Two Hemes Are Better than One

Psi factors are fatty acid-derived molecules that regulate the balance between sexual and asexual life cycles in many fungi; these psi factors are generated by enzymes known as psi factor-producing oxygenases (Ppos). Of the three Ppos in Aspergillus nidulans, PpoA is intriguing because it is predicted to contain two distinct heme domains, an N-terminal peroxidase domain and a C-terminal P450 heme thiolate domain. In this Paper of the Week, Florian Brodhun and colleagues cloned and expressed A. nidulans PpoA in Escherichia coli to better characterize its biochemical properties. Their biochemical and site-directed mutagenesis studies revealed that PpoA uses both domains to catalyze two separate reaction steps; the peroxidase domain first oxidizes linoleic acid to (8R)-hydroperoxyoctadecadienoic acid (8-HPODE), then the C-terminal thiolate domain isomerizes 8-HPODE to 5,8-dihydroxyoctadecadienoic acid (5,8-DiHODE). This mechanism and structure is quite different from that of a related fatty acid dioxygenase in Gaeumannomyces graminis, which has only one predicted heme domain and converts the 8-HPODE intermediate to (7S,8S)-DiHODE. In essence, PpoA is similar to other enzymatic partnerships in which separately expressed fatty acid dioxygenases and cytochrome P450s form bioactive products, such as the cyclooxygenase prostaglandin H synthase working with the cytochrome P450s thromboxane synthase and prostacyclin synthase to produce bioactive molecules that control hemostasis. In PpoA, though, the roles of fatty acid peroxide production and its isomerization are combined in a single fusion protein, making this enzyme somewhat unique.

Figure 1.

Hypothetical catalytic mechanism of A. nidulans PpoA.