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Isoforms of Glutamine Synthetase in the Marine Coccolithophorid Emiliania huxleyi (Prymnesiophyceae)

1997; Elsevier BV; Volume: 118; Issue: 4 Linguagem: Inglês

10.1016/s0305-0491(97)00279-4

1879-1107

Claire Maurin, Yves Le Gal,

Marine and coastal ecosystems

Two isoenzymes of glutamine synthetase (EC 6.3.1.2), GS1 and GS2, have been purified from cells of Emiliania huxleyi using Cibacron blue dye ligand chromatography and gel filtration, separated by ion-exchange chromatography on Mono-Q and partly characterized. Each enzyme is a homohexamer with a molecular mass of 402 kDa for GS1 and 501 kDa for GS2. The molecular mass of the subunits of GS1 and GS2 was estimated to be 61 and 78 kDa, respectively. As in higher plants, GS1 is slightly more thermostable than GS2 and much less stimulated by thiols than GS2. For these reasons, GS1 was designated as the cytosolic enzyme and GS2 as the chloroplastic one. Although the Kms for NH2OH are about the same, GS2 possesses a much higher affinity for glutamine than GS1. As in bacteria, ATP appears to play an important role in the allosteric regulation of GS2. l-Ala and CTP are potent inhibitors of GS1 activity. CTP, carbamoyl-phosphate and l-Ala exert a cumulative inhibitory effect on GS1 activity. GS2 is also inhibited to some extent by l-Ala and l-His. NH2-terminal sequence analysis of GS2 did not show any homology with bacteria, cyanobacteria or higher plants.