Normal and Abnormal Heme Biosynthesis

Lash, Timothy

Abstract

The sequential action of uroporphyrinogen decarboxylase and coproporphyrinogen oxidase transforms uroporphrinogen-III via coproporphyrinogen-III into proto- porphyrinogen-IX. Although this section of the heme biosynthetic pathway has been studied in detail, a number of ambiguities remain. The conversion of uroporphyrinogen-III to coproporphyrinogen-III could take place by 24 different pathways. Although there is evidence to support the involvement of a preferred pathway, recent findings have thrown this into doubt. The stepwise conversion of coproporphyrinogen-III to protoporphyrinogen-IX has been shown to involve a single pathway, although the mechanism for the oxidative decarboxylations of the propionate sidechains is presently unknown. The substrate specificity of coproporphyrinogen oxidase will be examined in detail and a model for the active site of this enzyme will be developed. All four of the pentacarboxylate porphyrins related to uroporphyrin-III will be synthesized and the action of coproporphyrinogen oxidase on the related porphyrinogens will be studied. This work may lead to the identification of novel abnormal pathways for heme biosynthesis. These studies will have clinical implications, as abnormal porphyrin metabolites are excreted in the porphyrias (diseases of porphyrin metabolism). In related studies, the mechanism of the oxidative decarboxylation of coproporphyrinogen-III will be examined. Trace amounts of a hydroxypropionate porphyrinogen are known to accumulate in incubations of coproporphyrinogen-III with coproporphyrinogen oxidase. The investigators will isolate the corresponding porphyrin and examine the stereochemistry of this system. Finally, substrate specificity studies will be carried out on uroporphyrinogen decarboxylase. Little is presently known about enzyme- substrate recognition for uroporphyrinogen decarboxylase and this work will provide important insights into the action of this poorly understood enzyme.