Kinetic Mechanism of the Dechlorinating Flavin-dependent Monooxygenase HadA.

The accumulation of chlorophenols (CPs) in the environment, due to their wide use as agrochemicals, has become a serious environmental problem. These organic halides can be degraded by aerobic microorganisms, where the initial steps of various biodegradation pathways include an oxidative dechlorinating process in which chloride is replaced by a hydroxyl substituent. Harnessing these dechlorinating processes could provide an opportunity for environmental remediation, but detailed catalytic mechanisms for these enzymes are not yet known. To close this gap, we now report transient kinetics and product analysis of the dechlorinating flavin-dependent monooxygenase, HadA, from the aerobic organism Ralstonia pickettii DTP0602, identifying several mechanistic properties that differ from other enzymes in the same class. We first overexpressed and purified HadA to homogeneity. Analyses of the products from single and multiple turnover reactions demonstrated that HadA prefers 4-CP and 2-CP over CPs with multiple substituents. Stopped-flow and rapid-quench flow experiments of HadA with 4-CP show the involvement of specific intermediates (C4a-hydroperoxy-FAD and C4a-hydroxy-FAD) in the reaction, define rate constants and the order of substrate binding, and demonstrate that the hydroxylation step occurs prior to chloride elimination. The data also identify the non-productive and productive paths of the HadA reactions and demonstrate that product formation is the rate-limiting step. This is the first elucidation of the kinetic mechanism of a two-component flavin-dependent monooxygenase that can catalyze oxidative dechlorination of various CPs, and as such it will serve as the basis for future investigation of enzyme variants that will be useful for applications in detoxifying chemicals hazardous to human health.

RpA, an extracellular protease similar to the metalloprotease of serralysin family, is required for pathogenicity of Ralstonia pickettii.

AIM: To investigate the biochemical and functional properties of an extracellular protease, RpA, in Ralstonia pickettii WP1 isolated from water supply systems. METHODS AND RESULTS: An extracellular protease was identified and characterized from R. pickettii WP1. A mutant strain WP1M2 was created from strain WP1 by mini-Tn5 transposition. The culture filtrates from WP1M2 had a lower cytotoxic effect than the parental WP1 on several mammalian cell lines. Cloning and sequence analysis revealed the Tn5 transposon inserted at a protease gene (rpA) which is 81% homologous to prtA and aprX genes of Pseudomonas fluorescens. The rpA gene encodes a 482-residue protein showing sequence similarity to metalloproteases of the serralysin family. The RpA protein was expressed in Escherichia coli using a pET expression vector and purified as a 55 kDa molecular weight protein. Furthermore, the protease activity of RpA was inhibited by protease inhibitor and heat treatment. CONCLUSIONS: The in vitro cytotoxic activity of R. pickettii culture filtrates was attributed to RpA protease. SIGNIFICANCE AND IMPACT OF THE STUDY: An extracellular protease, RpA, was identified from R. pickettii WP1 isolated from water supply system. The RpA metalloproteases is required for the pathogenicity of R. pickettii to mammalian cell lines.

Transcriptional repression of the poly(3-hydroxybutyrate) depolymerase in Ralstonia pickettii T1 by a tetR-like gene.

Ralstonia pickettii T1 secretes a poly(3-hydroxybutyrate) (PHB) depolymerase (PhaZ) and a 3-hydroxybutyrate (3HB)-oligomer hydrolase, and extracellularly degrades PHB to produce 3HB. However, it is not clear how the expression of phaZ is regulated. In this study, the mechanism by which phaZ expression is controlled in R. pickettii T1 was examined using a mutant made by the random insertion of a transposon, Tn5. The mutant produced a larger amount of PhaZ than the wild type in nutrient broth or a minimal salt (SM) medium supplemented with succinate. However, there was essentially no difference in the activity or amount of PhaZ in the culture supernatant between the wild type and mutant when the two were grown on 3HB. The gene disrupted by the insertion of Tn5 (epdR) was cloned and its nucleotide sequence was determined. In a BLAST search, epdR showed a high degree of similarity to genes for TetR transcriptional regulators of several bacteria. The introduction of epdR into the wild type and mutant grown on the three media described above decreased the amount of PhaZ. These results indicated EpdR to be involved in the repression of phaZ in R. pickettii T1. A quantitative RT-PCR analysis indicated that mRNA levels corresponded with the activity detected and the amounts of PhaZ in the wild type and mutant. Furthermore, the amount of epdR transcript was inversely proportional to the amount of phaZ transcript. In addition, the existence of a positive element acting on phaZ expression was suggested, because in the mutant lacking EpdR, the amount of phaZ transcript varied in cells grown in SM-3HB, SM-succinate or nutrient broth. Based on the above results, a model for the regulation of PhaZ expression in R. pickettii T1 is proposed.

Gradually accumulating beneficial mutations to improve the thermostability of N-carbamoyl-D-amino acid amidohydrolase by step-wise evolution.

