Comparison of the CLH reaction and a proposed in vivo degradation pathway of chlorophyll a. A. CLH catalyzes the hydrolysis of the ester bond of chlorophyll to form chlorophyllide and phytol. B. An in vivo degradation pathway of chlorophyll a proposed by Hörtensteiner and coworkers . MCS denotes magnesium dechelating substances. PPH denotes pheophytinase.