Photochemical and Enzymatic Synthesis of Malic Acid from Pyruvic Acid and HCO₃^- with Combination System of Zinc Chlorin-e₆ and Malic Enzyme in Aqueous Medium

Abstract

Photochemical and enzymatic synthesis of malic acid from pyruvic acid and HCO₃^- was studied with malic enzyme (ME) and NADP⁺ photoreduction with ferredoxin-NADP⁺-reductase (FNR) by visible light photo-sensitization of the chlorophyll derivative, zinc chlorin-e₆ (ZnChl-e₆), in the presence of NADH as an electron-donating reagent and methylviologen (MV²⁺) as an electron-relay reagent. NADPH was produced by irradiation of the reactant solution containing NADH, ZnChl-e₆, MV²⁺, NADP⁺ and FNR in Bis-Tris buffer (pH 8.0). After 180-min irradiation, 8.3 mmol·dm^-3 NADPH was produced under the optimum conditions with 10 mmol·dm^-3 of NADP⁺. The reduction ratio of NADP⁺ to NADPH was about 83%. Malic acid was produced by irradiation with visible light (>390 nm) of the reactant solution containing NADH, ZnChl-e₆, methylviologen (MV²⁺), pyruvic acid, NaHCO₃, NADP⁺, ferredoxin-NADP⁺-reductase (FNR) and ME. Malic acid production was 2.0 mmol·dm^-3 after 3-h irradiation under the optimum conditions with NADH, ZnChl-e₆, MV²⁺, FNR, pyruvic acid, NaHCO₃, NADP⁺ and ME. The ratio of HCO₃^- and pyruvic acid to malic acid was about 20% under the optimum conditions.