To further enhance repeated batch reactions with immobilized N-carbamoyl-D-amino acid amidohydrolase (DCase), which can be used for the industrial production of D-amino acids, the stability of high soluble mutant DCase-M3 from Ralstonia pickettii CGMCC1596 was improved by step-wise evolution. In our previous report, six thermostability-related sites were identified by error-prone PCR. Based on the above result, an improved mutant B5 (Q12L/Q23L/H248Q/T262A/T263S) was obtained through two rounds of DNA shuffling, showing a 10°C increase in the T (50) (defined as the temperature at which heat treatment for 15 min reduced the initial activity by 50%) compared with the parental enzyme DCase-M3. Furthermore, several thermostability-related sites (Met(31), Asn(93), Gln(207), Asn(242), Glu(266), Thr(271), Ala(273)) on B5 were identified using amino acid consensus approach based on sequence alignment of homologous DCases. These sites were further investigated by iterative saturation mutagenesis (ISM), and a combinational mutant D1 (Q12L/Q23L/Q207E/N242G/H248Q/T262A/T263S/E266D/T271I/A273P) that enhanced the T(50) by about 16°C over DCase-M3 was obtained. Oxidative stability assay showed that the most heat-resisting mutant displayed only a slight increase in resistance to hydrogen peroxide. Comparative characterization showed that D1 not only maintained its characteristic high solubility but also shared similar k(cat) and K(m) values and optimum reaction pHs with the parental enzyme. The significantly improved mutants in the immobilized form are expected to be applied in the industrial production of D-p-hydroxyphenylglycine.

Adsorption characteristics of P(3HB) depolymerase as evaluated by surface plasmon resonance and atomic force microscopy.

Molecular recognition of poly[(R)-3-hydroxybutyrate] (P(3HB)) depolymerase from Ralstonia pickettii T1 to the surfaces of biodegradable aliphatic polyesters such as P(3HB) and poly(L-lactic acid) (PLLA) was examined from the viewpoints of kinetics and dynamics. To determine the kinetic parameters on the interaction between the substrate-binding domain (SBD) of P(3HB) depolymerase and various polymer substrates with different chemical structures, surface plasmon resonance (SPR) measurements were performed. On the other hand, using an atomic force microscopic (AFM) cantilever tip functionalized with the SBD of P(3HB) depolymerase, the mechanical parameters such as unbinding force to the polymer surfaces were measured. Both the SPR and AFM measurements showed that the SBD has a high affinity to P(3HB) and PLLA. From the results of kinetics and dynamics, the energy potential landscape of SBD-polymer interaction was disclosed on the basis of a phenomenological model, and the mechanism of the interaction was discussed.

Display of functionally active PHB depolymerase on Escherichia coli cell surface.

The display of PHB depolymerase (PhaZ(RpiT1) ) from R. pickettii T1 on the surface of E. coli JM109 cells is realized using OprI of P. aeruginosa as the anchoring motif. The fusion protein is stably expressed and its surface localization is verified by immunofluorescence microscopy. The displayed PhaZ(RpiT1) retains its cleaving ability for soluble substrates as well as its ability to adsorb to the PHB surface, and also remains catalycically active in the degradation of insoluble polyester materials, in spite of the possible suppression of the enzyme movement on the polymer surface. The results demonstrate that PhaZ(RpiT1) -displaying E. coli shows potential for use as a whole-cell biocatalyst for the production of (R)-3-hydroxybutyrate monomers from insoluble PHB materials.

Secretion pathway for the poly(3-hydroxybutyrate) depolymerase in Ralstonia pickettii T1.

The extracellular poly(3-hydroxybutyrate) depolymerase from Ralstonia pickettii T1 has been purified, its function and character investigated in detail, and its gene cloned and sequenced. However, the mechanism by which this enzyme is secreted has not been elucidated. A mutant unable to degrade poly(3-hydroxybutyrate), N17, was obtained with the random insertion of a mini-transposon, Tn5. Western analysis using antiserum against the poly(3-hydroxybutyrate) depolymerase of Ralstonia pickettii T1, revealed that N17 accumulated the poly(3-hydroxybutyrate) depolymerase in the periplasm and cytoplasm, and did not secrete the enzyme into the external medium. It was also found that 3-hydroxybutyrate-oligomer hydrolase was secreted but inactive. The disrupted gene in N17, depO, was analyzed by Southern hybridization and its nucleotide sequence was determined. One complete open reading frame was found in the cloned 2.3-kbp DNA fragment. From a BLAST search, this gene product was found to be homologous to PulO of Ralstonia eutropha JMP134 (60% identity) and XcpA of Pseudomonas aeruginosa (60% identity). These proteins are prepilin peptidase/N-metyltransferases, a component of the Type II secretion pathway. DepO also had the four cysteines highly conserved in most prepilin peptidases at the same positions. The transcript of depO was examined by Northern hybridization using depO as a probe. In the total RNA of Ralstonia pickettii T1 in the early stationary phase, a band at 2.6-kb was detected, suggesting depO to be a functional gene. In this study, it was found that poly(3-hydroxybutyrate) depolymerase was secreted by the Type II pathway.