The Mechanism of Photoreduction of Protochlorophyll(ide)

The protochlorophyll(ide)-protein complexes of higher plants are found in the membranes of dark grown plastids (etioplasts). Their chromophore (-protochlorophyll(ide)-) is a porphyrin. Light absorption by the chromophore results in its reduction to a chlorin (chlorophyll(ide)). This occurs in vivo in a time which does not exceed 10-4 s. Because, after light absorption, the apoprotein behaves as a catalyst, and because protochlorophyll(ide) behaves as a specific substrate for this catalyst, the reaction may be termed "photoenzymatic". The apoprotein controls the stereospecific addition of 2 hydrogen atoms, in the trans-configuration, at the double bound between carbons 7 and 8 in cycle IV of the tetrapyrrole ring (Fig. 1). The protochlorophyll(ide)-protein complex may be extracted in a suitable buffer from the etiolated leaf in a form which preserves its phtotenzymatic activity.