Enzyme substrate recognition in oxygen sensing: how the HIF trap snaps.

The transcriptional activator HIF (hypoxia-inducible factor) is a focal point of biomedical research because many situations in physiology and in pathology coincide with hypoxia. The effects of HIF activation may be a facet of normal growth, as in embryonic development, they may counterbalance a disease, as seen in the stimulation of erythropoiesis in anaemia, and they may be part of the pathological processes, as exemplified by tumour angiogenesis. The oxygen-sensitive alpha-subunits of HIF are primarily regulated by the enzymatic hydroxylation that induces rapid proteasomal degradation. The HIFalpha hydroxylases belong to a superfamily of dioxygenases that require the co-substrates oxygen and 2-oxoglutarate as well as the cofactors Fe2+ and ascorbate. The regulation of enzyme turnover by the concentration of the cosubstrate oxygen constitutes the interface between tissue oxygen level and the activity of HIF. The HIFalpha prolyl hydroxylases, termed PHDs/EGLNs (prolyl hydroxylase domain proteins/EGL nine homologues), bind to a conserved Leu-Xaa-Xaa-Leu-Ala-Pro motif present in all substrates identified so far. This recognition motif is present twice in HIF1alpha, which gives rise to a NODD [N-terminal ODD (oxygen-dependent degradation domain)] containing Pro402 of HIF1alpha and a CODD (C-terminal ODD) where Pro564 is hydroxylated. PHD1/EGLN2 and PHD2/EGLN1 hydroxylate both ODDs with higher activity towards CODD, whereas PHD3/EGLN3 is specific for CODD. The reason for this behaviour has been unclear. In this issue of the Biochemical Journal, Villar and colleagues demonstrate that distinct PHD/EGLN domains, that are remote from the catalytic site, function in substrate discrimination. This elegant study improves our understanding of the interaction of the oxygen-sensing PHDs/EGLNs with their substrates, which include, but are not limited to, the HIFalpha proteins.