The duality of LysU, a catalyst for both Ap4A and Ap3A formation

Heat shock inducible lysyl-tRNA synthetase of Escherichia coli (LysU) is known to be a highly efficient diadenosine 5′,5‴-P 1,P4-tetraphosphate (Ap4A) synthase. However, we use an ion-exchange HPLC technique to demonstrate that active LysU mixtures actually have a dual catalytic activity, initially producing Ap4A from ATP, before converting that tetraphosphate to a triphosphate. LysU appears to be an effective diadenosine 5′,5‴-P1,P 3-triphosphate (Ap3A) synthase. Mechanistic investigations reveal that Ap3A formation requires: (a) that the second step of Ap4A formation is slightly reversible, thereby leading to a modest reappearance of adenylate intermediate; and (b) that phosphate is present to trap the intermediate (either as inorganic phosphate, as added ADP, or as ADP generated in situ from inorganic phosphate). Ap3A forms readily from Ap4A in the presence of such phosphate-based adenylate traps (via a 'reverse-trap' mechanism). LysU is also clearly demonstrated to exist in a phosphorylated state that is more physically robust as a catalyst of Ap 4A formation than the nonphosphorylated state. However, phosphorylated LysU shows only marginally improved catalytic efficiency. We note that Ap3A effects have barely been studied in prokaryotic organisms. By contrast, there is a body of literature that describes Ap3A and Ap4A having substantially different functions in eukaryotic cells. Our data suggest that Ap3A and Ap4A biosynthesis could be linked together through a single prokaryotic dual 'synthase' enzyme. Therefore, in our view there is a need for new research into the effects and impact of Ap3A alone and the intracellular [Ap3A]/[Ap4A] ratio on prokaryotic organisms. © 2006 The Authors.link_to_OA_fulltex

The discovery and development of Eg5 inhibitors for the clinic

The mitotic kinesin Eg5 (also known as kinesin spindle protein, KSP, Kif11, a member of the kinesin-5 family) represents an attractive oncology drug target in the ongoing development of anti-mitotic drugs that selectively block mitosis through disruption to the mitotic spindle. In this state-of-the-art review, we outline the progress that has been made in the development of Eg5 inhibitors for clinical use. We evaluate the preclinical development and attributes of key Eg5 inhibitors that have undergone clinical evaluation or extensive preclinical optimisation, and discuss the medicinal chemistry strategies utilised in their design to overcome the challenges encountered during lead optimisation. We critically analyse the progress that has been made towards delivering clinical benefits, and the wider implications this has in the utility of mitotic kinesin inhibitors as prospective oncology drugs