In Aspergillus nidulans Ppos [psi (precocious sexual inducer)-producing oxygenases] are required for the production of so-called psi factors, compounds that control the balance between the sexual and asexual life cycle of the fungus. The genome of A. nidulans harbours three different ppo genes: ppoA, ppoB and ppoC. For all three enzymes two different haem-containing domains are predicted: a fatty acid haem peroxidase/dioxygenase domain in the N-terminal region and a P450 haem-thiolate domain in the C-terminal region. Whereas PpoA was shown to use both haem domains for its bifunctional catalytic activity (linoleic acid 8-dioxygenation and 8-hydroperoxide isomerization), we found that PpoC apparently only harbours a functional haem peroxidase/dioxygenase domain. Consequently, we observed that PpoC catalyses mainly the dioxygenation of linoleic acid (18:2Δ9Z,12Z), yielding 10-HPODE (10-hydroperoxyoctadecadienoic acid). No isomerase activity was detected. Additionally, 10-HPODE was converted at lower rates into 10-KODE (10-keto-octadecadienoic acid) and 10-HODE (10-hydroxyoctadecadienoic acid). In parallel, decomposition of 10-HPODE into 10-ODA (10-octadecynoic acid) and volatile C-8 alcohols that are, among other things, responsible for the characteristic mushroom flavour. Besides these principle differences we also found that PpoA and PpoC can convert 8-HPODE and 10-HPODE into the respective epoxy alcohols: 12,13-epoxy-8-HOME (where HOME is hydroxyoctadecenoic acid) and 12,13-epoxy-10-HOME. By using site-directed mutagenesis we demonstrated that both enzymes share a similar mechanism for the oxidation of 18:2Δ9Z,12Z; they both use a conserved tyrosine residue for catalysis and the directed oxygenation at the C-8 and C-10 is most likely controlled by conserved valine/leucine residues in the dioxygenase domain.
在裸曲霉(Aspergillus nidulans)中,Ppos[psi(早熟性诱导剂)产生氧合酶]是产生所谓的psi因子所必需的,这些因子是控制真菌有性和无性生命周期平衡的化合物。A. nidulans 的基因组中含有三种不同的 ppo 基因:ppoA、ppoB 和 ppoC。这三种酶都有两个不同的含血红素结构域:N 端区域的脂肪酸血红素过氧化物酶/二氧 化酶结构域和 C 端区域的 P450 血红素硫酸盐结构域。研究表明,PpoA 利用这两个血红素结构域进行双功能催化活性(亚油酸 8-二氧 化和 8-过氧化氢异构化),而我们发现 PpoC 显然只含有一个功能性血红素过氧化物酶/二氧 化酶结构域。因此,我们观察到 PpoC 主要催化亚油酸(18:2Δ9Z,12Z)的二氧化反应,生成 10-HPODE(10-氢过氧化十八碳二烯酸)。未检测到异构酶活性。此外,10-HPODE 还以较低的速率转化为 10-KODE(10-酮基十八碳二烯酸)和 10-HODE(10-羟基十八碳二烯酸)。与此同时,10-HPODE 会分解成 10-ODA(10-十八碳烯酸)和挥发性 C-8 醇,这些物质是蘑菇特有风味的主要成分。除了这些原理上的差异,我们还发现 PpoA 和 PpoC 可以将 8-HPODE 和 10-HPODE 转化为相应的环氧醇:12,13-环氧-8-HOME(其中 HOME 是羟基十八碳烯酸)和 12,13- 环氧-10-HOME。通过使用定点诱变,我们证明这两种酶在氧化 18:2Δ9Z,12Z 时具有相似的机制;它们都使用一个保守的酪氨酸残基进行催化,C-8 和 C-10 的定向氧合很可能是由二氧化酶结构域中保守的缬氨酸/亮氨酸残基控制的。