Transition states of plasmodium falciparum and human orotate phosphoribosyltransferases

Yong Zhang, Minkui Luo, Vern L. Schramm

Research output: Contribution to journalArticlepeer-review

47 Scopus citations


Orotate phosphoribosyltransferases (OPRT) catalyze the formation of orotidine 5′-monophosphate (OMP) from α-D- phosphoribosylpyrophosphate (PRPP) and orotate, an essential step in the de novo biosynthesis of pyrimidines. Pyrimidine de novo biosynthesis is required in Plasmodium falciparum, and thus OPRT of the parasite (PfOPRT) is a target for antimalarial drugs. De novo biosynthesis of pyrimidines is also a feature of rapidly proliferating cancer cells. Human OPRT (HsOPRT) is therefore a target for neoplastic and autoimmune diseases. One approach to the inhibition of OPRTs is through analogues that mimic the transition states of PfOPRT and HsOPRT. The transition state structures of these OPRTs were analyzed by kinetic isotope effects (KIEs), substrate specificity, and computational chemistry. With phosphonoacetic acid (PA), an analogue of pyrophosphate, the intrinsic KIEs of [1′- 14C], [1, 3- 15N 2], [3- 15N], [1′- 3H], [2′- 3H], [4′- 3H], and [5′- 3H 2] are 1.034, 1.028, 0.997, 1.261, 1.116, 0.974, and 1.013 for PfOPRT and 1.035, 1.025, 0.993, 1.199, 1.129, 0.962, and 1.019 for HsOPRT, respectively. Transition state structures of PfOPRT and HsOPRT were determined computationally by matching the calculated and intrinsic KIEs. The enzymes form late associative D N*A N transition states with complete orotate loss and partially associative nucleophile. The C1′-O PA distances are approximately 2.1 Å at these transition states. The modest [1′- 14C] KIEs and large [1′- 3H] KIEs are characteristic of D N*A N transition states. The large [2′- 3H] KIEs indicate a ribosyl 2′-C-endoconformation at the transition states. p-Nitrophenyl β-D-ribose 5′-phosphate is a poor substrate of PfOPRT and HsOPRT but is a nanomolar inhibitor, supporting a reaction coordinate with strong leaving group activation.

Original languageEnglish (US)
Pages (from-to)4685-4694
Number of pages10
JournalJournal of the American Chemical Society
Issue number13
StatePublished - Apr 8 2009

ASJC Scopus subject areas

  • Catalysis
  • General Chemistry
  • Biochemistry
  • Colloid and Surface Chemistry


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