42 THE BACTERIAL PHOTOCHEMICAL APPARATUS 



Clearly oxygen cannot participate in the reaction in photosynthetic 

 bacteria growing anaerobically and presumably the acceptor is gener- 

 ated by the photosynthetic apparatus. Since i?/)s. s/?/^ero^■(ies is so active 

 in forming protoporphyrin under appropriate conditions it is a promis- 

 ing candidate for enzyme studies; analysisof this system might clarify 

 the general mechanism by which these bacteria perform other reactions 

 which are obligatorily linked to oxygen in aerobic organisms. 



Formation of iron and magnesium protoporphyrins . 



Soluble enzyme systems have been purified from animal tissues 

 which catalyse heme synthesis from protoporphyrin and ferrous ions 

 (28,29), and it is probable that similar enzymes are present in bac- 

 teria. Burnham (30) has obtained heme synthesis from protoporphyrin 

 and iron citrate with crude extracts of Rps. spheroides incubated 

 anaerobically in the light with succinate. Ferrichrome or related iron- 

 binding factors may participate coenzymically in this reaction. In the 

 Rps. spheroides systems, ferrichrome replaces iron citrate and ex- 

 periments with Fe^^-labelled ferrichrome have shown that the iron 

 is transferred to protoporphyrin to form labelled heme. Purification 

 of the enzyme is required to establish whether ferrichrome is an ob- 

 ligatory cofactor. 



The participation of magnesium protoporphyrin as an intermediate 

 in chlorophyll synthesis was indicated many years ago by the isolation 

 of chlorophyll- less mutants of Chlorella which accumulated this metal 

 complex (31). There is no information about the enzymic mechanism 

 of magnesium protoporphyrin synthesis. 



Synthesis of chlorophylls. 



The pathway from protoporphyrin to chlorophyll and bacterio- 

 chlorophyll has received little analysis at an enzymic level and present 

 knowledge is derived mainly from the compoiinds accumulating in 

 cultures of mutant strains of Chlorella and Rps. spheroides (Table 3; 

 Scheme 2). An early step is the formation of magnesium proto- 

 porphyrin monomethyl ester. The specific incorporation of C^"^- 

 formate by Chlorella into the methyl ester group of chlorophyll sug- 

 gested that this group is derived from a one-carbon unit (32). In Rps. 

 spheroides l-C^^-methionine labels the methyl group of bacterio- 

 chlorophyll specifically; inhibition of methyl transfer from methionine 

 by ethionine could account for the inhibition of bacteriochlorophyll 

 synthesis by this analogue (33). Chromatophore preparations from this 

 organism form magnesium protoporphyrin monomethyl ester when in- 

 cubated with S-adenosylmethionine and magnesium protoporphyrin (39). 

 Neither protoporphyrin nor the corresponding porphyrinogen is 

 methylated in this system, showing that the biosynthetic sequence is: 



Protoporphyrin ^ magnesium protoporphyrin *- magnesium 



protoporphyrin monomethyl ester 



