Characterization of Glutamine Synthetase Isoforms from Chlorella

Abstract

ABSTRACr Ion-exchange chromatography of extracts derived from Chlorella so-rokiniana mutant strain (oxygen resistant) yielded two separate activity peaks of glutamine synthetase (GS). GS, and GS,, were purified 220-and 187-fold and have molecular weights of approximately 398,000 and 360,000, respectively. Both enzymes are composed of eight identical subunits with a subunit molecular weight of 47,000 for GS, and 43,000 for GS,,. The amino acid composition, catalytic, and immunological properties for both enzymes are similar. In algae and plant leaves, two molecular forms of GS3 (L-glutamate:ammonia ligase [adenosine 5'-diphosphate forming], EC 6.3.1.2) have been found (2, 5, 15). The enzymes are present in different intracellular compartments. GS, is present in the cytosol and GS,, is located in the chloroplasts in plant leaves (15). In algae, the intracellular compartmentation of both en-zymes remains to be determined. A detailed comparison of purified GS, and GS,, is of interest considering the important roles both enzymes play in photosynthetic organisms. Not only is GS involved in primary ammonia assimilation but it also is responsible for the detoxification of NH3 produced during pho-torespiration via the glycine/serine pathway of glycolate metab-olism (1 1). It is presently being debated whether GS,, GS,,, or both enzymes are involved in the latter process (2, 15). In this paper, we report the purification and characterization ofGS, and GS,, from a mutant of C. sorokiniana which has an increased capacity for the metabolism of glycolate via the glycine/serine pathway. In addition, this organism exhibits an efficient GS-mediated assimilation of NH3 produced during photorespiration