to restore N 2 O conversion after oxygen-dependent inhibition of N 2 O reduction.
Since NGCs encode a multitude of electron transport enzymes (Fig. 2; Table 2), it
is conceivable that these proteins serve in different dedicated and functionally
non-redundant ETCs.
Figure 3 presents models of the ETCs involved in clade I and clade II N 2 O
respiration as well as hypotheses of the respective NosZ active site maturation and
maintenance processes [19]. Notably, the deduced bioenergetic framework in clade
I and clade II N 2 O respiration is assumed to be equivalent. In both cases, proton
motive quinol oxidation by N 2 O is thought to be catalysed by the Q cycle mechanism of a membrane-bound Rieske/cytochrome bc complex (QcrABC complex)
comprising a dihaem cytochrome b (QcrB), an iron–sulphur protein (QcrA, the
so-called Rieske protein) and a mono- or dihaem cytochrome c named QcrC
(Fig. 3) [19, 36, 45, 86–88]. QcrABC receives electrons from the membrane-bound
quinone/quinol pool, which may consist of ubiquinone/ubiquinol (UQ/UQH 2 )
and/or menaquinone/menaquinol (MK/MKH 2 ). Quinone species are reduced by
other pmf-generating enzymes such as hydrogenases, formate dehydrogenases or
NADH dehydrogenases whose activity completes the reactions shown in Eqs. 1–3
(Sect. 2). The ETC between QcrC and NosZ/cNosZ is thought to be mediated by
soluble periplasmic monohaem cytochromes c or, in addition for clade I NosZ
enzymes and depending on the organism, by small copper proteins such as (pseudo)
azurins or other cupredoxins (Fig. 3a) [45, 59, 69, 89]. The Qcr complex is usually
involved in multiple ETCs, and thus, the small redox mediator proteins form a
periplasmic electron transport branching point [45, 90]. In denitrifying clade I
organisms, electrons are delivered to different terminal reductases including NO
reductase (Nor), the NO-forming nitrite reductases NirS or NirK or the N 2 O
reductase NosZ [36]. In case of NosZ, it is thought that the electron entry site is the
Cu A centre. In fact, a cytochrome c has been shown to function as an electron donor
for NosZ in denitrifying P. pantotrophus or Marinobacter hydrocarbonoclasticus
cells [89, 91]. For W. succinogenes, interaction between a periplasmic monohaem
cytochrome c and cNosZ has been shown experimentally [16, 92]. In either case,
there is no indication that NosZ exchanges electrons by direct interaction with the
Qcr complex, and such a feature is thought to be counterproductive for the cell in
the light of the function of the Qcr complex as an electron hub. Overall, the Qcr
complex would be the only coupling site within a linear electron transport chain
from the quinone/quinol pool to N 2 O and a proton per electron ratio (H
+
/e
−
) of 1 is
expected.
The models put forward in Fig. 3 for the regular ETCs that connect the
quinone/quinol pool and NosZ/cNosZ involve N 2 O reductase as the only Nos
protein and thereby keep the length of the highly similar pmf-generating ETCs to a
minimum. In contrast, clade I and clade II organisms are expected to differ significantly in terms of the auxiliary electron transport processes mentioned above.
Figures 3a, b depict the proposed functions for the other Nos protein, namely
NosD, -Y, -F, -L, -R, -X, -C, -B, -G, -H, -C1 and -C2 (Table 2) [19]. Note that these
models are in line with the absence of any functional redundancy in Nos protein
inventories belonging to either clade I or clade II.
196
J. Simon
Since NGCs encode a multitude of electron transport enzymes (Fig. 2; Table 2), it
is conceivable that these proteins serve in different dedicated and functionally
non-redundant ETCs.
Figure 3 presents models of the ETCs involved in clade I and clade II N 2 O
respiration as well as hypotheses of the respective NosZ active site maturation and
maintenance processes [19]. Notably, the deduced bioenergetic framework in clade
I and clade II N 2 O respiration is assumed to be equivalent. In both cases, proton
motive quinol oxidation by N 2 O is thought to be catalysed by the Q cycle mechanism of a membrane-bound Rieske/cytochrome bc complex (QcrABC complex)
comprising a dihaem cytochrome b (QcrB), an iron–sulphur protein (QcrA, the
so-called Rieske protein) and a mono- or dihaem cytochrome c named QcrC
(Fig. 3) [19, 36, 45, 86–88]. QcrABC receives electrons from the membrane-bound
quinone/quinol pool, which may consist of ubiquinone/ubiquinol (UQ/UQH 2 )
and/or menaquinone/menaquinol (MK/MKH 2 ). Quinone species are reduced by
other pmf-generating enzymes such as hydrogenases, formate dehydrogenases or
NADH dehydrogenases whose activity completes the reactions shown in Eqs. 1–3
(Sect. 2). The ETC between QcrC and NosZ/cNosZ is thought to be mediated by
soluble periplasmic monohaem cytochromes c or, in addition for clade I NosZ
enzymes and depending on the organism, by small copper proteins such as (pseudo)
azurins or other cupredoxins (Fig. 3a) [45, 59, 69, 89]. The Qcr complex is usually
involved in multiple ETCs, and thus, the small redox mediator proteins form a
periplasmic electron transport branching point [45, 90]. In denitrifying clade I
organisms, electrons are delivered to different terminal reductases including NO
reductase (Nor), the NO-forming nitrite reductases NirS or NirK or the N 2 O
reductase NosZ [36]. In case of NosZ, it is thought that the electron entry site is the
Cu A centre. In fact, a cytochrome c has been shown to function as an electron donor
for NosZ in denitrifying P. pantotrophus or Marinobacter hydrocarbonoclasticus
cells [89, 91]. For W. succinogenes, interaction between a periplasmic monohaem
cytochrome c and cNosZ has been shown experimentally [16, 92]. In either case,
there is no indication that NosZ exchanges electrons by direct interaction with the
Qcr complex, and such a feature is thought to be counterproductive for the cell in
the light of the function of the Qcr complex as an electron hub. Overall, the Qcr
complex would be the only coupling site within a linear electron transport chain
from the quinone/quinol pool to N 2 O and a proton per electron ratio (H
+
/e
−
) of 1 is
expected.
The models put forward in Fig. 3 for the regular ETCs that connect the
quinone/quinol pool and NosZ/cNosZ involve N 2 O reductase as the only Nos
protein and thereby keep the length of the highly similar pmf-generating ETCs to a
minimum. In contrast, clade I and clade II organisms are expected to differ significantly in terms of the auxiliary electron transport processes mentioned above.
Figures 3a, b depict the proposed functions for the other Nos protein, namely
NosD, -Y, -F, -L, -R, -X, -C, -B, -G, -H, -C1 and -C2 (Table 2) [19]. Note that these
models are in line with the absence of any functional redundancy in Nos protein
inventories belonging to either clade I or clade II.
196
J. Simon