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Applied and Environmental Microbiology, November 2006, p. 7331-7338, Vol. 72, No. 11
0099-2240/06/$08.00+0     doi:10.1128/AEM.01187-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Effects of Mutations in the Substrate-Binding Domain of Poly[(R)-3-Hydroxybutyrate] (PHB) Depolymerase from Ralstonia pickettii T1 on PHB Degradation

Bioengineering Laboratory, RIKEN Institute, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan,¹ Polymer Chemistry Laboratory, RIKEN Institute, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan,² Division of Biotechnology and Macromolecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan³

Received 23 May 2006/ Accepted 30 August 2006

Poly[(R)-3-hydroxybutyrate] (PHB) depolymerase from Ralstonia pickettii T1 (PhaZ_RpiT1) adsorbs to denatured PHB (dPHB) via its substrate-binding domain (SBD) to enhance dPHB degradation. To evaluate the amino acid residues participating in dPHB adsorption, PhaZ_RpiT1 was subjected to a high-throughput screening system consisting of PCR-mediated random mutagenesis targeted to the SBD gene and a plate assay to estimate the effects of mutations in the SBD on dPHB degradation by PhaZ_RpiT1. Genetic analysis of the isolated mutants with lowered activity showed that Ser, Tyr, Val, Ala, and Leu residues in the SBD were replaced by other residues at high frequency. Some of the mutant enzymes, which contained the residues replaced at high frequency, were applied to assays of dPHB degradation and adsorption, revealing that those residues are essential for full activity of both dPHB degradation and adsorption. These results suggested that PhaZ_RpiT1 adsorbs on the surface of dPHB not only via hydrogen bonds between hydroxyl groups of Ser in the enzyme and carbonyl groups in the PHB polymer but also via hydrophobic interaction between hydrophobic residues in the enzyme and methyl groups in the PHB polymer. The L441H enzyme, which displayed lower dPHB degradation and adsorption abilities, was purified and applied to a dPHB degradation assay to compare it with the wild-type enzyme. The kinetic analysis of the dPHB degradation suggested that lowering the affinity of the SBD towards dPHB causes a decrease in the dPHB degradation rate without the loss of its hydrolytic activity for the polymer chain.

Published ahead of print on 8 September 2006.

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