Structural and enzymatic analyses of Anabaena heterocyst-specific alkaline invertase InvB.

Anabaena sp. PCC 7120 encodes two alkaline/neutral invertases, namely InvA and InvB. Following our recently reported InvA structure, here we report the crystal structure of the heterocyst-specific InvB. Despite sharing an overall structure similar to InvA, InvB possesses a much higher catalytic activity. Structural comparisons of the catalytic pockets reveal that Arg430 of InvB adopts a different conformation, which may facilitate the deprotonation of the catalytic residue Glu415. We propose that the higher activity may be responsible for the vital role of InvB in heterocyst development and nitrogen fixation. Furthermore, phylogenetic analysis combined with activity assays also suggests the role of this highly conserved arginine in plants and cyanobacteria, as well as some proteobacteria living in highly extreme environments.