Living on Acetylene. A Primordial Energy Source - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 27

Figure 5 Activities of AH variants relative to their W content. For better comparability, the
specific activity of the AH variants was divided by the W content in nmol. Data adapted from [ 27 ]
(see also Table 1 ).

4.7 Density Functional Theory Calculations on the Substrate
Binding Mode and Amino Acid Protonation States

Several groups directed a major computational effort towards the understanding of
the reaction mechanism of AH. Seiffert et al. [ 21 ] used density functional theory
(DFT) based atomic charge calculation to determine the titration curves of all
residues in AH. Notably, out of 34 Asp and 58 Glu residues, only three showed a
highly aberrant titration behavior: Asp298, Glu494, and Asp13. Asp13 that forms a
hydrogen bond to the water ligand of the W center stayed protonated in the pH
range 0-24, when the [4Fe-4S] cluster in close proximity was reduced. Whereas a
partially deprotonated Asp13 was found at high pH values when the [4Fe-4S]
cluster and the W center were fully oxidized [ 21 ].
Antony and Bayse [ 34 ], Vincent et al. [ 37 ], and Liao et al. [ 35 ] applied DFT
methods to calculate the energies of acetylene adduct formation and the energetic
barriers between intermediate states of several possible reaction mechanisms of
model complexes that mimic the active site of AH. Antony and Bayse [ 34 ] used
models of molybdenum- and tungsten-oxo dithiocarbamates (dtc) and dithiolates
(dtl) that have been shown to form adducts with alkynes [ 31 , 32 ] or even had
AH-like activity [ 33 ]. Furthermore, a truncated model of the active site of AH
consisting of a W ion coordinated by four dithiolene sulfur atoms, a cysteine sulfur
and a water molecule was used to study the binding of water versus acetylene [ 34 ].
In these models, the free energy difference G complex for the formation of an

acetylene metal complex favored the tungsten complexes over their molybdenum
analogues, due to a more favorable interaction of the 5d W orbitals with the

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molecule orbitals of acetylene. Additionally, the models showed that a thiolate
ligand (Cys141 in AH) made the complex formation more exergonic than an oxo
ligand in this position would do [ 34 ]. When calculating the

G values for the
substitution of the water by acetylene, the acetylene complex was favored by
~10 kcal/mol [ 34 ].

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