Porphyrins

Abstract

Porphyrins are a small but important group of secondary plant substances. We dealt with their structure when we discussed the chlorophylls The biosynthesis of the porphyrins is interlinked with the citric acid cycle and amino acid metabolism. (Fig. 132). The precursor succinyl CoA is derived from the citric acid cycle and the precursor glycine from amino acid metabolism. The two are combined to give an unstable intermediate which loses CO₂ to form δ-aminolevulinic acid. Two molecules of δ-aminolevuhnic acid are combined to yield porphobilinogen, a pyrrole system, which is the building block of the porphyrins. Four such porphobilinogen molecules become linked in a series of steps to form the porphynn system. The first porphyrin to appear in the course of biosynthesis is uroporphyrinogen III. Of the subsequent intermediates protoporphynn IX is worthy of mention. This is because introduction of Mg⁺⁺ on the one hand leads to Mg-protoporphyrin IX and then further to the chlorophylls, whereas introduction of Fe⁺⁺ on the other yields Feprotoporphyrin IX and, subsequently, to cell hemins, haem, the cytochromes, cytochrome oxidases, peroxidases, and catalases.