To examine the role of the distal His42 residue in the catalytic mechanism of pea cytosolic ascorbate peroxidase, two site-directed variants were prepared in which His42 was replaced with alanine (H42A) or glutamic acid (H42E). Electronic spectra of the ferric derivatives of H42A and H42E (pH , l ¼ M, °C) revealed wavelength maxima [kmax (nm): 397, 509, % 540sh, 644 (H42A); 404, 516, % 538sh, 639 (H42E)] consistent with a predominantly fiveco-ordinate high-spin iron. The specific activity of H42E for oxidation of L-ascorbate ( ± UÆmg)1) was % 30-fold lower than that of the recombinant wild-type.
