7 - Enzymes of Glutamate Formation: Glutamate Dehydrogenase, Glutamine Synthetase, and Glutamate Synthase

Abstract

This chapter discusses enzymes of glutamate formation—glutamate dehydrogenase (GDH), glutamine synthetase, and glutamate synthase. The amino acid, glutamate, and its amide called glutamine are the primary products of inorganic nitrogen assimilation and as such occupy central positions in intermediary nitrogen metabolism. The chapter describes the characteristics, properties, and behavior of glutamate dehydrogenase, glutamine synthetase, and glutamate synthase. Mitochondrial GDH enzymes are isolated and characterized from a number of plant tissues including pea roots, lettuce leaves, mung bean seedlings, and fronds of Lemna minor. These enzymes react with both nicotinamide adenine dinucleotide hydride (NADH) and nicotinamide adenine dinucleotide phosphate in the aminating direction but show greater activity with NADH. Glutamine synthetase is found throughout the plant and animal kingdoms and is characterized from a number of sources including bacteria, algae, fungi, and higher plants and animals. Angiosperm glutamine synthetase exhibits an absolute requirement for divalent metal cations, which can be satisfied by Mg2+ and, to a lesser degree, Mn2+ and Co2+. The catalytic activity of glutamate synthases is unstable.