Stephen G. Withers

Professor
( Joint with Biochemistry )

Office: Chemistry A333
Office Phone: (604) 822-3402
Lab(s): Chemistry A307/315/319/323/342/344
Lab Phone(s): 604-822-4626, 604-822-3161, 604-822-8847

FAX: (604) 822-8869
Email: withers@chem.ubc.ca

Curriculum Vitae: B.Sc., Bristol (1974); Ph.D., Bristol (M.L. Sinnott, 1977); Postdoctoral, Alberta (N.B. Madsen and B.D. Sykes, 1978-82); Fellow of the Chemical Institute of Canada; Merck, Sharpe and Dohme Award of the CIC (1989); Corday Morgan Medal of the Royal Society of Chemistry (1990); Rutherford Medal, Royal Society of Canada (1992), Khorana Chair of Biological Chemistry, UBC (1997), Hoffman LaRoche Award of the Canadian Society for Chemistry, 1998, Charmian Medal of the Royal Society of Chemistry, U.K., 2001, Whistler Award, International Carbohydrate Organisation, 2002. Scientific Director of the Protein Engineering Network of Centres of Excellence (PENCE)2000-05; Jacob Biely Faculty Research Prize (UBC) 2005; Director, Centre for High-Throughput Biology (CHiBi) 2008 - present; Canada Research Chair in Chemical Biology, 2008 - present.

Bioorganic: Enzyme mechanisms; glycosyl transfer mechanisms; carbohydrate chemistry; fluorinated sugars; hydrogen bonding and specificity; applications of 31P and 19F-NMR to enzymology; biological electrospray ionisation mass spectrometry; amylases and cellulases; mutagenesis; directed evolution; glycosyl transferases; polysaccharide lyases

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Research/Teaching Interests

The primary research interests of this laboratory centre upon the structures and mechanisms of enzymes which catalyse glycoside hydrolysis and other glycosyl transfers. These include glucosidases, galactosidases, amylases, cellulases and alpha-glucan phosphorylases. The approaches adopted to this work involve a combination of classical biochemistry (enzyme purification, enzyme kinetics, labeling and peptide isolation/sequencing, etc.), synthetic organic chemistry (synthesis of substrate analogues and inhibitors) and biophysical chemistry (NMR studies of enzyme-ligand complexes). More recently these approaches have been coupled with the powerful techniques of protein crystallography, site-directed mutagenesis and electrospray mass spectrometry in collaboration with members of the Biochemistry and Microbiology departments. These collaborations extend beyond UBC with our involvement in the Protein Engineering Centre of Excellence.

Deoxy- and deoxyfluoro-sugar analogues have been used to probe the specificities, mechanisms and structures of several enzymes. This has allowed the measurement of individual hydrogen bond strengths within a protein/sugar complex and has provided considerable insight into how such binding energy is used in catalysis. The high electronegativity of fluorine has been used in specific sugar analogues to trap enzymic intermediates for several of these enzymes in a mechanism-based manner. In several cases this intermediate has been characterised by ¹⁹F- NMR and X-ray crystallography and the identity of the enzymic nucleophile determined by classical protein chemistry coupled with mass spectrometry.

We are currently applying a combination of these techniques to probethe structures and mechanisms of several oligosaccharide-degrading enzymes such as human pancreatic alpha-amylase and the cellulase system of Cellulomonas fimi. In both cases X-ray crystal structures have been determined. The ultimate intent of the amylase project is to design novel inhibitors of this enzyme which might be useful in controlling starch breakdown in the gut, thus blood sugar levels. The study of cellulases is a major component of the Protein Engineering Network, and considerable progress has been made in determination of important amino acid residues at the active sites of several such enzymes, and in the engineering of their structures. Analysis of these mutants has provided valuable insights into the structures and mechanisms of these enzymes and has allowed the design of mutants of altered specificity and mechanism. An exciting recent development has been the generation of glycosynthases: mutant glycosidases that synthesise, but do not degrade oligosaccharides. The approach has been patented and we are now trying to broaden the synthetic scope through directed evolution of these catalysts.