Flavonol 3-O-glucosyltransferase

This enzyme is involved in the biosynthesis of secondary metabolites. The primary function of this enzyme within its pathway is binding a glucoside onto a flavonol molecule, forming a flavonol 3-O-glucoside.[5] It is through this mechanism that the enzyme converts anthocyanidins to anthocyanins as a part of the phenylpropanoid pathway. One specific example would be this enzymes actions on pelargonidin. Flavonol 3-O-glucosyltransferase binds the glucoside to this protein, making pelargonidin 3-O-glucoside. This enzyme is also involved in the flavone glycoside pathway, and daphnetin modification in some organisms. The role of the enzyme in these pathways is, again, to bind a glucoside to the substrate to construct a flavonol 3-O-glucoside.[6]

The structure of pelargonidin.

The structure of pelargonidin 3-O-glucoside.

This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is UDP-glucose:flavonol 3-O-D-glucosyltransferase. Other names in common use include:

• GTI,
• uridine diphosphoglucose-flavonol 3-O-glucosyltransferase,
• UDP-glucose:flavonol 3-O-glucosyltransferase, and
• UDP-glucose:flavonoid 3-O-glucosyltransferase (UFGT).

Among those, UFGT is divided into UDP-glucose: Flavonoid 3-O-glucosyltransferase (UF3GT) and UDP-glucose: Flavonoid 5-O-glucosyltransferase (UF5GT), which are responsible for the glucosylation of anthocyanins to produce stable molecules.[7]

Some of the inhibitors of this enzyme include CaCl₂, CoCl₂, Cu⁺², CuCl₂, KCl, Mg⁺², and Mn⁺².[8] The primary active site residue of this enzyme is Asp181, as determined by studies of how mutations affect enzyme capacity.[9] There are several documentations of the crystalline structure of flavonol 3-O-glucosyltransferase (2C1X, 2C1Z, and 2C9Z),[10] and, based on these renderings of the enzyme, there is only one subunit in the quaternary structure of the molecule.