A functional role for a flexible loop containing Glu182 in the class II fructose-1,6-bisphosphate aldolase from...

Class II fructose 1,6-bisphosphate aldolases (FBP-aldolases) catalyse the zinc-dependent, reversible aldol condensation of dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P) to form fructose 1,6-bisphosphate (FBP). Analysis of the structure of the enzyme from Escherichia coli in complex with a transition state analogue (phosphoglycolohydroxamate, PGH) suggested that substrate binding caused a conformational change in the beta5-alpha7 loop of the enzyme and that this caused the relocation of two glutamate residues (Glu181 and Glu182) into the proximity of the active site. Site-directed mutagenesis of these two glutamate residues (E181A and E182A) along with another active site glutamate (Glu174) was carried out and the mutant enzymes characterised using steady-state kinetics. Mutation of Glu174 (E174A) resulted in an enzyme which was severely crippled in catalysis, in agreement with its position as a zinc ligand in the enzyme's structure. The E181A mutant showed the same properties as the wild-type enzyme indicating that the residue played no major role in substrate binding or enzyme catalysis. In contrast, mutation of Glu182 (E182A) demonstrated that Glu182 is important in the catalytic cycle of the enzyme. Furthermore, the measurement of deuterium kinetic isotope effects using [1(S)-(2)H]DHAP showed that, for the wild-type enzyme, proton abstraction was not the rate determining step, whereas in the case of the E182A mutant this step had become rate limiting, providing evidence for the role of Glu182 in abstraction of the C1 proton from DHAP in the condensation direction of the reaction. Glu182 lies in a loop of polypeptide which contains four glycine residues (Gly176, Gly179, Gly180 and Gly184) and a quadruple mutant (where each glycine was converted to alanine) showed that flexibility of this loop was important for the correct functioning of the enzyme, probably to change the microenvironment of Glu182 in order to perturb its pK(a) to a value suitable for its role in proton abstraction. These results highlight the need for further studies of the dynamics of the enzyme in order to fully understand the complexities of loop closure and catalysis in this enzyme.