Title

The biological significance of fungal enzymes that degrade polycaprolactone

Date of Completion

January 1997

Keywords

Biology, Microbiology

Degree

Ph.D.

Abstract

Increasing plastic waste and decreasing landfill capacity have stimulated interest in biodegradable plastics. The major plastics now in use show very limited biodegradability. The few available synthetics that are readily biodegradable are expensive to produce, or have physical properties making them unsuitable for most applications, or both. However, understanding how microbial enzymes act on the available biodegradable synthetics may be useful in designing new biodegradable plastics with improved properties.^ Polycaprolactone (PCL), a synthetic polyester with a melting point too low for practical use as a plastic, is degraded by depolymerases secreted by a variety of microorganisms, including plant pathogens. Lack of knowledge about the natural substrates and biological functions of these enzymes has greatly slowed their understanding and application.^ This work shows that some PCL depolymerases produced by plant pathogenic Fusarium species are cutinases, the first demonstration of the biological role of PCL depolymerases. Wild-type strains but not a cutinase-negative mutant strain degraded PCL and used it as a source of carbon and energy, and showed induction of secreted PCL depolymerase when grown in the presence of cutin. Cutinases are serine hydrolases that degrade cutin, the polyester component of plant cuticle. The hydrolysis products of cutin include C-16 and C-18 omega hydroxy fatty acids, which are inducers of cutinase. PCL oligomers are structurally similar to some of these natural inducers, and a hydrolysate of PCL induced cutinase (PCL depolymerase) synthesis almost as well as cutin itself. PCL is thus both an inducer and a substrate of cutinase.^ A second PCL depolymerase secreted by the Fusarium strains is not cutinase. The second depolymerase is induced by Tween 80 and tributyrin, but not by PCL or cutin, and shows interfacial activation, indicating that the enzyme is a lipase. For this enzyme, PCL is a substrate but not an inducer. These results suggest that in some cases biodegradation of synthetic polymers may not occur due to lack of induction of degradative enzymes. ^