Mapping Single Molecular Binding Kinetics of Carbohydrate-Binding Module with Crystalline Cellulose by Atomic Force Microscopy Recognition Imaging

We studied the binding kinetics of family 3 carbohydrate-binding module (CBM3a) molecules to crystalline cellulose fibrils extracted from the poplar cell wall by atomic force microscopy (AFM) recognition imaging. The free CBM3a molecules of different concentrations were added to the buffer solution to bind to the crystalline cellulose sample immobilized on the AFM substrate. During in-situ AFM imaging, the CBM molecules were observed to bind to cellulose efficiently and regularly, especially in the first 60-120 min. A 1:1 single-molecule binding model was used to study the kinetics of the CBM3a-cellulose interaction. The saturation time when the concentration of occupied binding sites is 99% of the maximum bound CBM3a concentration at the end of reaction, t(0.99), was determined by fitting different concentrations of CBM3a against reaction time using the high resolution AFM images and the single-molecule kinetics equations. Based on the experimental data and kinetics calculations, the minimal effective initial CBM3a concentration was estimated to be 5.1 × 10(-7) M at 287 min reaction time. This study provides an in-depth understanding of the binding mechanism of CBM with crystalline cellulose at single molecule level.