Functional and molecular modelling studies of two hereditary fructose intolerance-causing mutations at arginine 303 in human liver aldolase

Abstract

We have identified a novel hereditary fructose intolerance mutation in the aldolase B gene (i.e. liver aldolase) that causes an arginine-to-glutamine substitution at residue 303 (Arg303 → Gln). We previously described another mutation (Arg303 → Trp) at the same residue. We have expressed the wild-type protein and the two mutated proteins and characterized their kinetic properties. The catalytic efficiency of protein Gln303 is approx. 1/100 that of the wild-type for substrates fructose 1,6-bisphosphate and fructose 1-phosphate. The Trp303 enzyme has a catalytic efficiency approx. 1/4800 that of the wild-type for fructose 1,6-bisphosphate; no activity was detected with fructose 1-phosphate. The mutation Arg303 → Trp thus substitution impairs enzyme activity more than Arg303 → Gln. Three-dimensional models of wild-type, Trp303 and Gln303 aldolase B generated by homology-modelling techniques suggest that, because of its larger size, tryptophan exerts a greater deranging effect than glutamine on the enzyme's three-dimensional structure. Our results show that the Arg303 → Gln substitution is a novel mutation causing hereditary fructose intolerance and provide a functional demonstration that Arg303, a conserved residue in all vertebrate aldolases, has a dominant role in substrate binding during enzyme catalysis.