Ammonia-oxidizing chemoautotrophic
Nitrosomonas europaea ATCC 25978 exhibited remarkable oxaloacetate-oxidative activity. Citrate(
si)-synthase [EC 4.1.3.7.] was purified as an electrophoretic homogeneous protein. The molecular weight of the enzyme was estimated to be about 295, 000 (α
xβ
y) by gel filtration, suggesting that the enzyme consisted of two different subunits (α; 65, 000, β; 75, 000), as demonstrated by SDS-PAGE. The N-terminal sequences of α and β type enzyme proteins were Ala-Leu-Val-Ser-Leu-Arg-Gln-Leu-Leu. The isoelectric point of the enzyme was pH 5.2. The enzyme was stable up to 35°C and in a pH range between 6.0-8.0. The optimum pH for the reaction was 6.5-8.5 and the optimum temperature was 30-40°C. The apparent
Km values for oxaloacetate and acetyl-CoA were about 0.025 and 0.080mM, respectively. The enzyme was not inhibited by ATP (1mM), NADH (1mM) or 2-oxoglutamic acid (10mM). Activation by metal ions (0.1mM) was not observed.
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