SPLINTERING OF POLY(3-HYDROXYBUTYRATE) SINGLE-CRYSTALS BY PHB-DEPOLYMERASE-A FROM PSEUDOMONAS-LEMOIGNEI

With the aim of improved understanding of the mechanism of depolymeras e action, folded chain lamellar single crystals were partially degrade d with PWB-depolymerase A from Pseudomonas lemoignei. Enzymatically de graded single crystals of bacterial poly(3-hydroxybutyrate), PHB, were observed by transmission electron microscopy and were found to be spl intered parallel to their long axes. Prior to degradation, the crystal s were lamellar with macroscopic dimensions of approximately 2.5 mu m by 20 mu m and the classical baseplane single crystal diffraction patt ern corresponding to bacterial PHB was recorded. When observed by TEM, the partially degraded crystals had been splintered longitudinally, t o a needlelike morphology. The needlelike fragments of PHB still yield ed the same crystalline baseplane diffraction pattern. These results s upport an ''edge attack'' model for the degradation of PHB single crys tals and explain, at a molecular level. the lack of decrease in molecu lar weight during the degradation since the direction of chain folding is parallel to the long axis of the crystals. The proposed mechanism explains the conversion of PHB spherulite lamellae into a needlelike m orphology and suggests that PHB-depolymerase A has both endo and exo a ctivity.