Sucrose biosynthesis in a prokaryotic organism: Presence of two sucrose-phosphate synthases in Anabaena with remarkable differences compared with the plant enzymes

Biosynthesis of sucrose-6-P catalyzed by sucrose-phosphate synthase (SPS), and the presence of sucrose-phosphate phosphatase (SPP) leading to the formation of sucrose, have both been ascertained in a prokaryotic organism: Anabaena 7119, a filamentous heterocystic cyanobacterium. Two SPS activities (SPS-I and SPS-II) were isolated by ion-exchange chromatography and partially purified. Four remarkable differences between SPSs from Anabaena and those from higher plants were shown: substrate specificity, effect of divalent cations, native molecular mass, and oligomeric composition. Both SPS-I and SPS-II accept Fru-6-P (K(m) for SPS-I = 0.8 ± 0.1 mM; K(m) for SPS-II = 0.7 ± 0.1 mM) and UDP-Glc as substrates (K(m) for SPS-I = 1.3 ± 0.4 mM; K(m) for SPS-II = 4.6 ± 0.4 mM), but unlike higher plant enzymes, they are not specific for UDP-Glc. GDP-Glc and TDP-Glc are also SPS-I substrates (K(m) for GDP-Glc = 1.2 ± 0.2 mM and K(m) for TDP-Glc = 4.0 ± 0.4 mM), and ADP-Glc is used by SPS-II (K(m) for ADP-Glc = 5.7 ± 0.7 mM). SPS-I has an absolute dependence toward divalent metal ions (Mg(2+) or Mn(2+)) for catalytic activity, not found in plants. A strikingly smaller native molecular mass (between 45 and 47 kDa) was determined by gel filtration for both SPSs, which, when submitted to SDS/PAGE, showed a monomeric composition. Cyanobacteria are, as far as the authors know, the most primitive organisms that are able to biosynthesize sucrose as higher plants do.

蔗糖磷酸合酶（sucrose-phosphate synthase, SPS）催化蔗糖-6-磷酸的生物合成，而蔗糖磷酸磷酸酶（sucrose-phosphate phosphatase, SPP）介导的反应可生成蔗糖，这两条代谢途径均在原核生物念珠藻7119（Anabaena 7119）——一种丝状异形胞蓝藻——中得到证实。研究人员通过离子交换层析分离得到两种SPS活性组分（SPS-I与SPS-II），并对其进行了部分纯化。与高等植物来源的SPS相比，念珠藻来源的酶存在四项显著差异：底物特异性、二价阳离子的调控作用、天然分子质量以及寡聚体组成。SPS-I与SPS-II均以果糖-6-磷酸（Fru-6-P）和UDP-葡萄糖（UDP-Glc）为底物：SPS-I对Fru-6-P的米氏常数（K(m)）为0.8 ± 0.1 mM，对UDP-Glc的K(m)为1.3 ± 0.4 mM；SPS-II对Fru-6-P的K(m)为0.7 ± 0.1 mM，对UDP-Glc的K(m)为4.6 ± 0.4 mM。但与高等植物的SPS不同，二者并非仅特异性识别UDP-Glc：GDP-葡萄糖（GDP-Glc）与TDP-葡萄糖（TDP-Glc）同样可作为SPS-I的底物（GDP-Glc的K(m)为1.2 ± 0.2 mM，TDP-Glc的K(m)为4.0 ± 0.4 mM），而ADP-葡萄糖（ADP-Glc）则可被SPS-II利用（其K(m)为5.7 ± 0.7 mM）。SPS-I的催化活性绝对依赖二价金属离子（Mg²+或Mn²+），这一特性在高等植物来源的SPS中并未被发现。通过凝胶过滤层析测定，两种SPS的天然分子质量均仅为45~47 kDa，远小于高等植物来源的酶；经十二烷基硫酸钠-聚丙烯酰胺凝胶电泳（SDS/PAGE）分析后，二者均呈现单亚基组成。据本文作者所知，蓝藻是目前已知能够像高等植物一样生物合成蔗糖的最原始生物类群。