Evolution of Enzymatic Activity in the Tautomerase Superfamily: Mechanistic and Structural Consequences of the L8R Mutation in 4-Oxalocrotonate Tautomerase,

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Abstract
4-Oxalocrotonate tautomerase (4-OT) and trans-3-chloroacrylic acid dehalogenase (CaaD) are members of the tautomerase superfamily, a group of structurally homologous proteins that share a β−α−β fold and a catalytic amino-terminal proline. 4-OT, from Pseudomonas putida mt-2, catalyzes the conversion of 2-oxo-4-hexenedioate to 2-oxo-3-hexenedioate through the dienol intermediate 2-hydroxymuconate in a catabolic pathway for aromatic hydrocarbons. CaaD, from Pseudomonas pavonaceae 170, catalyzes the hydrolytic dehalogenation of trans-3-chloroacrylate in the trans-1,3-dichloropropene degradation pathway. Both reactions may involve an arginine-stabilized enediolate intermediate, a capability that may partially account for the low-level CaaD activity of 4-OT. Two active-site residues in 4-OT, Leu-8 and Ile-52, have now been mutated to the positionally conserved and catalytic ones in CaaD, αArg-8, and αGlu-52. The L8R and L8R/I52E mutants show improved CaaD activity (50- and 32-fold increases in kcat/Km, respectively) and diminished 4-OT activity (5- and 1700-fold decreases in kcat/Km, respectively). The increased efficiency of L8R-4-OT for the CaaD reaction stems primarily from an 8.8-fold increase in kcat, whereas that of the L8R/I52E mutant is due largely to a 23-fold decrease in Km. The presence of the additional arginine residue in the active site of L8R-4-OT does not alter the pKa of the Pro-1 amino group from that measured for the wild type (6.5 ± 0.1 versus 6.4 ± 0.2). Moreover, the crystal structure of L8R-4-OT is comparable to that of the wild type. Hence, the enhanced CaaD activity of L8R-4-OT is likely due to the additional arginine residue that can participate in substrate binding and/or stabilization of the putative enediolate intermediate. The results also suggest that the evolution of new functions within the tautomerase superfamily could be quite facile, requiring only a few strategically placed active-site mutations.

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