Manganese-substituted carbonic anhydrase as a new peroxidase

Carbonic anhydrase is a zinc metalloenzyme that catalyzes the hydration of carbon dioxide to bicarbonate. Replacing the active-site zinc with manganese yielded manganese-substituted carbonic anhydrase (CA[Mn]), which shows peroxidase activity with a bicarbonate-dependent mechanism. In the presence of bicarbonate and hydrogen peroxide, (CA[Mn]) catalyzed the efficient oxidation of o-dianisidine with k_cat/K_M = 1.4 × 10 ⁶M^-1S^-1, which is comparable to that for horseradish peroxidase, k_cat/K_M = 57 × M ^-1 S^-1. CA[Mn] also catalyzed the moderately enantioselective epoxidation of olefins to epoxides (E = 5 for p-chlorostyrene) in the presence of an amino-alcohol buffer, such as N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES). This enantioselectivity is similar to that for natural heme-based peroxidases, but has the advantage that CA[Mn] avoids the formation of aldehyde side products. CA[Mn] degrades during the epoxidation limiting the yield of the epoxidations to <12%. Replacement of active-site residues Asn62, His64, Asn67, Gln92, or Thr200 with alanine by site-directed mutagenesis decreased the enantioselectivity demonstrating that the active site controls the enantioselectivity of the epoxidation.