(4-Hydroxyphenyl)pyruvate
dioxygenase (HPPD) is an alpha-keto-
acid-dependent
dioxygenase which catalyzes the conversion of (4-hydroxyphenyl)pyruvate (
HPP) to homogentisate as part of
tyrosine catabolism. While several di- and tri-
ketone alkaloids are known as inhibitors of HPPD and used commercially as
herbicides, one such inhibitor, [2-nitro-4-(trifluoromethyl)benzoyl]-1,3-cyclohexanedione (NTBC), has also been used therapeutically to treat
type I tyrosinemia and
alkaptonuria in humans. To gain further insight into the mechanism of inhibition by NTBC, a combination of CD/MCD spectroscopy and DFT calculations of HPPD/Fe(II)/NTBC has been performed to evaluate the contribution of the Fe(II)-NTBC bonding interaction to the high affinity of this
drug for the
enzyme. The results indicate that the bonding of NTBC to Fe(II) is very similar to that for
HPP, both involving similar pi-backbonding interactions between NTBC/
HPP and Fe(II). Combined with the result that the calculated binding energy of NTBC is, in fact, approximately 3 kcal/mol less than that for
HPP, the bidentate coordination of NTBC to Fe(II) is not solely responsible for its extremely high affinity for the
enzyme. Thus, the pi-stacking interactions between the aromatic rings of NTBC and two phenyalanine residues, as observed in the crystallography of the HPPD/Fe(II)/NTBC complex, appear to be responsible for the observed high affinity of
drug binding.