Cation-π Interactions Contribute to Substrate Recognition in γ-Butyrobetaine Hydroxylase Catalysis

gamma-Butyrobetaine hydroxylase (BBOX) is a nonheme Fe-II- and 2-oxoglutarate-dependent oxygenase that catalyzes the stereoselective hydroxylation of an unactivated C-H bond of gamma-butyrobetaine (gamma BB) in the final step of carnitine biosynthesis. BBOX contains an aromatic cage for the recognition of the positively charged trimethylammonium group of the gamma BB substrate. Enzyme binding and kinetic analyses on substrate analogues with P and As substituting for N in the trimethylammonium group show that the analogues are good BBOX substrates, which follow the efficiency trend N+ > P+ > As+. The results reveal that an un-charged carbon analogue of gamma BB is not a BBOX substrate, thus highlighting the importance of the energetically favorable cation-pi interactions in productive substrate recognition.