TETRAPYRROLES IN PHOTOSYNTHETIC BACTERIA 



JUNE LASCELLES 



Microbiology Unit, Department of Biochemistry, University of Oxford 



Tetrapyrroles in the form of chlorophylls and cytochromes have a 

 vital function in the transformation of radiant energy into a form 

 available for the metabolism of photosynthetic forms of life. It is 

 therefore important to know how photosynthetic organisms synthesize 

 and control the formation of these substances. The significance of 

 this information is not confined solely to photosynthetic organisms. 

 The unique capacity of photosynthetic bacteria and algae to make tetra- 

 pyrroles has led to their profitable exploitation by biochemists inter- 

 ested in wider aspects of tetrapyrrole metabolism and now it is not 

 unusual to find cultures of photosynthetic bacteria in laboratories pre- 

 viously devoted to ducks and rabbits. 



In this paper the distribution, biosynthesis, and control of formation 

 of tetrapyrroles is discussed with particular emphasis on those areas 

 where information is still sadly lacking. 



TYPES OF TETRAPYRROLES FORMED 

 BY PHOTOSYNTHETIC BACTERIA 



Chlorophylls . 



These comprise by far the major proportion of tetrapyrrole deriva- 

 tives in photosynthetic bacteria. It seems that bacteriochlorophyll 

 (Fig. 1) is the only form of chlorophyll in the Thiorhodaceae and 

 Athiorhodaceae (see "Note added in proof," p. 52). The structure 

 proposed by Fischer (1) has recently been confirmed (2), 



In the green sulfur bacteria two different types of chlorophyll 

 have been identified, designated Chlorobium chlorophyll 650 and 660 

 according to the red absorption maxima of the extracted pigments in 

 ether (3), The structure of these is being currently investigated; the 

 650 pigment is a derivative of 2-desvinyl-2-Q'-hydroxyethylpyropheo- 

 phorbide a and the 660 compound is a derivative of 5-methyl-2- 

 desvinyl-2-Q'-hydroxyethylpyropheophorbide a. Both pigments lack the 

 type of cyclopentanone ring typical of other chlorophylls and have no 

 carbomethoxyl groups (4,5), Also, the alcohol side chain differs from 

 phytol (C20H39OH) and appears tobetrans-trans-farnesol (C15H25OH) 

 (6). 



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