Abstract
α-glucuronidases, components of an ensemble of enzymes central to the recycling of photosynthetic biomass, remove the α-1,2 linked 4- O-methyl glucuronic acid from xylans. The structure of the α-glucuronidase, GlcA67A, from Pseudomonas cellulosa reveals three domains, the central of which is a (β/α) 8 barrel housing the catalytic apparatus. Complexes of the enzyme with the individual reaction products, either xylobiose or glucuronic acid, and the ternary complex of both glucuronic acid and xylotriose reveal a “blind” pocket which selects for short decorated xylooligosaccharides substituted with the uronic acid at their nonreducing end, consistent with kinetic data. The catalytic center reveals a constellation of carboxylates; Glu292 is poised to provide protonic assistance to leaving group departure with Glu393 and Asp365 both appropriately positioned to provide base-catalyzed assistance for inverting nucleophilic attack by water.
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