482 



L. nOGORAD 



These remarks have dwelt on the problem of the "fork in the road." 

 Once the PBG has been set on the "correct" fork and the "proper" 

 tetrapyrrole has been formed, many enzymatic steps must occur 



(X)3TBG 



z-i 



Ac P I Ac P Ac P 



^2 H 



&!iicii 



H 



N 

 H 



1+ 1P3G 



Upoporphypin I 



+ 1PBG 

 E-2 



Ac P 



P Ac 



I| 



NHjC N 



H2 H 



H 



iM 



.Ac 



B H ^2 



E-5 



HN, 



CH2 

 /=tAc 



PBG = Porphobilinogen 



Ac- -CH2-COOH 



TJpopopphypin ]1I 



P = -CH2-CH2-COOH 



Fig. 5. Hypothetical scheme for the enzymatic synthesis of uroporphyrins 

 (6). According to this hypothesis "E-1" would correspond to PBG deaminase, 

 "E-2" to the isomerizing enzyme. 



before the goal— photosynthetically active chlorophyll and biologi- 

 cally active cytochromes— is reached. Detailed maps of a suggested 

 route to this goal, which should perhaps be marked "carry extra 

 water, food, and gasohne," are available elsewhere (e.g., 6,1.3). 



Addendum 



The intermediate in the appearance of uroporphyrin from the color- 

 less compound produced by the action of porphobilinogen deaminase 

 on PBG and characterized by its strong absorption at about 500 mn 

 has been identified as a stage in the oxidation of uroporphyrinogen to 

 porphyrin. The strong 500 m^ band, sometimes accompanied by weak 

 absorption bands at 538, 561, and 612 m/x, appears during the oxida- 



