A reappraisal of the mechanism of the photoenzyme protochlorophyllide reductase based on studies with the heterologously expressed protein.

It is widely believed that protochlorophyllide reductase is a flavoenzyme effecting catalysis by a radical mechanism. Here the cyanobacterial reductase has been isolated from Escherichia coli overexpressing the Synechocystis gene. The purified enzyme, while retaining full activity, has no detectable flavine. No radical derived ESR signal was observed during catalysis or on photoexcitation under non-catalytic conditions. Mechanistic implications of the findings are discussed.