Cross-linked aggregation of glutamate decarboxylase to extend its activity range toward alkaline pH

Abstract

BACKGROUNDGamma()-aminobutyric acid (GABA) is produced through an -decarboxylation reaction of L-monosodium glutamate (MSG) using glutamate decarboxylase (GAD). The pH rise caused by the reaction inactivates the enzyme catalyst, which is active only under acidic conditions, and consequently leads to low reaction conversions. Employment of conventional acids and buffers inevitably forms salts, which result in serious problems in separation and purification of GABA. It is essential to render GAD active even at neutral and alkaline pHs. In the present study, we first apply a cross-linked aggregation method in order to extend the active range of GAD toward alkaline pH. RESULTSGAD from Escherichia coli was prepared as cross-linked enzyme aggregate (CLEA) in which the enzyme was precipitated using ammonium sulfate (60% saturation) and then cross-linked with glutaraldehyde (2%) in sodium acetate buffer (0.2molL(-1), pH 4.6). The cross-linked aggregation extended an active pH range of GAD from 5.5 up to 8.0; as a result, the reaction conversion of 1molL(-1) MSG into GABA was improved from 13% to 22%. Moreover, the CLEA of GAD was easily recovered after the reaction and reused retaining >95% of its initial activity during the first four cycles and >60% activity at the 10th cycle. CONCLUSIONCross-linked aggregation could make GAD active even at neutral and alkaline pHs. It is shown to be a useful method capable of facilitating recovery and reuse of the enzyme as well as increasing the reaction conversion by extending the active pH range of GAD. (c) 2015 Society of Chemical Industry