Higher plant sucrose-phosphate synthase: Structure, function and regulation

523-529Sucrose-phosphate synthase (SPS) in higher plants catalyzes the penultimate and pivotal step of sucrose synthesis. SPS has a broad pH optimum for its activity and 'catalyzes a order bi bi reaction. SPS shows hyperbolic substrate saturation kinetics for both the substrates, UDP-glucose and fructose-6-phosphate. The enzyme is allosterically activated by glucose-6-phosphate and inhibited by Pi. These two effectors change the enzyme's affinity for both the substrates. SPS is believed to be a homodimer of 120-138 kd subunits with no interchain disulfide bridges. It has a conserved N-terminal portion with variable C-terminal part. Cloning of SPS gene has been accomplished from some plant species. SPS shows high sequence homology among the plant species. A glycine-rich region in the N-terminal portion is a distinctive feature of SPS from monocot plant species but is absent in dicots. Rice SPS gene has been mapped on chromosome 1. Genome organization study reveals that rice SPS gene consists of 12 exons and 11 introns. Light modulates SPS activity by covalent modification (coarse control) and allosteric regulation (fine control). In dark SPS is phosphorylated at a specific serine residue (Ser-158 in case of spinach SPS) by a specific protein kinase and becomes less active. Light activation of SPS activity involves dephosphorylation by a type 2A protein phosphatase. Covalent modification alters the kinetic properties of the enzyme. The diurnal and circadian rhythm of SPS activity is because of light regulation of de novo expression of SPS-phosphatase. The tissue metabolites, glucose-6-phosphate and Pi, not only alter the SPS activity but also affect the enzymes involved in covalent modulation of SPS. Glucose-6-phosphate inhibits SPS protein kinase and Pi inhibits SPS-phosphatase. The fluctuation of leaf Pi concentrations during light-dark transitions has been proposed to play a major role in the signal amplification of SPS regulation. The prospects of generation of transgenic plants overexpressing SPS have also been discussed