Functional Modification of the Substrate-Binding Site for Isomaltulose Production Based on Predicted Structure of Sucrose Isomerase from Pantoea dispersa UQ68 J

摘要

Isomaltulose (α-D-glucopyranosyl-1, 6-D-fructofuranose) is a naturally occurring isomer of sucrose commonly referred to as palatinose. As a nutritional sugar found in honey with trace amount [1], it can prevent tooth decay and control insulin levels in the bloodstream. Furthermore, it is healthy for diabetics and non-diabetics, because it is digested more slowly than sucrose in intestine. Therefore, it is a potential functional sugar for human consumption [2]. Compared with chemical synthesis of isomaltulose, the bioconversion of isomaltulose could be carried out at ambient temperature and atmospheric pressure, thus avoiding the use of more extreme conditions causing problems with isomerization, racemization, epimerization and rearrangement [3]. The reaction process of sucrose isomerases (SIs) consists of sucrose binding, sucrose hydrolysis, glucosyl-enzyme intermediate occurrence, fructose release and sucrose isomer formation [4]. The immobilized microbial cells using chitosan embedding of Serratia plymuthica can only convert 40% of sugarcane to isomaltulose [5], SmuA from Propionibacterium rubrum produces 85% of isomaltulose, together with a minor amount of trehalulose, monosaccharide and isomaltose [6]. Therefore, improving isomaltulose production while controlling the amounts of side reaction products remains the considerable industrial problem.