Biosynthesis of porphyrins and related macrocycles. Part VIII. Enzymic decarboxylation of uroporphyrinogen-III: structure of an intermediate, phyriaporphyrinogen-III, and synthesis of the corresponding porphyrin and of two isomeric porphyrins

Abstract

A porphyrin formed when porphobilinogen (1) is incubated with an enzyme system from avian erythrocytes is proved to be heptacarboxylic and of the type III series; its properties correspond with those of phyriaporphyrin-III. Two of the four possible structures for this porphyrin are eliminated by preparing it from [2,11-¹³C₂]porphobilinogen and carrying out a ¹³C n.m.r. analysis of the product, with use of a praseodymium shift reagent.

Synthesis of all four heptacarboxylic porphyrins shows that the natural product has undergone specific decarboxylation of the acetic acid side chain on ring D to form (after aromatisation) the ring D methylporphyrin [as (12)]. Our results are consistent with the view that the initial enzymic decarboxylation on the pathway from uroporphyrinogen-III (2) to coproporphyrinogen-III (4) occurs at ring D.

Studies are outlined leading to effective large-scale syntheses of key pyrrolic building blocks for this and other work and several methods of general interest in this area are reported.