Vitexin Inhibits Polyubiquitin Synthesis by the Ubiquitin-conjugating Enzyme E2-25K

An extract of bark from the tropical rainforest plant Byrsonima crassifolia was screened for inhibition of diubiquitin formation by the human ubiquitin-conjugating enzyme E2-25K. Activity assays with both the full-length enzyme and a truncated, active catalytic UBC domain revealed that the extract contained inhibitory properties. Separation of the extract into individual components and additional screens identified vitexin as the active inhibitor. An IC₅₀ for vitexin was calculated to be approximately 0.5 mM. Molecular modeling simulations were used to predict the mode of inhibition and NMR spectra were used to confirm the binding site of vitexin to E2-25K.