The strict molybdate-dependence of glucose-degradation by the thermoacidophile Sulfolobus acidocaldarius reveals the first crenarchaeotic molybdenum containing enzyme - An aldehyde oxidoreductase

In order to investigate the effects of trace elements on different metabolic pathways, the thermoacidophilic Crenarchaeon Sulfolobus acidocaldarius (DSM 639) has been cultivated on various carbon substrates in the presence and absence of molybdate. When grown on glucose (but neither on glutamate nor casein hydrolysate) as sole carbon source, the lack of molybdate results in serious growth inhibition. By analysing cytosolic fractions of glucose adapted cells for molybdenum containing compounds, an aldehyde oxidoreductase was detected that is present in the cytosol to at least 0.4% of the soluble protein. With Cl₂Ind (2,6- dichlorophenolindophenol) as artificial electron acceptor, the enzyme exhibits oxidizing activity towards glyceraldehyde, glyceraldehyde-3- phosphate, isobutyraldehyde, formaldehyde, acetaldehyde and propionaldehyde. At its pH-optimum (6.7), close to the intracellular pH of Sulfolobus, the glyceraldehyde-oxidizing activity is predominant. The protein has an apparent molecular mass of 177 kDa and consists of three subunits of 80.5 kDa (α), 32 kDa (β) and 19.5 kDa (γ). It contains close to one Mo, four Fe, four acid- labile sulphides and four phosphates per protein molecule. Methanol extraction revealed the existence of 1 FAD per molecule and 1 molybdopterin per molecule, which was identified as molybdopterin guanine dinucleotide on the basis of perchloric acid cleavage and thin layer chromatography. EPR- spectra of the aerobically prepared enzyme exhibit the so-called 'desulpho- inhibited'-signal, known from chemically modified forms of molybdenum containing proteins. Anaerobically prepared samples show both, the signals arising from the active molybdenum-cofactor as well as from the two [2Fe- 2S]-clusters. According to metal-, cofactor-, and subunit-composition, the enzyme resembles the members of the xanthine oxidase family. Nevertheless, the melting point and long-term thermostability of the protein are outstanding and perfectly in tune with the growth temperature of S. acidocaldarius (80 °C). The findings suggest the enzyme to function as a glyceraldehyde oxidoreductase in the course of the nonphosphorylated Entner- Doudoroff pathway and thereby may attribute a new physiological role to this class of enzyme.