CODH/ACS (carbon monoxide dehydrogenase/acetyl CoA synthase) An alternative to carbon fixation by the Calvin cycle is the Wood‐Ljungdahl (or acetyl CoA) pathway, adopted by acetogenic bacteria: convergent synthesis of acetyl CoA from CO2 via CO and a formate/methyltetrahydrofolate path. Methanogenic archaea reverse this reaction, turning acetyl CoA into CO2 and CH4. The reactions are catalyzed by a bifunctional enzyme: CODH/ACS CODH: CO2 + 2H+ + 2e– CO + H2O ACS: HSCoA + MeCoIII(corrin) + CO CH3COSCoA + CoI(corrin) + H+ α
β
C
A B
D
B
A β
C
• 310 kDa α2β2 heterodimeric protein
• β subunits spanned by 3 [4Fe4S], connecting novel [4Fe4SNi] (C clusters), sites of CODH activity
• αs house [4Fe4S]Ni2 (A clusters), sites of ACS activity
• hydrophobic tunnels connect A and C for CO transport within protein (mapped out with Xe!)
C C
B B
D α
CODH mechanism CO2 + 2H+ + 2e– CO + H2O • Cluster C active site is bimetallic NiFe edge • CO2 binds at Ni0, reduced to formyl group bridging to Fe, H‐bonds to nearby NHis and NLys • C‐O bond cleavage generates CO and [OH–] HisNH
H3NLys
Fe
0
H3NLys
O
O C O
Ni
HisNH
HisNH
O
Ni
II
C
Fe
Ni
HisNH
H2NLys
O
II
+
H3NLys
–
OH
2H 2e
Fe
– CO, H2O
Ni0 Fe
+ CO2
ACS mechanism HSCoA + MeCoIII(corrin) + CO NH2
CoA or HSCoA (Coenzyme A)
N
N
N
CH3COSCoA + CoI(corrin) + H+ O O
N
HO
O
OPOPO
O O
H H
O
OH
III
CO CH3Co
O
N
H
SH
N
H
I
O
OPO3–
H+ Co
SCCH3
acetylCoA or CH3COSCoA
• Cluster A is [4Fe4S] bridged by SCys to 2 nickel ions bound by CysGlyCys residues O
cys
S
S Ni
[4Fe4S]
N
Ni
S
N
O
O
• conformation of α chain oscillates between “closed” (Ni exposed to CO channel) and “open” (Ni exposed to Co(corrin) protein and HSCoA) • exact mechanism ambiguous, but spectroscopic evidence suggests only one Ni ion involved in the chemistry – uncertain roles for 2nd Ni and [4Fe4S] cluster cys
S
[4Fe4S]
O
C
S
cys
CO
S
[4Fe4S]
S
S
S
Ni
CoI
N
cys
N
cys
cys
Ni0
CH3CoIII
cys
Ni0
Ni
S
[4Fe4S]
N
S
[4Fe4S]
CH3
SCoA
cys
NiII
S
O S Ni
cys
CoAS
–
N
N
N
S
[4Fe4S]
N
Ni
cys
cys
O
S
CH3
cys
S
cys
NiII
N
cys
CoASCCH 3
O
C
Ni
cys
II
S
CH3
O
S
Ni
N
N
cys