Various Metallopheophorbides as Substrates for Chlorophyll Synthetase

Pheophorbide a was prepared from a mixture o f chlorophylls a and b by differential extrac­ tion with HC1 and saponification. The insertion o f the following metal ions was investigated: Mg, Zn, Co, Cu, Ni. In the enzyme test with chlorophyll synthetase, the metallopheophorbides fall into two categories: the Mgand Zn-complexes are good substrates, the Co-, Cuand Ni-complexes are neither substrates nor competitive inhibitors for the enzyme reaction. This corresponds to two categories o f complex structures: Mgand Zn-porphyrins prefer pentacoordinate square-pyramidal structures, Co-, Cuand Ni-porphyrins prefer tetracoordinate square-planar structures. A model for substrate binding to chlorophyll synthetase is proposed.