ammonia monooxygenase (AMO) (Suzuki et al. 1974). The basic catabolic reaction
carried out by AOB is represented in Eqs. 7.16, 7.17, 7.18, and 7.19.
NH 3 þ O 2 þ 2H
þ
þ 2e
À
!
AMO NH 2 OH þ H 2 O
ð7:16Þ
NH 2 OH þ H 2 O !
HAO NO
À
2 þ 5H
þ
þ 4e
À
ð7:17Þ
HCO
À
3 þ xe
À
! Biomass
ð7:18Þ
0:5O 2 þ 2H
þ
þ 2 À xe
À
ð
Þ!H 2 O
ð7:19Þ
The first step in nitrification is catalyzed by AMO enzyme which oxidizes
ammonia into hydroxylamine (NH 2 OH) with the requirement of two reducing
equivalents and concomitant reduction of one mole of O 2 into H 2 O. The second
step is catalyzed by hydroxylamine oxidoreductase (HAO) enzyme which oxidizes
hydroxylamine formed in the first step to nitrite, with the release of four electrons.
This reaction catalyzed by HAO is the only known source of generation of electrons
in AOB cellular biochemical reactions. HAO is a homotrimeric multi-subunit
periplasmic enzyme which requires eight c-type hemes (Arp et al. 2002). Out of
four electrons released by HAO, two electrons flow back to AMO, x electrons are
utilized during autotrophic biomass accumulation, and (2Àxe
À ) electrons further
Fig. 7.1 Transmission electron microscope image of Nitrosomonas europaea (Wu et al. 2018)
160
P. Chawley et al.