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Diabetes, Vol 49, Issue 2 195-201, Copyright © 2000 by American Diabetes Association
Study of the regulatory properties of glucokinase by site-directed mutagenesis: conversion of glucokinase to an enzyme with high affinity for glucose
MA Moukil, M Veiga-da-Cunha and E Van Schaftingen
Laboratoire de Chimie Physiologique, Christian de Duve Institute of Cellular Pathology and Universite Catholique de Louvain, Brussels, Belgium.
To identify the amino acids involved in the specific regulatory properties
of glucokinase, and particularly its low affinity for glucose, mutants of
the human islet enzyme have been prepared, in which glucokinase-specific
residues have been replaced. Two mutations increased the affinity for
glucose by twofold (K296M) and sixfold (Y214A), the latter also decreasing
the Hill coefficient from 1.75 to 1.2 with minimal change in the affinity
for ATP. Combining these two mutations with N166R resulted in a 50-fold
decrease in the half-saturating substrate concentration (S0.5) value, which
became then comparable to the Km of hexokinase II. The location of N166,
Y214, and K296 in the three-dimensional structure of glucokinase suggests
that these mutations act by favoring closure of the catalytic cleft. As a
rule, mutations changed the affinity for glucose and for the competitive
inhibitor mannoheptulose (MH) in parallel, whereas they barely affected the
affinity for N-acetylglucosamine (NAG). These and other results suggest
that NAG and MH bind to the same site but to different conformations of
glucokinase. A small reduction in the affinity for the regulatory protein
was observed with mutations of residues on the smaller domain and in the
hinge region, confirming the bipartite nature of the binding site for the
regulatory protein. The K296M mutant was found to have a threefold
decreased affinity for palmitoyl CoA; this effect was additive to that
previously observed for the E279Q mutant, indicating that the binding site
for long-chain acyl CoAs is located on the upper face of the larger domain.

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Copyright © 2000 by the American Diabetes Association.
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