Living on Acetylene. A Primordial Energy Source - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 32

acetaldehyde has a barrier of 14.1 kcal/mol (Figure 7 )[ 35 ]. When Liao et al. [ 35 ]
used their model to calculate the barriers for the nucleophilic attack of a water
ligand on acetylene [ 21 ] they found a barrier of 45.4 kcal/mol (compared to 43.9
reported by Vincent et al. [ 37 ]).
So far, a first shell mechanism is clearly favored in all DFT calculations.
But since the hydrophobic ring and therefore the putative binding site for acetylene
in the second shell mechanism [ 21 ] was not included in any of the models, it would
be interesting to see a calculation with this feature included.

5 Conclusions

The isolation of P. acetylenicus [ 17 ] and the purification of a first tungsten-
dependent acetylene hydratase [ 1 ] led to numerous speculations with regard to
the physiological role and origin of this enzyme. Hydrolytic transformations of
toxic compounds such as cyanides and nitriles were brought forward as one
possibility [ 17 ]. However, neither growth of P. acetylenicus nor a reaction of AH
with one of these compounds could be observed [ 1 , 17 , 22 ]. When the X-ray
structure of AH was solved [ 21 ], major structural rearrangements became obvious
in comparison to other members of the DMSO reductase family. The different
location of the access funnel and the absence of a loop region separation of the
[4Fe-4S] from the MGD cofactor in formate and nitrate reductases leads to a new
active site on the opposite face of the W ion [ 21 ]. At the end of the access funnel, a
ring of 6 bulky hydrophobic residues forms a binding pocket ideally suited for a
small hydrophobic molecule like acetylene [ 21 , 22 ].
Taking everything into account, AH appears to be an enzyme that is highly
adapted to the conversion of acetylene, thus, it might be a rather old enzyme, from
a past when acetylene was more abundant in the Earth's atmosphere [ 16 ].
With regard to its reaction mechanism, a clear statement is not yet possible.
Unfortunately, a high resolution structure of AH with acetylene or an intermediate
of the reaction bound at the active site, could not be obtained so far [ 21 , 27 ].
Computational attempts to model the reaction pathway led to new insights and a
deeper understanding of the atomic structure and possible substrate-active site
interaction on the atomic level. For example, it was possible to demonstrate why
certain potential substrates, such as ethylene, acetonitrile, and propyne do not react
with AH [ 36 ], under the assumption of a reaction pathway. However, with the
rapidly increasing computer power and shorter computing times, future calculations
will take into account important features of AHs active site including the hydro-
phobic ring and the putative binding pocket for acetylene within it.
Note that an acetylene molecule can be modeled into this pocket located directly
above the water ligand of the W ion at a distance of ~4 [ 21 , 22 ]. Once these

important features have been included into the calculations, a second shell
mechanism might become favorable, which is clearly supported by the X-ray data

Å