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 247

SO 2 4 2H þ þ G 0' þ ATP ! APS þ PP i ʔ ¼ þ 46 : 0kJ = mol 41 ½ ð 6 Þ

The sulfate activation reaction is thermodynamically unfavorable and depletion
of end products (APS and PP i ) is required to favor APS production. Hydrolysis of
PP i by a cytoplasmic inorganic pyrophosphatase (EC 3.6.1.1) producing inorganic
phosphate, enhances the production of APS, reaction ( 7 ).

G 0'

PP i þ H 2 O ! 2P i ʔ ¼ 21 : 9kJ = mol 41 ½ ð 7 Þ

Metal ions (Zn 2+ ,Mn 2+ ,Co 2+ ,Mg 2+ ) are known to stabilize inorganic
pyrophosphatases of SRB with greatest activity observed with Mg 2+ [ 61 , 62 ].
SRB use APS reductase to catalyze the production of HSO 3 from APS. The

reaction of APS/AMP +HSO 3 is slightly exergonic with E 0'

¼

60 mV [ 41 ]. Pro-
duction of H 2 S from APS is a two step process. APS reductase catalyzes the two
electron reduction of APS to hydrogen sulfite (reaction 8 )[ 41 ] which is followed by
the six electron reduction converting hydrogen sulfite to sulfide by dissimilatory
sulfite reductase (reaction 9 ).

HSO 3 þ H þ G 0' APS þ H 2 ! AMP þ ʔ ¼ 68 : 9kJ = mol 41 ½ ð 8 Þ

HSO 3 þ HS þ G 0' 3H 2 ! 3H 2 O ʔ ¼ 171 : 7kJ = mol 41 ½ ð 9 Þ

While dissimilatory sulfate reduction proceeds, SRB also use the assimilatory
sulfate reduction pathway to synthesize the amino acid cysteine. In both sulfate
reduction pathways, sulfate is activated to APS by ATP sulfurylase.
Initially, it was proposed that a trithionate pathway was used to metabolize
intermediates produced by the dissimilatory sulfate reduction and several reports
with cell-free systems supported this [ 63 - 71 ]. However, there is no convincing
evidence for the presence of trithionate reductases in SRB, either by genome
analysis or biochemically, and thiosulfate reductases have been reported only in a
few SRB. It is probable that under general growth conditions trithionate and
thiosulfate are not produced as intermediates in dissimilatory sulfite reduction
[ 72 - 74 ].

2.2 ATP Sulfurylase

ATP sulfurylase (ATPS) is also known as adenylylsulfate pyrophosphorylase or
sulfate adenylyltransferase (EC 2.7.7.4) and is a product of the ppa gene. The
pathway utilizing ATPS is indicated in Figure 3 .
For optimal activity, ATPS from Desulfotomaculum ( Dst. ) (formerly Clostrid-
ium ) nigrificans and D. desulfuricans strain 8303 require Mg 2+ to neutralize the