Kinetic and stereochemical studies of manno-oligosaccharide hydrolysis catalysed by beta-mannanases from Trichoderma reesei.

Abstract

The two beta-mannanases from Trichoderma reesei with pI of 4.6 and 5.4, respectively, have been characterised by NMR spectroscopy. Following the kinetics of manno-oligosaccharide degradation with complete progress-curve analysis the stereospecificity and degradation pattern have been delineated. It was found that degradation of mannotriose and mannopentaose proceeds with retention of the anomeric configuration. Mannotriose degradation proceeds by almost random release of mannose. For mannopentaose there is initially no mannose formed showing that only the two middle mannosidic linkages are attacked. Progress-curve analysis shows that there is preference (70%) for cleavage of mannopentaose in such a way that mannobiose is released from the reducing end. The final product composition from the mannotriose degradation showed that transglycosylation has to be taken into account. Model calculation and progress-curve analysis showed that the transglycosylation rate is the fastest of all the rates in this system, 15 s-1 compared with mannohexaose and mannotetraose hydrolysis rates of 2 s-1 and mannotriose hydrolysis rate of 0.03 s-1 at 50 degrees C.