[3] Glutamate dehydrogenases from hyperthermophiles

This chapter focuses on a key enzyme of nitrogen metabolism in hyperthermophiles, glutamate dehydrogenase (GDH). GDHs are widely distributed enzymes involved in ammonia assimilation and catabolism of glutamate in microorganisms. GDH catalyses the reversible oxidative deamination of glutamate to 2-oxoglutarate and ammonia using NAD or NADP as cofactors. GDH represents an enzymatic link between major catabolic and biosynthetic pathways via the tricarboxylic acid (TCA) cycle intermediate 2-oxoglutarate. Analysis of GDH structures has provided the basis for comparative studies of extreme protein thermostability in this relatively complex enzyme. The chapter provides an overview of the purification and properties of thermostable GDHs and a review of the contributions that these studies have made to the understanding of the basis for exceptional enzyme stability.