Cytosol heme biosynthetic enzymes

Figure 6. The biosynthetic pathway for the production of heme. The enzymes and metabolites are located in the mitochondria (M) and the cytosol (C). Heme negatively regulates ALA synthase at several points, three of which are shown in the figure. Exogenous ALA induces excess formation of heme precursors, including PpIX that can be utilized for photosensitization. (N) nucleus where transcriptional control occurs.

The enzymes of the heme biosynthetic chain have a special distribution [Sano and Granick, 26]. Of the eight enzymes (Fig. 2), the first enz)Tne, ALA-synthetase, is found mainly in the mitochondria. The next four enzymes that convert ALA to coproporphyrinogen isomer III are localized in the cytosol that is, they are soluble enzymes. The last three enzymes are in the mitochondria—the two that oxidize, decarb-oxylate, and dehydrate coproporphyrinogen to protoporphyrinogen. 

Fig. 2. Distribution of the enzymes of the heme biosynthetic chain between mitochondria and cytosol.

As a result of such methods it was concluded that four of the eight enzymes of the heme biosynthetic chain are localized inside the inner membrane of the mitochondrion, whereas the other four enzymes reside in the cytosol. 

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