Biosynthesis of porphyrins and related macrocycles. Part V. Structural integrity of the type III porphyrinogen macrocycle in an active biological system; studies on the aromatisation of protoporphyrinogen-IX

Abstract

Coproporphyrin-III ester (23), specifically labelled with ¹⁴C or ³H in the propionic acid side chains of rings A and B, has been prepared by partial synthesis (40% overall yield) from readily available protoporphyrin-IX (9). The derived samples of coproporphyrinogen-III [(4) and (5)] have been converted enzymically into two samples of protoporphyrin-IX ester [(10) and (11)], shown by degradation to be labelled at sites equivalent to those in the original samples of coproporphyrinogen-III. These results and those of other workers establish that the type III porphyrinogen macrocycle, having been biosynthesised from four porphobilinogen (PBG) units, does not subsequently undergo enzymic ‘scrambling’ of the constituent pyrrole rings. This result is of crucial importance for further biosynthetic studies (see following paper).

Experiments based on isotope effects have been carried out with [11-³H₁]PBG which provide strong evidence that the final step on the biosynthetic pathway, viz. the aromatisation of protoporphyrinogen-IX (6) to the corresponding porphyrin (9), is an enzyme-catalysed process. Synthetic routes are described which provide [11-³H]- and [11-²H]-PBG.