Heme biosynthesis and its regulation: towards understanding and improvement of heme biosynthesis in filamentous fungi

Abstract

Heme biosynthesis in fungal host strains has acquired considerable interest in relation to the production of secreted heme-containing peroxidases. Class II peroxidase enzymes have been suggested as eco-friendly replacements of polluting chemical processes in industry. These peroxidases are naturally produced in small amounts by basidiomycetes. Filamentous fungi like Aspergillus sp. are considered as suitable hosts for protein production due to their high capacity of protein secretion. For the purpose of peroxidase production, heme is considered a putative limiting factor. However, heme addition is not appropriate in large-scale production processes due to its high hydrophobicity and cost price. The preferred situation in order to overcome the limiting effect of heme would be to increase intracellular heme levels. This requires a thorough insight into the biosynthetic pathway and its regulation. In this review, the heme biosynthetic pathway is discussed with regards to synthesis, regulation, and transport. Although the heme biosynthetic pathway is a highly conserved and tightly regulated pathway, the mode of regulation does not appear to be conserved among eukaryotes. However, common factors like feedback inhibition and regulation by heme, iron, and oxygen appear to be involved in regulation of the heme biosynthesis pathway in most organisms. Therefore, they are the initial targets to be investigated in Aspergillus niger.

Fig. 1

Chemical heme biosynthesis pathway. Biosynthesis is initiated in mitochondria with the condensation of glycine and succinyl CoA to 5′-aminolevulinic acid (ALA) by 5′-aminolevulinic acid synthase (ALAS). ALA is subsequently exported into the cytosol to be processed to uroporphyrinogen III (UroIII), the final common intermediate between heme and siroheme synthesis. For heme biosynthesis, UroIII is decarboxylated by UroIII decarboxylase (UROD) to coproporphyrinogen III, which in turn is redirected to mitochondria. Heme biosynthesis is finalized in mitochondria in three subsequent enzymatic reactions. Siroheme synthesis also derives from uroporphyrinogen III synthesis. Siroheme is synthesized in four subsequent reactions by one multifunctional (CysG in E. coli) or two enzymes (Met1p and Met8p in S. cerevisiae). ALAS 5′-aminolevulinic acid synthase, ALAD 5′-aminolevulinic acid dehydratase, PBGD porphobilinogen deaminase, UROS uroporphyrinogen III synthase, UROD uroporphyrinogen III decarboxylase, CPO coproporphyrinogen III oxidase, PPO protoporphyrinogen oxidase, FC ferrochelatase