The Mg insertion step in chlorophyll biosynthesis.

A developing chloroplast preparation obtained from greening cucumber cotyledons is able to bring about the synthesis of Mg-protoporphyrin-IX and/or Mg-protoporphyrin-IX monomethyl ester. l-glutamate, δ-aminolevulinic acid, and protoporphyrin-IX can serve as precursors for Mg-protoporphyrin synthesis. However, when δ-aminolevulinic acid or protoporpyrin are used, no Mg-protoporphyrin is formed unless l-glutamate is also added. Mg-Protoporphyrin synthesis with δ-aminolevulinic acid plus l-glutamate, or proto-porphyrin plus l-glutamate, is much more active than with l-glutamate alone. Therefore, it is apparent that l-glutamate plays a role in the Mg chelation step in chloroplasts. α-Keto-glutarate can replace l-glutamate in this role; glutamine cannot. ATP is also required for Mg chelation. The role of l-glutamate in the Mg insertion step is not yet understood, except that l-glutamate itself does not need to be converted to porphyrins in this process, because Mg-protoporphyrin can be synthesized from protoporphyrin and l-glutamate even in the presence of the δ-aminolevulinic acid dehydratase inhibitor, levulinate.