Simple kinetic model for the heterogeneous enzymatic hydrolysis of natural poly(3-hydroxybutyrate)

The kinetics of the enzymatic degradation of bacterial poly(3-hydroxybutyrate) (PHB) is studied using PHB-depolymerase A from Pseudomonas lemoignei (Tris-HCl buffer, pH = 8, T = 37 degrees C). Biodegradation experiments are performed on PHB in the form of both compression-molded films and powder suspension. From WAXS and DSC measurements the two substrates show the same crystalline fraction. The rate of hydrolysis of PHB films is determined by gravimetry and also through spectrophotometric quantification of the hydrolysis products at lambda = 210 nm. For the suspension of PHB particles, a turbidimetric determination of the biodegradation rate is applied. A simple two-step kinetic model is proposed, which predicts that the hydrolysis rate per unit substrate surface area reaches a plateau at high enzyme concentrations. The model satisfactorily describes the enzymatic degradation results of PHB film and PHB powder suspension, provided that the remarkable changes of exposed area caused by enzymatic attack to the latter substrate are taken into account. Analysis of the enzymatic degradation results yields analogous hydrolysis rate constants for film (1.48 mu g cm(-2) min(-1)) and powder suspension (1.42 mu g cm(-2) min(-1)).