Glucosyltransferases catalyze glucosylation by transferring glucose from UDP-activated sugar donor to the acceptor substrates. This research is focused on the study of the effect of a single point mutation on enzyme activity, characterization of a flavonol specific 3-Oglucosyltransferase (Cp-3-O-GT) mutant- P145T, and further modification of the clone to cleave off tags from recombinant wild type and P145T mutant proteins in order to crystallize the proteins. Multiple sequence alignment and homology modeling was done to identify candidate residues for mutation. Cp-3-O-GT was modeled with a flavonoid 3-O-GT from Vitis vinifera (VvGT) that can glucosylate both flavonols and anthocyanidins. We identified a proline residue at position 145 of Cp-3-O-GT that corresponded to a threonine residue in VvGT and designed a Cp-3-O-GT- P145T mutant to test the hypothesis that that mutation of proline by threonine in Cp-3-O-GT could alter substrate or regiospecificity of Cp-3-O-GT. While the mutant P145T enzyme did not glucosylate anthocyanidins, it did glucosylate flavanones and flavones in addition to flavonols. This is significant because flavanones and flavones do not contain a 3-OH group. HPLC was performed to identify the reaction products. Early results indicated that the mutant protein glucosylates naringenin at the 7-OH position forming prunin. Results are being used to revisit and refine the structure model. In other related work, a thrombin cleavage site was inserted into wild type and recombinant P145Tenzyme and we are currently working on transformation into yeast for recombinant protein expression. Cleaving off tags is a pre-requisite to future efforts to crystallize the proteins. Solving the crustal structures will make a significant contribution to the structural and functional study of plant flavonoid GTs in general and Cp-3-O-GT in particular.