Kinetic characterization and identification of a novel inhibitor of hypoxia-inducible factor prolyl hydroxylase 2 using a time-resolved fluorescence resonance energy transfer-based assay technology

The human hypoxia-inducible factor prolyl hydroxylases 1, 2, and 3 (HIF-PHD1, -2, and -3) are thought to act as proximal sensors of cellular hypoxia by virtue of their mechanism-based dependence on molecular oxygen. These 2-oxoglutarate (2-OG) and non-heme iron-dependent oxygenases constitutively hydroxylate HIF, resulting in high-affinity binding to Von Hippel-Lindau protein (pVHL). Some reported affinities for the HIF-PHDs for 2-OG and iron approach the estimated physiological concentrations for these cofactors, suggesting that the system as described is not catalytically optimal. Here we report the enzymatic characterization of full-length recombinant human HIF-PHD2 using a novel and sensitive catalytic assay. We demonstrated submicromolar affinities for 2-OG and ferrous iron and HIF-PHD2 K_m values for oxygen that are greater than atmospheric oxygen levels, suggesting that molecular oxygen is indeed the key regulator of this pathway. In addition, we observed enhancement of HIF-PHD2 catalytic activity in the presence of ascorbic acid with only minor modifications of HIF-PHD2 requirements for 2-OG, and a detailed pH study demonstrated optimal HIF-PHD2 catalytic activity at pH 6.0. Lastly, we used this sensitive and facile assay to rapidly perform a large high-throughput screen of a chemical library to successfully identify and characterize novel 2-OG competitive inhibitors of HIF-PHD2.