Abstract
A novel type of hydrolase was purified from culture fluid of Paucimonas (formerly Pseudomonas) lemoignei. Biochemical characterization revealed an unusual substrate specificity of the purified enzyme for amorphous poly((R)-3-hydroxyalkanoates) (PHA) such as native granules of natural poly((R)-3-hydroxybutyrate) (PHB) or poly((R)-3-hydroxyvalerate) (PHV), artificial cholate-coated granules of natural PHB or PHV, atactic poly((R,S)-3-hydroxybutyrate), and oligomers of (R)-3-hydroxybutyrate (3HB) with six or more 3HB units. The enzyme has the unique property to recognize the physical state of the polymeric substrate by discrimination between amorphous PHA (good substrate) and denatured, partially crystalline PHA (no substrate). The pentamers of 3HB or 3HV were identified as the main products of enzymatic hydrolysis of native PHB or PHV, respectively. No activity was found with any denatured PHA, oligomers of (R)-3HB with five or less 3HB units, poly(6-hydroxyhexanoate), substrates of lipases such as tributyrin or triolein, substrates for amidases/nitrilases, DNA, RNA, casein, N-alpha-benzoyl-l-arginine-4-nitranilide, or starch. The purified enzyme (M(r) 36,209) was remarkably stable and active at high temperature (60 degrees C), high pH (up to 12.0), low ionic strength (distilled water), and in solvents (e.g. n-propyl alcohol). The depolymerase contained no essential SH groups or essential disulfide bridges and was insensitive to high concentrations of ionic (SDS) and nonionic (Triton and Tween) detergents. Characterization of the cloned structural gene (phaZ7) and the DNA-deduced amino acid sequence revealed no homologies to any PHB depolymerase or any other sequence of data banks except for a short sequence related to the active site serine of serine hydrolases. A classification of the enzyme into a new family (family 9) of carboxyesterases (Arpigny, J. L., and Jaeger, K.-E. (1999) Biochem. J. 343, 177-183) is suggested.
Highlights
Age compounds that are synthesized during unbalanced growth by many bacteria
Purification of Extracellular nPHB Depolymerase PhaZ7— Extracellular nPHB depolymerase activity was followed during growth of P. lemoignei on acetate, succinate, 3HB, and dPHB
Low (Ͻ1 unit/ ml) or medium (1–2.5 units/ml) activities of nPHB depolymerase were found during exponential growth on acetate and 3HB or on succinate and dPHB, respectively. nPHB depolymerase activity was almost unchanged in the 3HB and PHB cultures at the beginning of the stationary phase (1–2.5 units/ ml), but the activities on succinate and acetate were increased significantly (Ͼ6 units/ml)
Summary
A number of extracellular PHB depolymerases have been isolated to date. Their structure and specificity have been reviewed recently [2, 3]. All available evidence shows that extracellular PHA depolymerases are inactive toward rubbery amorphous substrates such as nPHA granules and aPHB, recently unexpected high levels of a novel extracellular enzymatic activity of P. lemoignei that hydrolyzed both nPHB granules and aPHB was detected. This discovery prompted us to the present investigation on the activity of a newly isolated extracellular PHB depolymerase of P. lemoignei with a unique behavior, previously considered typical of intracellular PHB depolymerases
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