Evolution of polyhydroxyalkanoate (PHA) production system by "enzyme evolution": Successful case studies of directed evolution

Biotechnological studies towards the biosynthesis of polyhydroxyalkanoates (PHAs) biopolyesters have extensively progressed through the development of various metabolic engineering strategies. Historically, efficient PHA production has been achieved using the fermentation technology of naturally occurring PHA-producing bacteria based on external substrate manipulation (1st generation), and subsequent reinforcement with recombinant gene technology (2nd generation). More recently, "enzyme evolution" is becoming the 3rd generation approach for PHA production. A break-through in the chemical synthesis of macromolecules with desirable properties was achieved by the development of prominent chemical catalysts via "catalyst evolution", as represented by a series of Ziegler-Natta catalysts. Thus, one can easily accept the concept that the molecular evolution of the biocatalysts (enzymes) relevant to PHA synthesis will provide us with a chance to create novel PHA materials with high performance. The first trial of an in vitro enzyme evolution in PHA biosynthesis was reported by our group in 2001. The following literature data, as well as our own experimental results devoted to this new approach, have been accumulated over a short time. This review article focuses specifically on the concept and current case studies of the application of "enzyme evolution" to PHA biosynthesis.