Chapter 2 Enzymes and Energy • MHR 53Canadians in Biology
“Marvellous” Mutants:
The Enzymatic Synthesis
of OligosaccharidesDr. Stephen G. WithersUntil quite recently, scientists thought carbohydrates
played only simple biological roles as sources of energy
or as polymeric building blocks. Now many scientists
are discovering that oligosaccharides, or carbohydrates
in small clusters, can be used to recognize microbial
infection, cancer metastasis, and cellular inflammation.
Oligosaccharides also hold tremendous potential
as therapeutics for bacterial infections, inflammation,
and AIDS.
A major obstacle in glycobiology (the science of
understanding the role of carbohydrates in biological
events) has been to obtain large enough quantities of
oligosaccharides, from natural sources or by synthetic
methods, for research. The chemical synthesis of
oligosaccharides has not been economical on a large
scale, so biochemists have turned to enzymes as a
cheaper means for large-scale synthesis.Engineered Enzymes
Dr. Stephen Withers, a biochemist at the University of
British Columbia, has developed an enzyme to synthesize
oligosaccharides. Dr. Withers and his colleagues have
developed and patented “glycosynthases,” which arecreated through genetically engineered DNA. This DNA
programs an organism to produce mutant glycosynthase
enzymes that can synthesize high yields of
oligosaccharides.
Glycosynthases are mutated from glycosidase enzymes,
which normally function to hydrolyze oligosaccharides.
Under certain conditions, glycosidases can be made to
synthesize oligosaccharides, but only in low yields. The
problem with glycosidases is that they hydrolyze the
oligosaccharide product too quickly. Withers says, “We
designed glycosynthases as mutant enzymes that will not
hydrolyze the oligosaccharide product, but can still form
it. So, in essence, we shut down the hydrolysis pathway.”
Dr. Withers focusses his research on determining the
fundamental mechanisms of enzymes. “How do these
marvellous catalysts accelerate reactions by such huge
amounts?” he asks. “We really want to understand how
the enzymes do this.” Withers and his colleagues are
now trying to generate a library of glycosynthases to
synthesize a vast array of oligosaccharides.
By his own account, Withers has “always been fascinated
by science.” He grew up on a dairy farm outside the
village of Horton in Somerset, England. He traces his
interest in biological chemistry to his experiences
medicating sick animals on the family farm, and to the old
chemistry set he found in the attic of his childhood home.
He completed a Bachelor of Science degree at the
University of Bristol in the United Kingdom. He went on
to complete his doctoral degree in chemistry at both the
University of Bristol and at the Université de Paris-Sud
in Orsay, just south of Paris, France.
Withers came to Edmonton, Alberta, in 1977 for post-
doctoral studies. He says, “I came here because of the
high quality of science in Canada, and also because I
wanted to see some of the fantastic scenery, and kayak
some of the amazing white-water rivers!”
What are the long-term benefits of glycosynthase
research? Withers says it will allow “access to cheaper
oligosaccharides, which may then become viable as new
therapeutics, functional foods, and maybe even as bulk
commodities.” For example, he says there is interest in
manufacturing oligosaccharides as ingredients in infant
milk formula. Scientists have discovered that breast milk
contains certain oligosaccharides that are believed to
play a role in protecting infants against bacterial
infections. Specific oligosaccharides could also be used
as a treatment for E. colipoisoning to bind to or absorb
the toxin produced by these bacteria.