Biomedical Engineering Reference
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HCO
3
-
CO
2
?
PCE
TCE+Cl
-
TCE
VC/ETH+Cl
-
Out
(periplasm)
H
2
2H
+
H
+
?
H
+
Membrane
LP?
Q?
“Fdh”
TCE RD
Hup
PCE RD
NAD(P)
+
?
H
2
2H
+
In
(cytoplasm)
ADP
ATP
H
2
2H
+
Hym
Vhu
ATPase
Figure 2.6. Proteins detected with high abundance in proteomics studies and other components
potentially involved in organohalide respiration in Dhc strain 195 (modified after Morris et al.,
2006
). Legend: “Fdh”, protein complex annotated as formate dehydrogenase; Hym, Vhu, and Hup
hydrogenases; PCE and TCE RDases; Q, quinones potentially involved in electron transport; LP,
low potential electron carrier potentially involved in electron transport.
menaquinone derivatives were found in themembranes of
Dhc
strains BAV1 and FL2 (White et al.,
2005
). It is not clear whether these quinones take part in electron transport in
Dhc
, and it has been
proposed that their role is to quench radicals that form in the reductive dehalogenation process
(White et al.,
2005
). Moreover, quinones carry electrons at relatively high redox potentials near
0 V, whereas evidence obtained from investigations of the PCE RDase of the PCE-to-
cis
-DCE
dechlorinator
Dehalobacter restrictus
indicates that at least one of the two electrons passed on to
the RDasemust have a redox potential below~
360mV to reduce the cobalt in the corrinoid to the
+1 oxidation state (Holliger et al.,
2003
; Schumacher et al.,
1997
). Thus, quinones are unlikely
electron carriers involved in reductive dechlorination in
Dhc
(Figure
2.6
).
The exact mechanisms by which
Dhc
obtain energy for growth and maintenance from
reductive dechlorination reactions are not understood. It is presumed that somehow the
transport of electrons from the hydrogenases to the RDases leads to the generation of a proton
motive force that drives a membrane-bound F
1
F
o
ATPase to generate ATP (Figure
2.6
).
Preliminary studies with ionophores and other inhibitors in strains 195 and CBDB1 support
such a mechanism (Jayachandran et al.,
2004
; Nijenhuis and Zinder,
2005
). Peptides from an
F
1
F
o
ATPase are readily detected in membrane preparations from strain 195 (Morris et al.,
2006
), but this enzyme occurs in essentially all organisms and its presence does not prove any
particular mechanism for energy conservation.
The biochemistry of the reductive dechlorination reaction is not fully understood and the
majority of information was generated with the PCE RDases of
Sulfurospirillum multivorans
and
Dehalobacter restrictus
(Holliger et al.,
2003
). PceA of
Sulfurospirillum multivorans
contains a
modified cobalamin called norpseudo-B
12
(Kr¨utler et al.,
2003
). The requirement of
Dhc
for
vitamin B
12
for dechlorination and growth, preliminary
in vitro
biochemical data (i.e., the require-
ment for a low potential electron donor and reversible inhibition of dechlorination in cell extracts
by alkyl iodides), and the presence of corrinoid-binding motifs in some RDases suggest that
Dhc
RDases also contain a cobalamin cofactor (Adrian et al.,
2007b
;H¨lscheretal.,
2004
; Rosner et al.,
1997
). Unfortunately, the structure of these
Dhc
cobalamin cofactor(s) has not been resolved.
Since only low potential electron donors drive reductive dechlorination, the most plausible
pathway involves a Co(I) species in catalysis and the intermediate formation of a radical anion
(Banerjee and Ragsdale,
2003
). Mechanistic studies of reductive dechlorination have mainly
focused on chlorinated alkenes (Banerjee and Ragsdale,
2003
; McCauley et al.,
2005
; Schumacher
et al.,
1997
; Holliger et al.,
2003
); it remains to be seen if reductive dechlorination of chlorinated
alkanes and aromatic compounds involve similar cofactors and mechanisms.
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