| Root |
Created: 23 November 1998
| Haem | Haem type | Haem iron coordination | Axial iron ligands | Formal iron oxidation/spin states |
|---|---|---|---|---|
| 1 |  |
 |
 His;
N |
FeII (S=0);
FeIII (S=1/2) |
| 2 |  |
His |
FeII (S=2);
FeIII (S=5/2) |
|
| 3
4 |
|
His;
N |
FeII (S=0);
FeIII (S=1/2) |
Tetrahaem cytochrome c554 (cyt c554) is involved in the oxidation of ammonia to nitrite in a nitrifying chemoautotrophic bacterium Nitrosomonas europaea as the mobile electron carrier between hydroxylamine oxidoreductase (HAO; EC 1.7.3.4) and monohaem cytochrome c552 (cyt c552) [1] (1). Alternative schemes include the direct electron transfer from cyt c554 to a membranebound terminal oxidase [2] (2):
Cyt c554 contains four covalently bound haem groups,
designated 1 to 4 according to the position of the haem binding motif
CxxCH
in the sequence.
The reduction potentials of the four haems at pH 7.0 have been determined
[3] (see table below).
Spectroscopic changes indicate that the two electrons from HAO are taken up by
a highpotential (Em = +47 mV) haem pair
(one highspin and one lowspin)
[4].
NH2OH + H2O 
HAO
4e¯
2 cyt c554
4e¯
4 cyt c552
4e¯
terminal oxidase
(1)
NO2¯ + 5H+
4e¯
(2)
The 3D structure of cyt c554 has been determined
[1].
It consists mainly of 
helices,
with two stretches of 310helix and two short parallel
ßstrands. Haems 1 and 2 are located at the opposite ends of cyt
c554, while haems 3 and 4 are located in the middle of the
molecule. The four haems are arranged as two haem pairs, (1,3) and (2,4).
Within each pair, the haem planes are roughly parallel to each other
and the pairs are arranged roughly perpendicular to each other.
Haems 1, 3 and 4 are bisHis coordinated, lowspin haems; haem 2 is
pentacoordinate, highspin haem.
Cyt c554 is unique among other haem proteins in coordination
of haem 1: the iron sixth ligand is the
N
of the distal histidine
(His102).
Interestingly, the four haems of cyt c554 may be structurally
aligned with haems 3-6 of HAO and are connected to the
protein in the same order, although there are no significant sequence and
structure similarity between cyt c554 and HAO. Haem 2
corresponds to the pentacoordinate haem P460 which forms the active site of
HAO [1].
Some properties of the cyt c554 haems are summarised in the
table:
| Haem | Crystallography [1] | Spectroscopy / electrochemistry ¹ | |||||
|---|---|---|---|---|---|---|---|
| Haem iron ligands | Covalent links | Spin | Absorption spectral features (nm) [3] | Em (mV)
[3] ³ |
|||
| Proximal | Distal | Soret ² | |
||||
| 2 | His64 | Cys60; Cys63 | high | 424; 432 | 554 | +47 (+50) | |
| 1 | His13 | His102 | Cys11; Cys14 | low | 424; 432 | 554 | +47 (+50) |
| 3 | His92 | His179 | Cys88; Cys91 | low | 414; 424 | 554 | -147 (-120) |
| 4 | His138 | His27 | Cys134; Cys137 | low | 418; 432 | 554 | -276 (-225) |
¹
Note that there are no assignment between `crystallographic' and
`spectroscopic' parts of the table, especially as concerns haems 1, 3 and 4.
It is likely that haem 2 corresponds to +47 mV haem.
²
The Soret bands are split.
³
Spectroelectrochemical measurements; figures in brackets are from the
voltammetry measurements [3].
| C554_NITEU | Cytochrome c554 precursor (hydroxylamine oxidoreductaselinked cytochrome); Nitrosomonas europaea |
References
| Bibliography on structural studies of cytochrome c554 |
