Elaboration of a General Strategy for Inhibition of myo-Inositol 1-Phosphate Synthase: Active Site Interactions of Analogues Possessing Oxidized Reaction Centers

In addition to being an intermediate along the reaction coordinate
catalyzed by myo-inositol 1-phosphate
(MIP) synthase, myo-2-inosose 1-phosphate is a 3.6 μM
competititve inhibitor of the enzyme at pH 7.2. To
probe
the importance of the oxidized reaction center in
myo-2-inosose 1-phosphate to active site interactions and
gauge the
relative contribution of the keto form to these interactions, three
analogues have been examined for MIP synthase
inhibition. 2-Deoxy-myo-inositol 1-phosphate is used to
gauge the impact of removing the oxidized reaction center
of myo-2-inosose 1-phosphate.
1-Deoxy-1-(phosphonomethyl)-myo-2-inosose, with a
methylene group substituted
for the phosphate monoester oxygen of myo-2-inosose
1-phosphate, exists in neutral aqueous solution exclusively
in
its keto form. Dihydroxyacetone phosphate provides insights into
the minimum set of structural requirements for
inhibition of MIP synthase. 2-Deoxy-myo-inositol
1-phosphate was a 170 μM competitive inhibitor at pH 7.2,
which
is a 47-fold reduction in inhibitor potency relative to
myo-2-inosose phosphate. Competitive inhibition of MIP
synthase
by 1-deoxy-1-(phosphonomethyl)-myo-2-inosose was dependent
on solution pH with inhibition constants of 6.4, 37,
and 160 μM measured, respectively, at pH 8.0, 7.2, and 6.4.
Dihydroxyacetone phosphate was a 700 μM competitive
inhibitor of MIP synthase at pH 7.2. The measured inhibition
constants do not allow MIP synthase inhibition to be
unambiguously assigned to the keto form of myo-2-inosose
1-phosphate. However, the examined analogues of
myo-2-inosose 1-phosphate do establish the importance of active site
interactions with an oxidized reaction center and
suggest that this approach may be a general strategy for inhibiting MIP
synthase.