Site-directed alteration of four active-site residues of a pyruvoyl-dependent histidine decarboxylase

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Abstract
Site-directed mutagenesis has been used to examine the chemical roles of four active-site residues in histidine decarboxylase (HDC) from Lactobacillus 30a. This protein is known to undergo an autoactivation in which chain cleavage between serines-81 and -82 leads to cofactor (pyruvoyl) formation at position 82. Conversion of Ser-81 to Ala virtually eliminates productive cleavage. It is proposed that the residue plays a key role in stabilizing the transition state of the chain cleavage reaction. Conversion of Phe-83 to Met renders the proenzyme thermally less stable than wild type and appears to slightly increase the rate of autoactivation. The Km value for histidine is increased about 8-fold, confirming crystallographic evidence that Phe-83 is involved in substrate binding. Both wild-type and F83M enzymes show constant Km and steadily increasing kcat values as a function of temperature. Lys-155 and Tyr-262, by virtue of their positions in the active site of HDC, have been proposed to possibly play specific roles in either autoactivation or catalysis by active HDC. Conversion to Gln and Phe respectively suggests that these residues have real but minor roles in those processes.

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