Flavonoid glycosyltransferases (GTs), enzymes integral to plant ecological responses and human pharmacology, necessitate rigorous structural elucidation to decipher their mechanistic function and substrate specificity, particularly given their role in the biotransformation of diverse pharmacological agents and natural products. This investigation delved into a comprehensive exploration of the flavonol 3-O GT from Citrus paradisi (Cp3GT), scrutinizing the impact of a c-terminal c-myc/6x histidine tag on its enzymatic activity and substrate specificity, and successfully achieving its purification to apparent homogeneity. This established a strong foundation for potential future crystallographic and other structure/function analyses. Through the strategic implementation of site-directed mutagenesis, a thrombin cleavage site was incorporated proximal to the tag, followed by cloning in Pichia pastoris, methanol-induced expression, and cobalt-affinity chromatography for initial purification stages. Notably, the recombinant tags did not exhibit a discernible influence on Cp3GT kinetics, substrate preference, pH optima, or metal interactions, maintaining its specificity towards flavonols at the 3-OH position and favoring glucosylation of quercetin and kaempferol. Subsequent purification steps, including MonoQ anion exchange and size-exclusion chromatography, yielded Cp3GT with ≥95% homogeneity. In silico molecular models of Cp3GT and its truncated variants, Cp3GTΔ80 and Cp3GTΔ10, were constructed using D-I-TASSER and COFACTOR to assess binding interactions with quercetin and kaempferol. Results indicated minimal interference of c-myc/6x-his tags with the native Cp3GT structure. This study not only lays a foundation for impending crystallographic studies, aiming to solidify the understanding of Cp3GT's stringent 3-O flavonol specificity, but also accentuates the potential of microbial expression platforms and plant metabolic engineering in producing beneficial compounds. To this end, a thorough review of four pivotal classes of plant secondary metabolites, flavonoids, alkaloids, betalains, and glucosinolates, was conducted. This will open avenues for further research and applications in biotechnological, medical, and agricultural domains.

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