Hydrogen Sulfide: A Toxic Gas Produced by Dissimilatory Sulfate and Sulfur Reduction and Consumed by Microbial Oxidation - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 249

[ 75 ]. The binding of cobalt to ATPS is unresolved at this time but may contribute to
stability of the polymeric structure [ 78 ].

2.3 Dissimilatory Adenylylsulfate Reductase

In the pathway of sulfate reduction, APS is reduced to sulfite by APS reductase
(APSR) (adenylylsulfate reductase, EC 1.8.99.2). A ʱʲ heterodimer forms the basic
enzyme unit with the active site FAD located in the AprA subunit and two
ferredoxin-like [4Fe-4S] clusters in the AprB subunit. The crystal structure of
APSR has been reported for D. gigas at 3.1

Å
[ 82 ]. Using homology modeling of 20 different species, Meyer and Kuever [ 83 ]
have compared the dissimilatory APSR (AprBA) of SRP and SOB and report that
the protein matrix around the [4Fe-4S] clusters and the FAD cofactor show high
similarity.
The D. gigas APSR has a hexamer structure consisting of six

Å [ 81 ] and for Ar. fulgidus at 1.6

ʱʲ heterodimers

[ 81 ]. The subunit of D. gigas APSR has a distinct region that binds FAD; another

area is the capping attributed to the specific molecular configuration and the third is
the helical domain [ 81 ]. FAD is attached to the

ʱ

ʱ subunit domain by six hydrogen

bonds and a shallow cleft is formed in the ʱ subunit by the action of the FAD and

subunit of D. gigas APSR has a domain for binding the
[4Fe-4S] cluster, a segment that contains the

ʲ

helical domains. The

-sheet protein and the C-terminal
region. In formation of the dimer, the domain of the

ʲ

subunit containing the
[4Fe-4S] clusters is buried in the shallow cleft occurring in the ʱ subunit. The
ʲ

ʲ

-sheet protein and the C-terminal domains stabilize the interaction between the ʱ

subunit and ʲ subunit of APSR. Cluster I [4Fe-4S] is buried in the ʲ subunit while

cluster II [4Fe-4S] is located on the surface of the subunit where it presumably

acquires electrons from the electron donor [ 81 ]. The redox potentials for the cluster
I and cluster II are 0 and

ʲ

400 mV, respectively [ 84 ]. The electron donor for the
dissimilatory APSR in SRB is the membrane QmoABC complex [ 85 ]. Using
deletion mutants of D. vulgaris H, it has been shown that the Qmo complex
( qmoABC genes) is not essential for sulfite or thiosulfate reduction [ 86 ]. The
proximity of AprBA genes to QmoABC for D. gigas and D. vulgaris H are given
in Figure 4 .
A comparison of the D. gigas APSR crystal structure with the structure of the Ar.
fulgidus enzyme reveals considerable similarity [ 81 ]. The subunits from Ar.

fulgidus APSR have the same three domains in each subunit as the D. gigas APSR.
In the

ʱ and ʲ

ʲ subunit from Ar. fulgidus APSR, the redox potential for cluster I [4Fe-4S] is

60 mV while that for cluster II [4Fe-4S] is -520 mV [ 82 ]. The C-terminus of the
D. gigas APSR ʲ subunit is longer than the C-terminus of Ar. fulgidus APSR.