[Glutamine synthetase of the rhizobacterium Azospirillum brasilense: specific features of catalysis and regulation]

Abstract

Data on Glutamine synthetase (GS) of Azospirillum brasilense, a rhizobacterium stimulating plant growth, have been reviewed. GS of the azospirillum is a type alpha12 dodecamer with oligomer and monomer having molecular weights of 630 and 52 kDa, respectively. Glutamine synthesis is performed in 12 active sites of the enzyme, depending, first and foremost, on the extent of GS adenylation and, secondarily, on the exact bivalent metal cations involved in the catalysis. Structural characteristics and catalytic properties of the completely unadenylated and moderately adenylated forms of GS of A. brasilense have been studied. The enzyme appears as a highly structured protein, with alpha helices and beta structures accounting for about 70% of the polypeptide chain length. Binding of Mg2+, Co2+, and Mn2+ to the protein globule changes both the secondary structure and the catalytic properties of the enzyme. The uses of nuclear gamma resonance emission spectroscopy demonstrates that the active center of GS of the azospirillum has two metal-binding sites differing in their affinity for Co2+. The activity and biosynthesis of GS of the azospirillum is regulated by wheat lectin (a molecular signal of the host plant), in addition to other means of regulation described for GSs.