As shown in Figure 11, condensation of glycine with suc- 

 cinic acid gives S-amino levulinic acid, which in turn con- 

 denses with itself to make a substituted pyrrole ring (por- 

 phobilinogen). Condensations and isomerizations, the exact 

 mechanisms of which are not known, lead to the formation 

 of the tetrapyrrole structure of uroporphyrinogen(III) from 

 four porphobilinogen molecules. 



The conversion of uroporphyrinogen to protoporphyrin 

 requires a number of decarboxylations of the substituent acyl 

 groups, oxidation of two of these groups to vinyl groups, and 

 dehydrogenation and aromatization of the pyrrole rings and 

 the methylene bridges connecting them. 



Protoporphyrin-9 is an important branching point: in- 

 corporation of Fe++ leads to heme and thence to the various 

 hematin pigments, whereas incorporation of Mg++ ion leads 

 ultimately to the synthesis of the chlorophylls. The latter 

 pathway must first accomplish the formation of the fifth 

 ring and the partial saturation of one of the pyrrole rings. 



Finally the phytol alcohol, probably formed as shown 

 in Figure 10, is attached to the pigment molecule as a phytyl 

 group, and the synthesis of chlorophyll is complete. At some 

 time, before or after this step, the alterations needed to make 

 the various forms of chlorophyll, and to incorporate it into 

 the structure of the photosynthetic apparatus are completed. 



64 



