Identification and characterization of a glutamate dehydrogenase in the unicellular cyanobacterium Synechocystis PCC 6803

Glutamate dehydrogenase activity has been detected in permeabilized cells of the unicellular cyanobacterium Synechocystis PCC 6803 at similar levels to those of glutamate synthase. The enzyme responsible has been purified by affinity chromatography on 2′,5′‐ADP‐Sepharose and identified as an NADPH‐specific glutamate dehydrogenase. The enzyme catalyzes preferentially glutamate formation rather than the reverse reaction, with K m values for NADPH, 2‐oxoglutarate and ammonia of 20 μM, 1.5 mM and 3.7 mM, respectively. It is composed of four identical subunits giving a total molecular mass of 208 kDa for the native protein. Its physiological role is discussed in terms of being an alternative pathway to the glutamine synthetase‐glutamate synthase cycle for ammonia assimilation.