In vitro C132-dealkoxycarbonylations of zinc chlorophyll a derivatives including C132-substitutes by a BciC enzyme

In Vitro Hydrolysis of Zinc Chlorophyllide a Homologues by a BciC Enzyme

“…The insertion of a methylene moiety increases the average distance between the central zinc atom and the carbonyl carbon atom in the C13 2 substituent and alters the configuration of the carbonyl group in the complex, thereby retarding the BciC enzymatic reaction. Consistently, the substitution of the methyl ester in zinc Chlide a with ethyl ester as one of the isomers of Zn-1a did not affect the BciC reaction, confirming the above explanation.…”

“…Both of the reactions are essentially similar, but their mechanisms must be different: the former could not be inhibited by additional methanol (≤1 M), but the latter was suppressed at the same concentration (vide supra). Additionally, during the in vitro BciC-catalyzed reaction of the natural substrate bearing the 13 2 -COOCH 3 , only the demethoxycarbonylated product has been observed, but no hydrolyzed product possessing the 13 2 -COOH could not be detected. ,,, To confirm whether the BciC enzyme catalyzed the conversion of 13 2 -COOH to 13 2 -H, the putative intermediate, the C13 2 -carboxylated compound was prepared as follows.…”

Predicted Structure of the BciC Enzyme Catalyzing the Removal of the C132-Methoxycarbonyl Group for Biosynthesis of Chlorosomal Chlorophylls: A Mechanism for Dual Catalytic Functions of Hydrolysis and Decarboxylation inside Its Active Site

“…The examined substrate was not a magnesium complex bearing the C17 2 -COOH group (Chlide- a ), but it was a zinc complex possessing the C17 2 -COOCH 3 group, because Chlide- a was more chemically labile during handling than Zn-Me-Pheide- a (Figure S5). Previously, we reported that the C13 2 -methoxycarbonyl group of the zinc complex bearing a methyl ester in the C17-propionate residue was removed by the enzymatic action of BciC. , It is noted that Zn-Me-Pheide- a was used as a model substrate for the BciC enzymatic reaction, which maintains the same E ring functional groups as in Chlide- a .…”

“…Previously, we reported that the C13 2 -methoxycarbonyl group of the zinc complex bearing a methyl ester in the C17-propionate residue was removed by the enzymatic action of BciC. 44,45 It is noted that Zn-Me-Pheide-a was used as a model substrate for the BciC enzymatic reaction, which maintains the same E ring functional groups as in Chlide-a.…”

“…If the latter release-and-capture reaction occurred, some C13 2 -COOH intermediates should have been detected in previous reports, in which in vitro BciC-catalyzed reactions were performed with various Chl derivatives (Figure S18). ,,, However, no C13 2 -COOH intermediate bearing the natural E ring had been detected, except in the BciC enzymatic reactions of Chl analogues possessing the 13 1 -hydroxy, 13 2 -methylene, or 13 2 -ethylene group (Figure S1). , Hence, the former mechanism is preferable where the C13 2 -COOH product was decarboxylated through the conformational rotation inside the active site of the BciC enzyme.…”