The series of reactions leading from protoporphyrin IX to protochlorophyllide have been studied over the last 15 years in the authors' laboratories at Davis and Clemson. Here, two crucial steps are emphasized, the discovery of the ATP requirement for Mg2+ chelation, and the oxidative cyclization of Mg-protoporphyrin IX monomethyl ester to protochlorophyllide. The in vitro systems for the chelation of Mg2+ and for the oxidative cyclization of Mg-protoporphyrin IX monomethyl ester both require membrane-associated and soluble heat-labile components. We speculate about the enzymological mechanisms of these important reactions, their sub-plastidic localization and the relationship of these individual steps to the broader questions of chloroplast and cell development.
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