Kinetic and structural characterisation of the ubiquinol-binding site and oxygen reduction by the trypanosomal alternative oxidase

The alternative oxidase (AOX) is a monotopic di-iron carboxylate protein which acts as a terminal respiratory chain oxidase in a variety of plants, fungi and protists. Of particular importance is the finding that both emerging infectious diseases caused by human and plant fungal pathogens, the majority of which are multi-drug resistant, appear to be dependent upon AOX activity for survival. Since AOX is absent in mammalian cells, AOX is considered a viable therapeutic target for the design of specific fungicidal and anti-parasitic drugs. In this work, we have mutated conserved residues within the hydrophobic channel (R96, D100, R118, L122, L212, E215 and T219), which crystallography has indicated leads to the active site. Our data shows that all mutations result in a drastic reduction in Vmax and catalytic efficiency whilst some also affected the Km for quinol and oxygen. The extent to which mutation effects inhibitor sensitivity was also investigated, with mutation of R118 and T219 leading to a complete loss of inhibitor potency. However, only a slight reduction in IC50 values was observed when R96 was mutated, implying that this residue is less important in inhibitor binding. In silico modelling has been used to provide insight into the reason for such changes, which we suggest is due to disruptions in the proton transfer network, resulting in a reduction in overall reaction kinetics. We discuss our results in terms of the structural features of the ubiquinol binding site and consider the implications of such findings on the nature of the catalytic cycle. Significance The alternative oxidase is a ubiquinol oxidoreductase enzyme that catalyses the oxidation of ubiquinol and the reduction of oxygen to water. It is widely distributed amongst the plant, fungal and parasitic kingdoms and plays a central role in metabolism through facilitating the turnover of the TCA cycle whilst reducing ROS production.