Members of the DRE-TIM metallolyase superfamily rely on an active-site divalent cation to catalyze various reactions involving the making and breaking of carbon-carbon bonds. While the identity of the metal varies, the binding site is well-conserved at the superfamily level with an aspartic acid and two histidine residues acting as ligands to the metal. Previous structural and bioinformatics results indicate that the metal can adopt an alternate architecture through the addition of an asparagine residue as a fourth ligand. This asparagine residue is strictly conserved in all members of the DRE-TIM metallolyase superfamily except fungal homocitrate synthase (HCS-lys) where it is replaced with isoleucine. The role of this additional metal ligand in α-isopropylmalate synthase from Mycobacterium tuberculosis (MtIPMS) has been investigated using site-directed mutagenesis. Substitution of the asparagine ligand with alanine or isoleucine results in inactive enzymes with respect to α-isopropylmalate formation. Control experiments suggest that the substitutions have not drastically affected the enzyme's structure indicating that the asparagine residue is essential for catalysis. Interestingly, all enzyme variants retained acetyl CoA hydrolysis activity in the absence of α-ketoisovalerate, similar to the wild-type enzyme. In contrast to the requirement of magnesium for α-isopropylmalate formation, hydrolytic activity could be inhibited by the addition of magnesium chloride in wild-type, D81E, and N321A MtIPMS, but not in the other variants studied. Attempts to rescue loss of activity in N321I MtIPMS by mimicking the fungal HCS active site through the D81E/N321I double variant were unsuccessful. This suggests epistatic constraints in evolution of function in IPMS and HCS-lys enzymes.
Keywords: DRE-TIM metallolyase superfamily; Enzyme catalysis; Isopropylmalate synthase; Metalloenzyme.
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