The response to hypoxia is primarily mediated by the hypoxia-inducible transcription factor (HIF). Levels of HIF are regulated by the oxygen-sensing HIF hydroxylases, members of the 2-oxoglutarate (2OG) dependent oxygenase family. JmjC-domain containing histone lysine demethylases (JmjC-KDMs), also members of the 2OG oxygenase family, are key epigenetic regulators that modulate the methylation levels of histone tails. Kinetic studies of the JmjC-KDMs indicate they could also act in an oxygen-sensitive manner. This may have important implications for epigenetic regulation in hypoxia. In this review we examine evidence that the levels and activity of JmjC-KDMs are sensitive to oxygen availability, and consider how this may influence their roles in early development and hypoxic disease states including cancer and cardiovascular disease.

In higher plants, molecular responses to exogenous hypoxia are driven by group VII ethylene response factors (ERF-VIIs). These transcriptional regulators accumulate in the nucleus under hypoxia to activate anaerobic genes but are destabilized in normoxic conditions through the action of oxygen-sensing plant cysteine oxidases (PCOs). The PCOs catalyze the reaction of oxygen with the conserved N-terminal cysteine of ERF-VIIs to form cysteine sulfinic acid, triggering degradation via the Cys/Arg branch of the N-degron pathway. The PCOs are therefore a vital component of the plant oxygen signaling system, connecting environmental stimulus with cellular and physiological response. Rational manipulation of PCO activity could regulate ERF-VII levels and improve flood tolerance, but requires detailed structural information. We report crystal structures of the constitutively expressed PCO4 and PCO5 from Arabidopsis thaliana to 1.24 and 1.91 Å resolution, respectively. The structures reveal that the PCOs comprise a cupin-like scaffold, which supports a central metal cofactor coordinated by three histidines. While this overall structure is consistent with other thiol dioxygenases, closer inspection of the active site indicates that other catalytic features are not conserved, suggesting that the PCOs may use divergent mechanisms to oxidize their substrates. Conservative substitution of two active site residues had dramatic effects on PCO4 function both in vitro and in vivo, through yeast and plant complementation assays. Collectively, our data identify key structural elements that are required for PCO activity and provide a platform for engineering crops with improved hypoxia tolerance.

The JmjC histone lysine demethylases (KDMs) are epigenetic regulators involved in the removal of methyl groups from post-translationally modified lysyl residues within histone tails, modulating gene transcription. These enzymes require molecular oxygen for catalytic activity and, as 2-oxoglutarate (2OG)-dependent oxygenases, are related to the cellular oxygen sensing HIF hydroxylases PHD2 and FIH. Recent studies have indicated that the activity of some KDMs, including the pseudogene-encoded KDM4E, may be sensitive to changing oxygen concentrations. Here, we report detailed analysis of the effect of oxygen availability on the activity of the KDM4 subfamily member KDM4A, importantly demonstrating a high level of O2 sensitivity both with isolated protein and in cells. Kinetic analysis of the recombinant enzyme revealed a high KMapp(O2) of 173 ± 23 μM, indicating that the activity of the enzyme is able to respond sensitively to a reduction in oxygen concentration. Furthermore, immunofluorescence experiments in U2OS cells conditionally overexpressing KDM4A showed that the cellular activity of KDM4A against its primary substrate, H3K9me3, displayed a graded response to depleting oxygen concentrations in line with the data obtained using isolated protein. These results suggest that KDM4A possesses the potential to act as an oxygen sensor in the context of chromatin modifications, with possible implications for epigenetic regulation in hypoxic disease states. Importantly, this correlation between the oxygen sensitivity of the catalytic activity of KDM4A in biochemical and cellular assays demonstrates the utility of biochemical studies in understanding the factors contributing to the diverse biological functions and varied activity of the 2OG oxygenases.