Oxygenases for oxygen sensing

In animals, cellular and physiological responses to oxygen level variations are regulated via the post-translational modification of the heterodimeric hypoxia-inducible transcription factor (HIF). Hydroxylation of the HIF-alpha subunit at either of two conserved prolyl residues enables binding to the von Hippel-Lindau protein (pVHL) elongin C/B complex (VCB) which targets HIF-a for degradation via the ubiquitin proteasome pathway. Hydroxylation of an asparaginyl residue in the C-terminal transcriptional activation domain of HIF-a reduces its interaction with the transcriptional coactivator p300. Thus, post-translational hydroxylation is used both to "make" (HIF-VCB) and "break" (HIF-p300) protein-protein interactions in the hypoxic response. The requirement for oxygen of the HIF prolyl and asparaginyl hydroxylases in catalysis links changes in oxygen concentration and transcription of the gene array that enables cells to adapt to hypoxia. All four identified human HIF hydroxylases are members of the Fe(II) and 2-oxoglutarate (2OG)-dependent family of oxygenases. Inhibition of HIF hydroxylases mimics the hypoxic response resulting in the upregulation of erythropoietin (EPO), vascular endothelial growth factor (VEGF), and other proteins of biomedicinal importance. We briefly review biochemical analyses on the HIF hydroxylases and discuss how their structural and mechanistic characteristics may make them suited to their oxygen-sensing role.