title: The surface structure of a complex substrate revealed by enzyme kinetics and Freundlich constants for α-amylase interaction with the surface of starch.
abstract: BACKGROUND
  Starch is a main source of carbohydrate in human diets, but differences are observed in postprandial glycaemia following ingestion of different foods containing identical starch contents. Such differences reflect variations in rates at which different starches are digested in the intestine. In seeking explanations for these differences, we have studied the interaction of α-amylase with starch granules. Understanding this key step in digestion should help with a molecular understanding for observed differences in starch digestion rates.
  
  
  METHODS
  For enzymes acting upon solid substrates, a Freundlich equation relates reaction rate to enzyme adsorption at the surface. The Freundlich exponent (n) equals 2/3 for a liquid-smooth surface interface, 1/3 for adsorption to exposed edges of ordered structures and 1.0 for solution-solution interfaces. The topography of a number of different starch granules, revealed by Freundlich exponents, was compared with structural data obtained by differential scanning calorimetry and Fourier transform infrared spectroscopy with attenuated total internal reflectance (FTIR-ATR).
  
  
  RESULTS
  Enzyme binding rate and FTIR-ATR peak ratio were directly proportional to n and ΔgelH was inversely related to n. Amylase binds fastest to solubilised starch and to granules possessing smooth surfaces at the solid-liquid interface and slowest to granules possessing ordered crystalline surfaces.
  
  
  CONCLUSIONS
  Freundlich exponents provide information about surface blocklet structures of starch that supplements knowledge obtained from physical methods.
  
  
  CONCLUSIONS
  Nanoscale structures at the surface of starch granules influence hydrolysis by α-amylase. This can be important in understanding how dietary starch is digested with relevance to diabetes, cardiovascular health and cancer.
authors.affiliation: King's College London, School of Medicine, Diabetes and Nutritional Sciences Division, Biopolymers Group, Franklin-Wilkins Building, 150 Stamford Street, London, SE1 9NH, UK.
authors.forename: Frederick J
authors.initials: FJ
authors.lastname: Warren
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Butterworth
authors.forename: Peter R
authors.initials: PR
authors.lastname: Ellis
mesh_terms: D002152:Calorimetry, Differential Scanning; D007700:Kinetics; D049329:Nanostructures; D017550:Spectroscopy, Fourier Transform Infrared; D013213:Starch / Q000737:chemistry*; D000516:alpha-Amylases / Q000737:chemistry*
chemical_list: D013213:Starch; D000516:alpha-Amylases