Extremely thermostable glutamate dehydrogenase from the hyperthermophilic archaebacterium Pyrococcus furiosus

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Abstract
The hyperthermophilic archaebacterium Pyrococcus furiosus contains high levels of NAD(P)‐dependent glutamate dehydrogenase activity. The enzyme could be involved in the first step of nitrogen metabolism, catalyzing the conversion of 2‐oxoglutarate and ammonia to glutamate. The enzyme, purified to homogeneity, is a hexamer of 290 kDa (subunit mass 48 kDa). Isoelectric‐focusing analysis of the purified enzyme showed a pI of 4.5. The enzyme shows strict specificity for 2‐oxoglutarate and l‐glutamate but utilizes both NADH and NADPH as cofactors. The purified enzyme reveals an outstanding thermal stability (the half‐life for thermal inactivation at 100°C was 12 h), totally independent of enzyme concentration. P. furiosus glutamate dehydrogenase represents 20% of the total protein; this elevated concentration raises questions about the roles of this enzyme in the metabolism of P. furiosus.