Replacement of the catalytic nucleophile cysteine-296 by serine in class II polyhydroxyalkanoate synthase from Pseudomonas aeruginosa-mediated synthesis of a new polyester: identification of catalytic residues

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Abstract
The class II PHA (polyhydroxyalkanoate) synthases [PHAMCL synthases (medium-chain-length PHA synthases)] are mainly found in pseudomonads and catalyse synthesis of PHAMCLs using CoA thioesters of medium-chain-length 3-hydroxyfatty acids (C6–C14) as a substrate. Only recently PHAMCL synthases from Pseudomonas oleovorans and Pseudomonas aeruginosa were purified and in vitro activity was achieved. A threading model of the P. aeruginosa PHAMCL synthase PhaC1 was developed based on the homology to the epoxide hydrolase (1ek1) from mouse which belongs to the α/β-hydrolase superfamily. The putative catalytic residues Cys-296, Asp-452, His-453 and His-480 were replaced by site-specific mutagenesis. In contrast to class I and III PHA synthases, the replacement of His-480, which aligns with the conserved base catalyst of the α/β-hydrolases, with Gln did not affect in vivo enzyme activity and only slightly in vitro enzyme activity. The second conserved histidine His-453 was then replaced by Gln, and the modified enzyme showed only 24% of wild-type in vivo activity, which indicated that His-453 might functionally replace His-480 in class II PHA synthases. Replacement of the postulated catalytic nucleophile Cys-296 by Ser only reduced in vivo enzyme activity to 30% of wild-type enzyme activity and drastically changed substrate specificity. Moreover, the C296S mutation turned the enzyme sensitive towards PMSF inhibition. The replacement of Asp-452 by Asn, which is supposed to be required as general base catalyst for elongation reaction, did abolish enzyme activity as was found for the respective amino acid residue of class I and III enzymes. In the threading model residues Cys-296, Asp-452, His-453 and His-480 reside in the core structure with the putative catalytic nucleophile Cys-296 localized at the highly conserved γ-turns of the α/β-hydrolases. Inhibitor studies indicated that catalytic histidines reside in the active site. The conserved residue Trp-398 was replaced by Phe and Ala, respectively, which caused inactivation of the enzyme indicating an essential role of this residue. In the threading model this residue was found to be surface-exposed. No evidence for post-translational modification by 4-phosphopantetheine was obtained. Overall, these data suggested that in class II PHA synthases the conserved histidine which was found as general base catalyst in the catalytic triad of enzymes related to the α/β-hydrolase superfamily, was functionally replaced by His-453 which is conserved among all PHA synthases.

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