Environmental Engineering Reference
In-Depth Information
nucleotide changes, and this results in different enzymatic activities: TriA is a
deaminase and AtzA a chlorohydrolase. DNA shuffling studies done using both
enzymes indicated that (1) leaving group specificity was likely imparted by
residue 328, (2) each parent enzyme is fairly well optimised for their respective
reactions and (3) a few amino acid changes increased TriA activity for displace-
ment of methylthioether and methoxy substituents on the s-triazine ring,
leaving groups displaced by TrzN (Raillard et al.
2001
; Seffernick & Wackett
2001
). While the original enzyme which evolved into one hydrolysing atrazine
remains unknown, taken together, these results strongly suggest that atrazine
chlorohydrolase and melamine deaminase recently diverged from a common
ancestral amidohydrolase superfamily enzyme. These enzymes include those
essential for microbial metabolism, such as cytosine deaminase and adenosine
deaminase. Like AtzA, these two superfamily member enzymes contain a mono-
nuclear active site metal, which is required for catalysis (Seffernick et al.
2002
).
Conclusions
The metabolic versatility of micro-organisms is truly astounding! Micro-
organisms often respond to anthropogenic inputs of chemicals by evolving the
ability to use these compounds as growth substrates. However, these com-
pounds create a powerful selection pressure for the evolution of enzymes with
new activities and the assembly of these enzymes into new or existing meta-
bolic pathways. While it was previously thought that such processes occurred
over long evolutionary time scales, rapid microbial growth rates and gene
introgression, facilitated by a plastic microbial genome, has resulted in more
rapid evolution of metabolic functionality than previously thought possible.
Such is the case for microbial acquisition of the ability to degrade atrazine,
a novel compound that has been on the planet for only about 50 years. As
with most halogenated compounds, the ability to use atrazine as a growth
substrate was due to the evolution of a suitable dechlorinating enzyme,
atrazine chlorohydrolase (AtzA) or the broad triazine hydrolase TrzN. Both of
these metalloenyzmes are members of the amidohydrolase superfamily and
have commonalities both in structure and function. Since substrate hydro-
lysis by AtzA and TrzN fails to release carbon or nitrogen from their respective
preferred substrates, atrazine and ametryn, respectively, subsequent hydrolytic
steps are required to produce metabolically useful compounds. Genes encoding
for these enzymatic steps were likely initially present in separate bacteria,
which lived in a consortium capable of growing on these substrates. While
syntrophic interactions like this are pervasive among micro-organisms in
most ecosystems, they are likely metabolically and energetically inefficient,
and selection pressure from competitive micro-organisms living in the same
environment likely led to the eventual recruitment of the six enzymatic
steps required for complete atrazine mineralisation into a single bacterium.
