Mechanistic studies on the biosynthesis of bacteriochlorophylls in photosynthetic bacteria

Ajaz, Abid Aziz (1983) Mechanistic studies on the biosynthesis of bacteriochlorophylls in photosynthetic bacteria. University of Southampton, Doctoral Thesis.

Abstract

In a preliminary study, experiments were performed to incorporate various precursors into the bacterio-chlorophylls-c of green sulphur bacteria and into the bacteriochlorophylls-a of purple non-sulphur bacteria. Out of all the precursors studied, 5-ALA, methionine and acetate were incorporated in larger amounts into bacteriochlorophylls- a and bacteriochlorophylls-c. The purple bacteria Rs. spheroides and Rm. rubrum incorporated highest amounts of 5-ALA into the bacteriochlorophylls-a than the green bacteria Chloropseudomonas ethylicum, Chlorobium thiosulphatophilum and Prosthecochloris aestuarii. Chloropseudomonas ethylicum was found to incorporate 5-ALA more efficiently into its bacteriochlorophylls-c than the other two green bacteria. 5-Aminolaevulinic acid, labelled with 180 at the oxygen atoms of C-1 carboxyl group, was prepared and was used to study the mechanisms of isoprenylation (at C-173 carboxyl group) and methylation (at C-133 carboxyl group) processes in the biosynthesis of bacteriochlorophylls-a in Rhodospirillum rubrum. During these studies it was discovered that both the esterification processes occur with the full retention of oxygen atoms of the respective carboxyl groups.In another study [3-RS.3H2J 2-oxoglutarate and [3-R.3H J2-oxoglutarate were prepared and converted into the respectively labelled succinates. The latter were used to prepare 12, 3-RS. 3H4J and [2, 3-R. 3H2] 5-aminolevulinic acid by the 5-ALA synthetase reaction. The species of labelled 5-ALA so obtained were incorporated into bacteriochlorophylls-a which were oxidized to obtain ring-B as ethylmethyl maleimide and ring-D as haematinicacidimide. The determination of radioactivity in the imides, showed that the pro-S hydrogen atom, present at C-3 of 5-ALA, was lost during the biosynthesis of bacteriochlorophylls-a, in the two anaerobic photosynthetic bacteria Rhodopseudomonas spheroides and Rhodospirillumrubrum. It was found that the stereochemistry of hydrogen removal, during the conversion of coproporphyrinogen into protoporphyrinogen-IX, in anaerobic organisms was also the same as previously found in the aerobic organisms. The results from this study are in agreement with the previously suggested mechanisms for the corresponding conversion of coproporphyrinogen-111 Enzyme ) protoporphyrinogen-IX in animals and plants. The probable mechanisms based upon our findings are discussed.

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