Submitted to: Plant Physiology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2002
Publication Date: 2/1/2003
Citation: KLOTZ, K.L., FINGER, F.L., SHELVER, W.L. CHARACTERIZATION OF TWO SUCROSE SYNTHASE ISOFORMS IN SUGARBEET ROOT. PLANT PHYSIOLOGY AND BIOCHEMISTRY. 2003. V. 41 P. 107-115. Interpretive Summary: Sugarbeet roots contain several enzymes that degrade sucrose, including acid invertases, alkaline invertases and sucrose synthases. These activities impact sucrose accumulation during production of the sugarbeet crop and sucrose retention during postharvest storage. The major sucrose degrading activity in sugarbeet roots throughout most of their development and during postharvest storage is sucrose synthase. Earlier research identified two sucrose synthase isoforms in sugarbeet roots. To aid in understanding the significance of these two sucrose synthase isoforms, the two isoforms were partially purified and some of their physical and kinetic properties determined. The two isoforms were similar in size, kinetic properties and exhibited similar responses to temperature. They differed, however, in their subunit composition and their response to changes in pH conditions. These differences 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.
Technical Abstract: Two sucrose synthase isoforms (ED 18.104.22.168) 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 weights 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 composed of four subunits of 86 kDa. The two isoforms also differed in their reactivity in response to varying pH conditions. The optimum pH for sucrose cleaving activity was observed at pH 6.0 and pH 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 the two isoforms and modulation of their activity by changes in cellular pH.