Structure to function studies on sucrose synthases from Anabaena variabilis

MINEN RI; FIGUEROA CM; ASENCION DIEZ MD; BALLICORA MA; IGLESIAS AA

Córdoba

Congreso; LII Reunión Anual de la SAIB; 2016

SAIB

Sucrose synthase (SucSase, EC 2.4.1.13)catalyzes the reversible conversion of NDP-glucose and fructose to sucrose andNDP. Plant SucSase has been extensively studied but little is known about the cyanobacterialenzyme. Anabaena variabilis (a filamentousheterocyst-forming cyanobacterium) has two genes encoding putative SucSases (susA and susB). In this work we show that recombinant SucSase A is active,whereas SucSase B has negligible catalytic activity. Homology modeling studiesusing the crystal structure of Nitrosomonaseuropaea SucSase as template showed that SucSase B lacks Pro132 (replacedby Ser) and Phe162 (replaced by Ala), which are important for the interactionbetween A:D and A:B subunits of the tetramer, respectively. Using gelfiltration chromatography we determined that SucSase A is a tetramer and SucSaseB is a dimer, suggesting that the quaternary structure is important for thecatalytic activity. We hypothesize that, after gene duplication, SucSase Bevolved to play a new (regulatory?) role, as it has been described for the plantADP-glucose pyrophosphorylase large subunit. Alternatively, SucSase B mightcatalyze a yet unidentified reaction, different from sucrosesynthesis/degradation. Experiments to test these hypotheses are currentlyunderway.