BIOGENESIS OF CHLOROPHYLL; EARLIER PRECURSORS 1769 



that the development of chlorophyll has been interrupted at an intermediate 

 stage l)y lack of a specific enzyme (caused by damage to a gene). He de- 

 rived from these observations the scheme of the l)iosynthesis of chloro- 

 phyll, which is represented in the right side of fig. 37B.4. 



Protoporphyrin 9 is the porphyrin most closely related to hemoglobin 

 (in fact, it is the heme minus iron). According to Granick, this porphyrin 

 is the common precursor of both the red blood pigment and the green plant 

 pigment. 



Granick speculated further on the probable mechanism of synthesis 

 of protoporphyrin 9 from ultimate, small building blocks — assumed to be 

 glycine and acetate. The first synthesized pyrrole derivative he assumed 

 to be (I) in fig. 37B.4. Uroporphyrin (II) was assumed to be the first 

 tetrapyrrole formed from this monopyrrole derivative, and coproporphyrin 

 (III), the intermediate between uroporphyrin and protoporphyrin 9. 



Additional results were reported by Bogorad and Granick (1952). 

 They found new Chlorella mutants, containing porphyrins with 2, 3, 4, 5 

 and 8 carboxyl groups. One of them, the monovinyl hydroxy dicarboxyl- 

 porphyrin, was suggested as a likely immediate precursor of protoporphyrin ; 

 its own precursor could be hematoporphyrin, also found in these mutants. 



The role of glycine in supplying nitrogen for the synthesis of porphin 

 derivatives was first demonstrated by Shemin and Rittenberg in 1940, by 

 experiments on human erythrocytes with N^^ tracer. The (indirect) 

 acetate origin of the carbon atoms (other than those supplied by the a- 

 carbons of glycine), also was supported by isotope tracer studies of Rit- 

 tenberg and co-workers. Salomon, Altman and Rosa (1950) showed that 

 the a-carbons of acetic acid and of glycine are used in the synthesis of chlo- 

 rophyll in the plant cell; this lends support to Granick's hypothesis that the 

 formation of chlorophyll follows — up to a certain point — the same path 

 as the synthesis of heme. 



Granick suggested, as a further hypothesis, that biosynthesis, as it 

 occurs in plants now, repeats the path of evolution. In other words, he 

 postulated that organisms synthesizing uroporphyrin, coproporphyrin, 

 protoporphyrin and, finally, protochlorophyll and chlorophyll, have 

 evolved in this order. He assumed that, at the time when any one 

 of these compounds represented the final product of synthesis, it was 

 used for some metabolic process by the organism; this particular process 

 fell into disuse as the synthesis advanced another step. More specifically, 

 Granick suggested that all porphin pigments presently encountered in 

 cells have been used, at some stage of evolution, as photochemical sensi- 

 tizers. He saw a confirmation of this view in the capacity of "vestigial" 

 pigments, such as chlorophyll c, to sensitize photosynthesis. (He had 

 found chlorophyll c to be a porphin, rather than a chlorin, cf. section 5 



