Electronic Journal of Biotechnology
ISSN: 0717-3458 |
© 2000 by Universidad
Católica de Valparaíso -- Chile |
Enzymes from Extremophiles:
the Structural Basis of Enzyme Stability and Activity in Extreme
Conditions
M.J. Danson*
Centre for Extremophile Research,
Department of Biology & Biochemistry
University of Bath, Bath, BA2 7AY, UK
D.W. Hough
Centre for Extremophile Research,
Department of Biology & Biochemistry
University of Bath, Bath, BA2 7AY, UK
L.C. Kurz
Department of Biochemistry
and Molecular Biophysics
Washington University School of Medicine, St. Louis, MO, USA
A.J. Mulholland
School of Chemistry
University of Bristol, Bristol, BS8 1TS, UK
*Corresponding Author
Keywords: extremophiles,
enzyme stability, protein structure.
Extremophiles are microorganisms
growing in environmental extremes of temperature (-2 to 15°C and 60-110°C),
salinity (2-5M NaCl), pH (<4 and >9), and/or pressure (>400 atmospheres).
Consequently, their cellular components are remarkably stable entities,
and they provide a unique source of hyperstable enzymes: 'Extremozymes'.
This lecture will focus on our current understanding of the structure,
function and stability of extremozymes, with particular emphasis on
their adaptation to extremes of temperature.
What is the structural
basis of enzyme stability to extreme temperatures?
The strategies used by extremozymes to achieve stability and activity
at both low and high temperatures have been investigated in our laboratory
by the determination and analysis of protein 3D-structures from organisms
spanning the biological range of temperatures. The dimeric enzyme, citrate
synthase, is currently our best-studied system, where we have determined
high-resolution structures of a series of homologues from organisms
growing optimally at 10, 37, 55, 80 and 100°C. Structural trends have
been identified, and these include both adaptations within individual
subunits and an apparent strengthening of the inter-subunit contacts.
These potential adaptations have been correlated with simulations of
protein inter-atomic forces and how these are thought to change with
temperature. Possible stabilising features have then been tested by
site-directed mutagenesis.
Does thermostability
limit catalytic activity?
We are now correlating these structural studies with investigations
of enzyme thermoactivity. At their organisms' growth temperatures, the
individual citrate synthases have similar catalytic activities, which
is surprising given that enzymatic activity increases 1.5-2.5 fold with
every 10°C rise in assay temperature, as long as thermal inactivation
is not occurring. Quantum mechanical - molecular mechanical simulations,
and kinetic and mechanistic studies of the 37°C and 55°C citrate synthases,
will be presented to address the relationship of stability and activity,
and how these two parameters might be independently manipulated.
[1] Danson, M.J. and Hough,
D.W. Structure, function and stability of enzymes from the Archaea,
Trends in Microbiology 6, 307-314, 1998.
[2] Hough, D.W. & Danson,
M.J. Extremozymes, Curr. Opinion Chem. Biol. 3, 39-46, 1999.
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