Carboxyl group modification significantly altered the kinetic properties of purified carboxymethylcellulase from Aspergillus niger

Carboxymethylcellulase (CMCase) from Aspergillus niger NIAB280 was purified by a combination of ammonium sulphate precipitation, ion-exchange, hydrophobic interaction and gel filtration chromatography on FPLC with 9-folds increase in specific activity. Native and subunit molecular weights were found to be 36 kDa each. The purified CMCase was modified by 1-ethyl-3(3-dimethylaminopropyl) carbodiimide (EDC) in the presence of glycinamide for 15 min (GAM15) and glycinamide plus cellobiose for 75 min (GAM75). Similarly, the enzyme was modified by EDC in the presence of ethylenediamine dihydrochloride plus cellobiose for 75 min (EDAM75). The neutralization (GAM15 and GAM75) and reversal (EDAM75) of negative charges of carboxyl groups of CMCase had profound effect on the specificity constant (k(cat)/K(m)), pH optima, pK(a)'s of the active-site residues and thermodynamic parameters of activation. The specificity constants of native, GAM15, GAM75, and EDAM75 were 143, 340, 804, and 48, respectively. The enthalpy of activation (DeltaH(#)) of Carboxymethylcellulose (CMC) hydrolysis of native (50 and 15 kJ mol(-1)) and GAM15 (41 and 16 kJ mol(-1)) were biphasic whereas those of GAM75 (43 kJ mol(-1)) and EDAM75 (41 k J mol(-1)) were monophasic. Similarly, the entropy of activation (DeltaS(#)) of CMC hydrolysis of native (-61 and -173 J mol(-1) K(-1)) and GAM15 (-91 and -171 J mol(-1) K(-1)) were biphasic whereas those of GAM75 (-82 J mol(-1) K(-1)) and EDAM75 (-106 J mol(-1) K(-1)) were monophasic. The pH optima/pK(a)'s of both acidic and basic limbs of charge neutralized CMCases increased compared with those of native enzyme. The CMCase modification in the presence of glycinamide and absence of cellobiose at different pH's periodically activated and inhibited the enzyme activity indicating conformational changes. We believe that the alteration of the surface charges resulted in gross movement of loops that surround the catalytic pocket, thereby inducing changes in the vicinity of active site residues with concomitant alteration in kinetic and thermodynamic properties of the modified CMCases.