Investigating the inhibitor and substrate diversity of the JmjC histone demethylases

Epigenetic control of gene expression by histone post-translational modifications (PTMs) is a complex process regulated by proteins that can 'read', 'write' or 'erase' these PTMs. The histone lysine demethylase (KDM) family of epigenetic enzymes remove methyl modifications from lysines on histone tails. The Jumonji C domain (JmjC) family is the largest family of KDMs. Investigating the scope and mechanisms of the JmjC KDMs is of interest for understanding the diverse functions of the JmjC KDMs in vivo, as well as for the application of the basic science to medicinal chemistry design. The work described in this thesis aimed to biochemically investigate the inhibitor and substrate diversity of the JmjC KDMs, it led to the identification of new inhibitors and substrates and revealed a potential combinatorial dependence between adjacent histone PTMs. Structure–activity relationship studies gave rise to an n-octyl ester form of IOX1 with improved cellular potency and selectivity towards the KDM4 subfamily. This compound should find utility as a basis for the development of JmjC inhibitors and as a tool compound for biological studies. The rest of this thesis focused on the biochemical investigations of potential substrates and inhibitors for KDM3A, a JmjC demethylase with varied physiological functions. Kinetic characterisation of reported KDM3A substrates was used as the basis for evaluations of novel substrates and inhibitors. Further studies found TCA cycle intermediates to be moderate co-substrate competitive inhibitors of KDM3A. Biochemical investigations were carried out to study potential protein-protein interactions of KDM3A with intraflagellar transport proteins (IFTs), non-histone proteins involved in the formation of sperm flagellum. Work then addressed the exploration of novel in vitro substrates for KDM3 (KDM3A and JMJD1C) mediated catalysis, including: methylated arginines, lysine analogues, acetylated and formylated lysines. KDM3A, and other JmjC KDMs, were found to catalyse novel arginine demethylation reaction in vitro. Knowledge gained from studies with unnatural lysine analogues was utilised to search for additional novel PTM substrates for KDM3A. These results constitute the first evidence of JmjC KDM catalysed hydroxylation of an Nε-acetyllysine residue. The H3 K4me3 position seems to be required for acetyllysine substrate recognition, implying a combinatorial effect between PTMs. Preliminary results provide evidence that JMJD1C, a KDM3 protein previously reported to be inactive, may catalyse deacetylation in vitro. An additional novel reaction, observed with both KDM3A and JMJD1C, is deformylation of NƐ-formyllysine residues on histone H3 fragment peptides. Interestingly, H3 K4 methylation was also observed to enhance the apparent deformylation of both KDM3A and JMJD1C catalysed reactions. Overall, findings in this thesis suggest that the catalytic activity of JmjC KDMs extends beyond lysine demethylation. In a cellular context, members of the KDM3 subfamily might provide a regulatory link between methylation and acylation marks. Such a link will further highlight the complex relationships between histone PTMs and the epigenetic enzymes that regulate them. The observed dependency of H3 K9 catalysis on H3 K4 methylation adds another layer of complexity to the epigenetic regulation by histone PTMs.