Substrate specificities of poly(hydroxyalkanoate)-degrading bacteria and active site studies on the extracellular...

The isolation of poly(3-hydroxyoctanoic acid)- and poly(6-hydroxyhexanoic acid)-degrading bacteria yielded 28 strains with abilities to degrade various polymers. The most versatile strains hydrolyzed five different polyesters comprising short chain length and medium chain length poly(hydroxyalkanoates). The new isolates together with previously isolated poly(hydroxyalkanoate)-degrading bacteria were classified into 11 groups with respect to their polymer-degrading specificities. All PHA depolymerases studied so far have been characterized by the lipase consensus sequence Gly-X-Ser-X-Gly in their amino acid sequence, which is a known sequence for serine hydrolases. When we replaced the central residue, Ser-172, in the corresponding sequence Gly-Ile-Ser-Ser-Gly of the extracellular poly(3-hydroxyoctanoic acid) depolymerase of Pseudomonas fluorescens GK13, with alanine the enzyme lost its activity completely. This result of the mutational experiment indicates that the poly(3-hydroxyoctanoic acid) depolymerase belongs to the family of serine hydrolases.