One molecule of molybdopterin guanine dinucleotide is associated with each subunit of the heterodimeric Mo-Fe-S protein transhydroxylase of Pelobacter acidigallici as determined by SDS/RAGE and mass spectrometry

Abstract

The molybdenum-containing iron-sulfur protein 1,2,3,5-tetrahydroxybenzene:1,2,3-trihydroxyben-zene hydroxyltransferase (transhydroxylase) of Pelobacter acidigallici was investigated by various techniques including mass spectrometry and electron paramagnetic resonance. Mass spectrometry confirmed that the 133-kDa protein is a heterodimer consisting of an α subunit (100.4 kDa) and a β subunit (31.3 kDa). The presence of a molybdenum cofactor was documented by fluorimetric analysis of the oxidized form A of molybdopterin. The enzyme contained 1.55 ± 0.14 mol pterin and 0.92 ± 0.25 mol molybdenum/mol enzyme (133 kDa). Alkylation of the molybdenum cofactor with iodoacetamide formed di(carboxamidomethyl)-molybdopterin. Upon acid hydrolysis, 1.4 mol 5'GMP/mol enzyme (133 kDa) was released indicating that molybdenum is bound by a molybdopterin guanine dinucleotide. The α and β subunits were separated by preparative gel electrophoresis. Both subunit fractions were free of molybdenum but contained equal amounts of a fluorescent form of the molybdenum cofactor. Mass spectrometry at various pH values revealed that an acid-labile cofactor was released from the large subunit and also from the small subunit. At X-band, 5 25 K, transhydroxylase (as isolated) showed minor EPR resonances with apparent g values around 4.3, 2.03 and, depending on the preparation, a further signal at g of approximately 1.98. This signal was still detectable above 70 K and was attributed to a Mo(V) center. Upon addition of dithionite, a complex set of intense resonances appeared in the region g 2.08 1.88. From their temperature dependence, three distinct sites could be identified: the Fe-S center I with gx,y,z at approximately 1.875, 1.942 and 2.087 (gav 1.968, detectable 20K); and the Mo(V) center consisting of a multiple signal around g 1.98 (detectable >70 K).