beta-Glutamate as a substrate for glutamine synthetase.

The conversion of beta-glutamate to beta-glutamine by archaeal and bacterial glutamine synthetase (GS) enzymes has been examined. The GS from Methanohalophilus portucalensis (which was partially purified) is capable of catalyzing the amidation of this substrate with a rate sevenfold less than the rate obtained with alpha-glutamate. Recombinant GS from the archaea Methanococcus jannaschii and Archaeoglobus fulgidus were considerably more selective for alpha-glutamate than beta-glutamate as a substrate. All the archaeal enzymes were much less selective than the two bacterial GS (from Escherichia coli and Bacillus subtilis), whose specific activities towards beta-glutamate were much smaller than rates with the alpha-isomer. These results are discussed in light of the observation that beta-glutamate is accumulated as an osmolyte in many archaea while beta-glutamine (produced by glutamine synthetase) is used as an osmolyte only in M. portucalensis.