The ammonia-oxidizing chemoautotrophic bacterium Nitrosomonas europaea possesses prominant succinate-reducing activity of succinyl-Coenzyme A synthetase (SCS, EC 6.2.1.5). SCS was purified as an electrophoretically homogeneous protein from Nitrosomonas europaea strain ATCC 25978 about 275-fold, with a 3.9% activity yield. The molecular mass of the native enzyme was estimated to be about 130 kDa by gel filtration, whereas SDS-PAGE gave two protein bands with M(r) values of 29 (alpha) and 36 kDa (beta). The isoelectric point of the enzyme was 5.3. The apparent K-m values of the enzyme for ATP, succinate and CoA were 0.4 mM, 5 mM and 0.1 mM, respectively. The pH and temperature optima of the SCS were about 5.0 and 55 degrees C, respectively. The SCS was stable in the pH range of 8.0-10.0 and up to 70 degrees C. The enzyme was thermostable; 50% of the enzyme activity was retained at 90-100 degrees C for 10 min. The SCS was activated by Mg2+ at 1.0-100 mM, but inhibited by Cu2+ (0.1 mM) and SDS (1.0 mM). The enzyme utilized ATP as the preferred substrate.