Living on Acetylene. A Primordial Energy Source - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 16

transfer. The acetylene hydratase from the strictly anaerobic bacterium Pelobacter
acetylenicus is so far the only known and characterized acetylene hydratase. With a
crystal structure solved at 1.26 Å resolution and several amino acids around the
active site exchanged by site-directed mutagenesis, many key features have been
explored to understand the function of this novel tungsten enzyme. However, the
exact reaction mechanism remains unsolved. Trapped in the reduced W IV state, the
active site consists of an octahedrally coordinated tungsten ion with a tightly bound
water molecule. An aspartate residue in close proximity, forming a short hydrogen
bond to the water molecule, was shown to be essential for enzyme activity.
The arrangement is completed by a small hydrophobic pocket at the end of an
access funnel that is distinct from all other enzymes of the DMSO reductase family.

Keywords acetylene ￿ hydration ￿ iron sulfur ￿ tungsten

Please cite as: Met. Ions Life Sci . 14 (2014) 15-35

1 Introduction

Acetylene (C 2 H 2 , IUPAC name ethyne) is only a minor trace gas in the composition
of the Earth's atmosphere, but notably it can be used as carbon and energy source by
several bacteria. One of these, Pelobacter acetylenicus , was isolated by Schink
in 1985 and the acetylene-converting enzyme of P. acetylenicus , the acetylene
hydratase (AH; EC 4.2.1.112), has been studied in great detail over the years.
AH is a hydrolyase, that catalyzes the addition of one molecule of water to the
C C triple bond of acetylene-forming acetaldehyde. Therefore, the conversion

of acetylene by AH is distinct from the only other known enzymatic reaction of
acetylene, the reduction of acetylene to ethylene by nitrogenase [ 1 ]. Although
the addition of a molecule of water to acetylene is formally not a redox reaction,
AH activity depends on the presence of a strong reducing agent like titanium(III)
citrate or sodium dithionite.

2 Acetylene

In biological systems, acetylene is well known as inhibitor of microbial processes
by interaction with the metal sites of several metallo enzymes, such as nitrogenase,
hydrogenase, ammonia monooxygenase, methane monooxygenase, assimilatory
nitrate reductase or nitrous oxide reductase [ 2 ]. Thus, acetylene has been employed
for the quantification of several important biological processes for a long time.
For instance, nitrogen fixation can be measured by determining the reduction of
acetylene to ethylene (C 2 H 4 ) by nitrogenase [ 3 ], and inhibition of N 2 O reductase by