We studied the mechanism of an observed significant enhancement in enzymatic saccharification of lignocelluloses at elevated pH of 5.5 � 6.0.  Four enzymatic hydrolysis lignin residues with different sulfonic acid group contents were isolated from lodgepole pine pretreated by either dilute acid or Sulfite Pretreatment to Overcome Recalcitrance of Lignocelluloses (SPORL). The adsorption isotherms of a commercial Trichoderma reesi cellulase cocktail (CTec2) to these lignin residues at 50oC were measured at a pH range of 4.5 � 6.0.  The zeta potentials of these lignin samples were also measured.  The study discovered that elevated pH significantly increased lignin surface charge (negative shown in the figure below), causing lignin to become more hydrophilic, coordinately reducing its affinity to cellulase and consequently the nonspecific binding of cellulase.  The reduction in nonspecific cellulase binding to lignin is also attributed to enhanced electrostatic interactions at elevated pH through the increased negative charges of cellulase enzymes with low pI As shown in the figure below.  The results validate the hypothesis that the gains in enzymatic saccharification efficiencies at elevated pH for different pretreated lignocelluloses are solely the result of reduced nonspecific cellulase binding to lignin.  This study contradicts a well-established concept of �optimal pH 4.8 � 5.0� for enzymatic hydrolysis using Trichoderma reesi cellulase that is widely accepted and exclusively practiced in numerous laboratories throughout the world.  Because an elevated pH can be easily implemented commercially without capital cost and with only minimal operating cost, this study has both scientific importance and practical significance.

Why enzymatic hydrolysis of lignocelluloses should be conducted at elevated pH 5.2-6.2

247th National Meeting (2014)

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