The Power of Two

Although the crystal structure of α-amino-β-carboxymuconate-ϵ-semialdehyde decarboxylase from Pseudomonas fluorescens was solved as a dimer, this enzyme is a mixture of monomer, dimer, and higher order structures in solution. In this work, we found that the dimeric state, not the monomeric state, is the functionally active form. Two conserved arginine residues are present in the active site: Arg-51 and an intruding Arg-239* from the neighboring subunit. In this study, they were each mutated to alanine and lysine, and all four mutants were catalytically inactive. The mutants were also incapable of accommodating pyridine-2,6-dicarboxylic acid, a competitive inhibitor of the native enzyme, suggesting that the two Arg residues are involved in substrate binding. It was also observed that the decarboxylase activity was partially recovered in a heterodimer hybridization experiment when inactive R51(A/K) and R239(A/K) mutants were mixed together. Of the 20 crystal structures obtained from mixing inactive R51A and R239A homodimers that diffracted to a resolution lower than 3.00 Å, two structures are clearly R51A/R239A heterodimers and belong to the C2 space group. They were refined to 1.80 and 2.00 Å resolutions, respectively. Four of the remaining crystals are apparently single mutants and belong to the P4 2 2 1 2 space group. In the heterodimer structures, one active site is shown to contain dual mutation of Ala-51 and Ala-239*, whereas the other contains the native Arg-51 and Arg-239* residues, identical to the wild-type structure. Thus, these observations provide the foundation for a molecular mechanism by which the oligomerization state of α-amino-β-carboxymuconate-ϵ-semialdehyde decarboxylase could regulate the enzyme activity. Background: α-Amino-β-carboxymuconate-ϵ-semialdehyde decarboxylase is a key enzyme that controls quinolinic acid levels. Results: Two arginines, including one from a neighboring subunit, are required for substrate binding. Conclusion: Dimerization and two arginine residues are required for activity. Significance: This study provides the first structurally proven example of a functionally active heterodimer hybrid resulting from a simple mixing of two inactive homodimer mutants.