Molecular Mechanisms of Oxygen Sensing
All aerobic organisms must balance oxygen supply and demand, and therefore need response systems when oxygen availability drops (hypoxia) to allow adaptation to hypoxic conditions. These adaptations might include mechanisms to deliver more oxygen, or conversely metabolic reconfiguration to use less oxygen. In plants and animals, these responses are mediated by transcription factors that upregulate genes to enable the hypoxic response, and these transcription factors are in turn regulated by oxygen-dependent enzymes. Thus under normal oxygen conditions, the enzymes catalyse post-translational modification of the transcription factors targetting them for degradation by the proteasome, while in hypoxia the enzymes lose catalytic activity and the transcription factors are stabilised to elicit the hypoxic response.
In animals the master regulator of the hypoxic response is the Hypoxia-Inducible Factor, HIF, whose levels and activity are controlled by the catalytic activity of HIF hydroxylases. These are Fe(II) and 2-oxoglutarate dependent dioxygenases whose activity is particularly sensitive to oxygen availability. Our work has focussed on understanding how the HIF hydroxylases are tailored at the molecular level to act as such efficient oxygen sensors. We are also interested in whether other Fe(II) and 2-oxoglutarate dependent dioxygenases, e.g. histone demethylases, could act in an oxygen-sensitive manner; this would have implications for their activity in hypoxic disease states such as cancer or ischaemic diseases.
In plants the hypoxic response is mediated by Group VII Ethylene Response Factors (ERF-VIIs), whose levels are regulated by the catalytic activity of Plant Cysteine Oxidases. These enzymes are not as well-characterised as the HIF hydroxylases, but are homologous to Fe(II)-dependent mammalian and bacterial cysteine dioxygenases. Supported by a BBSRC grant, we are investigating the structural, functional and kinetic characteristics of these enzymes to determine their oxygen sensing characteristics, but also whether their activity could be modulated to artificially elevate ERF-VII levels. This has been shown in some plants to enhance flood tolerance.