Mechanistic analysis of the unusual redox-elimination sequence employed by Thermotoga maritima BglT: a 6-phospho-beta-glucosidase from glycoside hydrolase family 4.
"Classical" glycosidases utilize either direct or double-displacement mechanisms involving oxocarbenium ion-like transition states to catalyze the hydrolysis of glycosidic bonds. By contrast, the mechanism of the glycosidases in glycoside hydrolase family 4 has been recently proposed to involve NAD+-mediated redox steps along with alpha,beta-elimination and addition steps via anionic intermediates. Support for this mechanism in BglT, a 6-phospho-beta-glucosidase in family 4, has been provided through mechanistic and X-ray crystallographic analyses [Yip, V. L.Y., et al. (2004) J. Am. Chem. Soc. 126, 8354-8355] in which primary deuterium kinetic isotope effects for the hydride abstraction at C3 and for the alpha-proton abstraction at C2 indicate that these two steps are both partially rate-limiting. Current data reveal that there is no secondary deuterium kinetic isotope effect associated with the rehybridization of the C1 sp3 center to a sp2 center. Furthermore, a flat linear free energy relationship was established with a series of aryl 6-phospho-beta-D-glucosides of varying leaving group abilities. Taken together, these data indicate that cleavage of the C1-O1 linkage does not occur during a rate-limiting step. Since the deprotonation at C2 is slow and partially rate-limiting while the departure of the leaving group is not, a stepwise E1(cb)-type mechanism rather than an E1 or a concerted E2-syn mechanism is proposed. Direct evidence for the role of NAD+ was obtained by reduction in situ using NaBH4 leading to an inactive enzyme that could be reactivated by the addition of excess NAD+. This was accompanied by the expected UV-vis spectrophotometric changes.
Yip
Vivian L Y
VL
Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1.
Withers
Stephen G
SG
D002384:Catalysis; D041961:Deuterium Exchange Measurement; D017666:Deuterium Oxide; D019298:Glucose-6-Phosphate / Q000031:analogs & derivatives; D005954:Glucosephosphate Dehydrogenase / Q000737:chemistry; D005959:Glucosidases / Q000737:chemistry* / Q000378:metabolism*; D006026:Glycoside Hydrolases / Q000737:chemistry* / Q000378:metabolism*; D007700:Kinetics; D005810:Multigene Family; D009243:NAD / Q000378:metabolism; D010084:Oxidation-Reduction; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics; D020124:Thermotoga maritima / Q000201:enzymology*; D000520:alpha-Glucosidases / Q000737:chemistry
D009243:NAD; D019298:Glucose-6-Phosphate; D005954:Glucosephosphate Dehydrogenase; D005959:Glucosidases; D006026:Glycoside Hydrolases; C049150:maltose-6'-phosphate glucosidase; D000520:alpha-Glucosidases; C066586:6-phospho-beta-glucosidase; D017666:Deuterium Oxide