Molecular and kinetic characterisation of sugarcane pyrophosphate: fructose-6-phosphate 1-phosphotransferase and its possible role in the sucrose accumulation phenotype

Abstract

The amount of pyrophosphate: fructose-6-phosphate 1-phosphotransferase (PFP) activity in sugarcane internodal tissue is inversely correlated with sucrose content. To help elucidate this apparent role of PFP in sucrose accumulation in sugarcane we have determined its molecular and kinetic properties. Sugarcane PFP was purified 285-fold to a final specific activity of 4.23 µmol min^-1 mg^-1 protein. It contained two polypeptides of 63.2 and 58.0 kDa respectively, at near equal amounts that cross-reacted with potato PFP-α and -β antiserum. In gel filtration analyses the native enzyme eluted in three peaks of 129, 245 and 511 kDa, corresponding to dimeric, tetrameric and octameric forms, respectively and fructose 2,6-bisphosphate (Fru 2,6-P₂) influenced this aggregation state. Both the glycolytic (forward) and gluconeogenic (reverse) reactions had relative broad pH optima between pH 6.7 and 8.0. The Fru 2,6-P₂ saturation curves were hyperbolic with approximate K_a values of 69 and 82 nm for the forward and reverse reactions, respectively. The enzyme showed hyperbolic saturation curves for all its substrates with K_m values comparable with that of other plant PFP, i.e. 150, 37, 39 and 460 µM for fructose 6-phosphate, inorganic pyrophosphate, fructose 1,6-bisphosphate and inorganic phosphate, respectively. Sugarcane PFP's molecular and kinetic characteristics differed slightly from that of other plant PFP in that: (i) Fru 2,6-P₂ directly induced the octameric state from the dimeric state; (ii) Fru 2,6-P₂ shifted the pH optimum for the forward reaction to a slightly more basic pH; and (iii) Fru 2,6-P₂ increased the V_max for the forward and reverse reactions by similar amounts.