title: No evidence for the occurrence of substrate inhibition of Arabidopsis thaliana sucrose synthase-1 (AtSUS1) by fructose and UDP-glucose.
abstract: Sucrose synthase (SuSy) catalyzes the reversible conversion of sucrose and NDP into the corresponding nucleotide-sugars and fructose. The Arabidopsis genome possesses six SUS genes (AtSUS1-6) that code for proteins with SuSy activity. As a first step to investigate optimum fructose and UDP-glucose (UDPG) concentrations necessary to measure maximum sucrose-producing SuSy activity in crude extracts of Arabidopsis, in this work we performed kinetic analyses of recombinant AtSUS1 in two steps: (1) SuSy reaction at pH 7.5, and (2) chromatographic measurement of sucrose produced in step 1. These analyses revealed a typical Michaelis-Menten behavior with respect to both UDPG and fructose, with Km values of 50 μM and 25 mM, respectively. Unlike earlier studies showing the occurrence of substrate inhibition of UDP-producing AtSUS1 by fructose and UDP-glucose, these analyses also revealed no substrate inhibition of AtSUS1 at any UDPG and fructose concentration. By including 200 mM fructose and 1 mM UDPG in the SuSy reaction assay mixture, we found that sucrose-producing SuSy activity in leaves and stems of Arabidopsis were exceedingly higher than previously reported activities. Furthermore, we found that SuSy activities in organs of the sus1/sus2/sus3/sus4 mutant were ca. 80-90% of those found in WT plants.
authors: Instituto de Agrobiotecnología, Universidad Pública de Navarra, Consejo Superior de Investigaciones Científicas, Gobierno de Navarra, Mutiloako Etorbidea Zenbaki Gabe, Nafarroa, Spain.
authors: Goizeder
authors: G
authors: Almagro
authors: Edurne
authors: E
authors: Baroja-Fernández
authors: Francisco José
authors: FJ
authors: Muñoz
authors: Abdellatif
authors: A
authors: Bahaji
authors: Ed
authors: E
authors: Etxeberria
authors: Jun
authors: J
authors: Li
authors: Manuel
authors: M
authors: Montero
authors: Maite
authors: M
authors: Hidalgo
authors: María Teresa
authors: MT
authors: Sesma
authors: Javier
authors: J
authors: Pozueta-Romero
mesh_terms: D017360:Arabidopsis / Q000201:enzymology*; D029681:Arabidopsis Proteins / Q000037:antagonists & inhibitors* / Q000378:metabolism; D005632:Fructose / Q000494:pharmacology*; D005964:Glucosyltransferases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D007700:Kinetics; D010936:Plant Extracts / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D013379:Substrate Specificity / Q000187:drug effects; D013395:Sucrose / Q000378:metabolism; D014532:Uridine Diphosphate Glucose / Q000494:pharmacology*
chemical_list: D029681:Arabidopsis Proteins; D010936:Plant Extracts; D011994:Recombinant Proteins; D005632:Fructose; D013395:Sucrose; D005964:Glucosyltransferases; C022699:sucrose synthase; C578062:sucrose synthase-1, Arabidopsis; D014532:Uridine Diphosphate Glucose