ISOLATION AND ENZYMATIC-PROPERTIES OF LEVANSUCRASE SECRETED BY ACETOBACTER-DIAZOTROPHICUS SRT4, A BACTERIUM ASSOCIATED WITH SUGAR-CANE

Acetobacter diazotrophicus, a nitrogen-fixing bacterium associated with sugar cane, secretes a levansucrase (sucrose-2,6-beta-D-fructan 6-beta-D-fructosyltransferase; EC 2.4.1.10). This enzyme is constitutively expressed and represents more than 70 % of the total proteins secreted by strain SRT4. The purified protein consists of a single 58 kDa polypeptide with an isoelectric point of 5.5. Its activity is optimal at pH 5.0, It catalyses transfructosylation from sucrose to a variety of acceptors including water (sucrose hydrolysis), glucose (exchange reaction), fructan (polymerase reaction) and sucrose (oligofructoside synthesis). In vivo the polymerase activity leads to synthesis of a high-molecular-mass fructan of the levan type. A. diazotrophicus levansucrase catalyses transfructosylation via a Ping Pong mechanism involving the formation of a transient fructosyl-enzyme intermediate. The catalytic mechanism is very similar to that of Bacillus subtilis levansucrase. The kinetic parameters of the two enzymes are of the same order of magnitude. The main difference between the two enzyme specificities is the high yield of oligofructoside, particularly 1-kestotriose and kestotetraose, accumulated by A. diazotrophicus levansucrase during sucrose transformation. We discuss the hypothesis that these catalytic features may serve the different biological functions of each enzyme.