Reaction and Mechanism Catalyzed By Chlorophyllase
Chlorophyllase catalyzes the hydrolysis of ester bond to yield chlorophyllide and phytol. It reacts via tranesterification or hydrolysis of a carboxylic ester in which its natural substrates are 13-OH-chlorophyll a, bacteriochlorophyll and chlorophyll a.
Hydrolysis of chlorophyll starts with the attack of a carbonyl group of chlorophyll by the oxygen of the hydroxyl group of the crucial serine residue of the chlorophyllase. This attack forms a tetrahedral transition state. The double bond of the attacked carbonyl reforms and the serine is then esterified to chlorophyllide. The phytol group consequently leaves the compound and replaces the serine residue on the chlorophyllase enzyme. The addition of water to the reaction cleaves the phytol off the enzyme. Next, through the reverse reaction, the oxygen on the hydroxy group from the water in the previous step attacks the carbonyl of the intermediate in order to form another tetrahedral transition state. The double bond of the carbonyl forms again and the serine residue returns to chlorophyllase and the ester of the chlorophyll is now a carboxylic acid. This product is chlorophyllide.
Chlorophyllide is then broken down to Pheophorbide A. After Pheophorbide a is formed, the poryphin ring is cleaved by Pheophorbide an oxide to form RCC causing the plant to lose its green color. RCC is then broken down into pFCC.
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