Nitrification by Heterotrophic Bacteria

Although the oxidation mechanism of nitrite to nitrate in the heterotrophic nitrifiers has not been known at all on the enzyme level, the oxidation mechanism of ammonia to nitrite has been partially clarified. Ammonia is oxidized to nitrite through hydroxylamine also in the heterotrophic bacteria. The oxidation of ammonia to hydroxylamine is catalyzed by ammonia monooxygenase as in the enzyme of Nitrosomonas europaea. The enzyme purified from Paracoccus pantotropha GB17 (formerly Thiosphaera pantotropha GB17 or Paracoccus denitrificans GB17) catalyzes the oxidation of ammonia to hydroxylamine and contains copper, but its activity is not inhibited by acetylene (Moir et al., 1996), unlike the enzyme of Nitrosomonas europaea. 

From several heterotrophic nitrifiers, enzymes are obtained which catalyze the reduction of ferricytochrome c in the presence of hydroxylamine like hydroxylamine oxidoreductase of N. europaea does, but they do not have heme C, unlike the N. europaea enzyme (Kurokawa et al., 1985 Wehrfritz et al., 1993, 1997). Hydroxylamine oxidoreductase purified from Alcaligenes faecalis strain TUD has non-heme iron and catalyzes the reduction of ferricyanide with hydroxylamine, but does not catalyze the reduction of ferricytochrome c with hydroxylamine (Otte et al., 1999). 

Alcaligenes faecalis has a hydroxylamine-oxidizing enzyme which differs from the enzyme of other heterotrophic nitrifying bacteria. In this bacterium, hydroxylamine is oxidized through pyruvic oxime (Fig. 3.6). Pyruvic acid derived from lactate, succinate, and other carboxylates reacts nonenzymatically with hydroxylamine formed from ammonia to form pyruvic oxime. This compound is oxidized to nitrite and pyruvic acid by the catalysis of pyruvic oxime dioxygenase (Ono et al., 1996, 1999). 

CH3C(= NOH)COOH + 02 Pvruvic oxime dioxygenase CJ CQCOOH + HN02 (3.8) 

Previously, a heterotrophic nitrifier, Achromobacter sp. was found to oxidize pyruvic oxime externally added to form nitrite (Quastel et al., 1952). However, as pyruvic oxime does not occur in Nature (Lang and Jagnow, 1986), the nitrification mechanism in which pyruvic oxime has to be acquired from the outside of the bacterial cells was denied. Ono et al. (1996) have shown that pyruvic oxime is formed inside the bacterial cells. Thus, an oxidation mechanism of hydroxylamine to nitrite in a heterotrophic nitrifier has been elucidated. 

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