore, the IET rate constant in the mutant at pH 6.0 is only about one-tenth that of the wild-type enzyme, suggesting that the OH group of Tyr-343 is vital for efficient IET in SO. The pH dependences of IET rate constants in the wild-type and mutant SO are consistent with the previously proposed coupled electron-proton transfer mechanism.

Microbial arsenic: from geocycles to genes and enzymes

Arsenic compounds have been abundant at near toxic levels in the environment since the origin of life. In response, microbes have evolved mechanisms for arsenic resistance and enzymes that oxidize As(III) to As(V) or reduce As(V) to As(III). Formation and degradation of organoarsenicals, for example methylarsenic compounds, occur. There is a global arsenic geocycle, where microbial metabolism and mobilization (or immobilization) are important processes. Recent progress in studies of the ars operon (conferring resistance to As(III) and As(V)) in many bacterial types (and related systems in Archaea and yeast) and new understanding of arsenite oxidation and arsenate reduction by respiratory-chain-linked enzyme complexes has been substantial. The DNA sequencing and protein crystal structures have established the convergent evolution of three classes of arsenate reductases (that is classes of arsenate reductases are not of common evolutionary origin). Proposed reaction mechanisms in each case involve three cysteine thiols and S-As bond intermediates, so convergent evolution to similar mechanisms has taken place.