Mechanism and Catalytic Site Atlas

Introduction

The reaction involves a complex mechanism via a base catalysed redox transfer and a triphosphate elimination. Each of the six active sites are located at the interface of three subunits: two subunits, A and A', from one trimer and one, B, from another trimer. A Glu107 forms a salt bridge and A Met70 and A Thr106 form hydrogen bonds with the substrate. The active site zinc ion is coordinated by A His23, A His48 and A His50 as well as by a water molecule in the resting state. However when the substrate is bound a pentavalent coordination occurs with two hydroxyl groups from the substrate acting as ligands allowing the energetically unfavourable conformation of the complex to exist. The intersubunit catalytic triad consists of A Cys42, B His89 and B Asp88. It is thought that the residues of the catalytic triad activate the nucleophile A Cys42 for proton abstraction from the substrate leading to an enol intermediate which is stabilised by electrostatic interaction with Zn(II). The next step is the stereospecific protonation of the intermediate catalysed by A Glu133 to produce a 6_R stereoisomer followed by formation of a keto group. A Cys42 is now deprotonated possibly by the catalytic triad to enable abstraction of a second proton followed by the triphosphate elimination and keto-enol tautomerisation yielding the product 2-amino-4-oxo-6-7-8- dihydroneopterin.