Photosynthetic Membranes of Porphyridium cruentum: An Analysis of Chlorophyll-Protein Complexes and Heme-Binding Proteins

Abstract

Three chlorophyll-protein complexes (CP I, CP III, CP IV) were electrophoretically separated from thylakoids of the eukaryotic red alga Porphyridium cruentum. CP I contained the primary photochemical reaction center of photosystem I as judged by its light-induced reversible absorbance change at 700 nanometers, by its fluorescence emission maximum at 720 nanometers (-196 degrees C), and by the molecular weight of its apoprotein (68,000 daltons). CP III and CP IV appeared to belong with photosystem II as suggested by the absence of light-reversible absorbance at 700 nanometers, by their fluorescence maximum at 690 nanometers (-196 degrees C), and by the presence of a chlorophyll-binding polypeptide with a molecular weight of about 52,000 daltons. CP IV when completely denatured had two additional polypeptides of about 40,000 and 48,000 daltons. All three chlorophyll-protein complexes contained carotenoids: the chlorophyll/carotenoid molar ratio of 15:1 for CP I, and 20:1 for CP III and CP IV. The thylakoid membranes of P. cruentum contained four cytochromes, detected by heme-dependent peroxidase activity, but there was no observed association with the electrophoretically separated chlorophyll-protein complexes.