The 6-OH group of D-inositol 1-phosphate serves as an H-bond donor in the catalytic hydrolysis of the phosphate ester by inositol monsphosphatase

Inositol monophosphatase plays a pivotal role in the biosynthesis of second ary messengers and is believed to be a target for lithium therapy. It is es tablished how a lithium ion works in inhibiting the enzyme but details of t he mechanism for the direct magnesium ion activated hydrolysis of the subst ate have been elusive. It is known that substrates require a minimal 1,2-di ol phosphate structural motif, which in D-myo-inositol 1-phosphate relates to the fragment comprising the 1-phosphate substrate analogues possessing 6 -substituents larger than the 6-hydroxy group of the substrate, for example , the 6-O-methyl analogue, are able to bind the enzyme in a congruous manne r to the substrate. It is demonstrated, however, that such compounds show n o substrate activity whatsoever. It is also shown that a 6-amino group is a ble to fulfil the role of the 6-hydroxy group of the substrate in conferrin g substrate activity and that a 6-methylamino group is similarly able to su pport catalysis. The results indicate that a 6-substituent capable of servi ng as a hydrogen-bond donor is required in the catalytic mechanism for hydr olysis. It has recently been shown that inositol is displaced from phosphor us with inversion of stereochemistry and we expect that the nucleophilic sp ecies is associated with Mg2+-1. It is proposed here that the role of the 6 -hydroxy group of the substrate is to H-bond with a water molecule or hydro xide ion located on Mg2+-2. From this analysis, it appears that the water m olecule bound to Mg2+-2 serves as a proton donor for the inositolate leavin g group in a process that stabilises the alkoxide product and retards the b ack-reaction.