Characterization of two sucrose synthase isoforms in sugarbeet root

Two sucrose synthase isoforms (EC 2.4.1.13) have been identified in developing sugarbeet (Beta vulgaris L.) roots. To aid in understanding the physiological significance of these multiple sucrose synthase isoforms, the two isoforms were partially purified and some of their physical and kinetic properties determined. Both isoforms were tetrameric proteins with native molecular masses of 320 kDa. The isoforms exhibited similar kinetic properties as well as similar changes in activity in response to changes in temperature. The isoforms differed, however, in their subunit composition. Sucrose synthase isoform I (SuSyI) was composed of two 84 kDa subunits and two 86 kDa subunits. Sucrose synthase isoform II (SuSyII) was a homotetramer with a subunit size of 86 kDa. The amino acid composition of the two subunits was similar, although differences in alanine, glycine, isoleucine and lysine content were noted. The activity of the two isoforms differed in response to varying pH conditions. The optimum pH for sucrose cleaving activity was observed at pH 6.0 and 6.5 for SuSyI and SuSyII, respectively. The optimum pH for sucrose synthesizing activity occurred at pH 7.5 and 7.0 for SuSyI and SuSyII, respectively. The observed differences in subunit composition and reactivity at different pH values suggest that multiple isoforms of sucrose synthase may provide a mechanism to regulate sucrose metabolism in sugarbeet root by differential regulation of expression of the two isoforms and modulation of their activity by changes in cellular pH.