EFFECTS OF SITE-DIRECTED MUTAGENESIS OF CONSERVED LYS606 RESIDUE ON CATALYTIC AND REGULATORY FUNCTIONS OF MAIZE C-4-FORM PHOSPHOENOLPYRUVATE CARBOXYLASE

Lys606, one of the two highly conserved lysine residues in maize C-4-f orm phosphoenolpyruvate carboxylase (PEPC), was converted to Asn, Glu or Arg by site-directed mutagenesis. Resulted mutant enzymes expressed using pET system [Dong, L.-Y. et al, (1997) Biosci. Biotech. Biochem. 61: 545] were purified by one step procedure through nickel-chelate a ffinity chromatography to a purity of about 95%. The replacement of Ly s606 by Arg had little effect on the kinetic and allosteric properties of the resulting mutant enzyme, In contrast, the maximum velocities ( V-max) were decreased to 22% and 2% of that of wild-type PEPC upon the substitution of Lys606 by Asn and Glu, respectively, The value of S-0 .5(HCO3-) was increased 21-25 fold by the replacements, whereas the S- 0.5(Mg2+) and S-0.5(PEP) values were increased only 5-8 fold. The exte nts of activation of mutant enzymes by glucose 6-phosphate and glycine were 2 to 3-fold higher than those of wild-type enzyme, The mutant en zymes showed less sensitivity to malate inhibition, compared with the wild-type enzyme, The results suggested that the Lys606 is not obligat ory for the enzyme activity, but may be: involved in the bicarbonate-b inding and contribute somehow to the allosteric regulatory properties.