Kinetics and energetics of ligand binding determined by microcalorimetry: Insights into active site mobility in a psychrophilic alpha-amylase

• Published in: Journal of Molecular Biology
• Main Authors: D'Amico, Salvino, Sohier, Jean, Feller, Georges
• Format: Article in Journal/Newspaper
• Language: English
• Published: Academic Press Ltd Elsevier Science Ltd 2006
• Subjects: extremophiles; psychrophiles; microcalorimetry; isothermal titration calorimetry; Life sciences; Biochemistry; biophysics & molecular biology; Sciences du vivant; Biochimie; biophysique & biologie moléculaire; Antarctic; Antarc*
• Online Access: https://orbi.uliege.be/handle/2268/15376; https://doi.org/10.1016/j.jmb.2006.03.004

peer reviewed A new microcalorimetric method for recording the kinetic parameters k(cat)/K-m and K-i of alpha-amylases using polysaccharides and oligosaccharides as substrates is described. This method is based on the heat released by glycosidic bond hydrolysis. The method has been developed to study the active site properties of the cold-active alpha-amylase produced by an Antarctic psychrophilic bacterium in comparison with its closest structural homolog from pig pancreas. It is shown that the psychrophilic a-amylase is more active on large macromolecular substrates and that the higher rate constants k(cat) are gained at the expense of a lower affinity for the substrate. The active site is able to accommodate larger inhibitory complexes, resulting in a mixed-type inhibition of starch hydrolysis by maltose. A method for recording the binding enthalpies by isothermal titration calorimetry in a low-affinity system has been developed, allowing analysis of the energetics of weak ligand binding using the allosteric activator chloride. It is shown that the low affinity of the psychrophilic a-amylase for chloride is entropically driven. The high enthalpic and entropic contributions of activator binding suggest large structural fluctuations between the free and the bound states of the cold-active enzyme. The kinetic and thermodynamic data for the psychrophilic a-amylase indicate that the strictly conserved side-chains involved in substrate binding and catalysis possess an improved mobility, responsible for activity in the cold, and resulting from the disappearance of stabilizing interactions far from the active site. (c) 2006 Elsevier Ltd. All rights reserved.