Macromolecular interactions: pectin, pectin methylesterase and beta-lactoglobulin

Abstract

Pectin is a complex polysaccharide found in the plant cell wall. Pathogenic bacteria express a set of enzymes called pectin methylesterase, which start the dismantling of the plant cell wall by de-esterifying homogalacturonan chains in pectin. The efficacy of pectin methylesterase in favouring plant infection may be due to their ability to act processively on pectin chains by catalysing many reactions cycles before dissociating from the polysaccharide. Herein, computational techniques were employed (i) to identify the key interactions between the pectin methylesterase from the bacterium Erwinia chrysanthemi and homogalacturonan oligomers and (ii) to find evidence that it is possible for the enzyme to act processively on pectin carbohydrates. Molecular dynamics simulations demonstrated that the protein binds the polysaccharide chain with different affinity at different sites along the binding interface and that such a strategy is fundamental for the processive catalysis. More importantly, an investigation of the conformational variations of the oligosaccharide lead to a mechanism by which the geometrical restraints impeding the processive action of the enzyme are removed. In a second research project, analytical ultracentrifugation experiments revealed the oligomeric nature of β-lactoglobulin A and B in solution. The experiments showed that both β-lactoglobulin variants are mostly dimeric all throughout the pH range investigated (2.5-7.5), although β-lactoglobulin had been considered mainly monomeric at low pH. Analytical ultracentrifugation experiments quantified, for the first time, the association kinetics of β-lactoglobulin dimer formation as a function of pH, whereas the dimer stability as a function of ionic strength was investigated by continuum electrostatic calculations. Besides the importance of pectin in plant physiology, complexes between pectin and whey proteins have found a wide use in the food and pharmaceutical industries. However, the basis of the interaction between pectin and whey proteins is still poorly understood. After the identification of β-lactoglobulin oligomeric state in solution, isothermal titration calorimetry experiments have been employed to investigate the binding between the protein and differently methylated pectin chains. The collected results suggested possible models by which β-lactoglobulin dimers and pectin interact.