The Production of Ammonia by Multiheme Cytochromes c - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 216

functions and enzymes equipped with different catalytic centers: (i) assimilatory
enzymes which host a special porphyrin cofactor in close neighborhood to a
[4Fe-4S] cluster, the so-called siroheme-[4Fe-4S] catalytic center, and
(ii) respiratory multiheme c -type cytochromes which contain multiple heme
c groups (Figure 2 ).
Both metal enzymes show highly complex EPR spectra, with characteristic
resonances ranging from g 10 and 3.8 (multiheme c -

type cytochromes), respectively. In short, sirohemes are Fe complexes of
isobacteriochlorin, a class of hydroporphyrins with eight carboxylic acid side
chains. They have been detected in assimilatory nitrite (NO 2 ! NH 3 ) and sulfite
(SO 2 3 ! H 2 S) reductases as well as in dissimilatory sulfite (SO 2 3 ! H 2 S)
reductases. A salient feature of the isobacteriochlorin skeleton is its ease of
oxidation and difficulty of reduction compared with those of porphyrins and
chlorins. This facile oxidation led to the proposal that the siroheme macrocycle
itself might be involved in the multi-electron, multi-proton transfer reactions ( 13 )
and ( 18 )[ 20 , 21 , 25 ].

18 to 1.5 (siroheme) to g

Figure 2 Structure of
heme c , the covalently
bound heme group found
in cytochromes c . During
cytochrome c biosynthesis,
two thioether bonds are
formed between two vinyl
groups of heme b and the
two sulfhydryl (

SH)

residues of the
apocytochrome heme
c binding motif (usually
Cys-X-X-Cys-His).

Since the early studies by Fujita on soluble cytochromes in Enterobacteriaceae
and the first characterization of the then cytochrome c 552 enzyme [ 39 ], research on
multiheme nitrite reductases, especially in view of their key role in the biogeo-
chemical nitrogen cycle (Figure 1 ) (transformation NO 2 !

NH 3 ), has attracted a
major interest as documented by the large number of publications and comprehen-
sive reviews in biology, ecology, and chemistry [ 19 , 23 , 24 , 40 - 42 ]. Nowadays,
the enzyme is known as pentaheme cytochrome c nitrite reductase (or NrfA) and is
the prototypic enzyme of respiratory nitrite reduction to ammonium (equation 13 )
[ 19 , 43 ].
Originally discovered in enteric bacteria, the range of NrfA-producing
organisms is ever increasing in many habitats suggesting that NrfA contributes
significantly to global nitrogen turnover. Part of the NH 4 is released as NH 3 leading
to loss of nitrogen, similar to the pathway of denitrification which generates the
gaseous compounds NO, N 2 O, and/or N 2 . On the other hand, since NH 4 can be