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

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The National Academy of Sciences of the USA

RESUMO

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 (Km for SPS-I = 0.8 ± 0.1 mM; Km for SPS-II = 0.7 ± 0.1 mM) and UDP-Glc as substrates (Km for SPS-I = 1.3 ± 0.4 mM; Km 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 (Km for GDP-Glc = 1.2 ± 0.2 mM and Km for TDP-Glc = 4.0 ± 0.4 mM), and ADP-Glc is used by SPS-II (Km for ADP-Glc = 5.7 ± 0.7 mM). SPS-I has an absolute dependence toward divalent metal ions (Mg2+ or Mn2+) 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.

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