Sucrose-fructose exchange

If a very low molar concentration of enzyme is present, and a large excess of nonradioactive fructose is added, the enzyme will catalyze no net reaction but will change back and forth repeatedly between the free enzyme and glucosyl enzyme. Each time, in the reverse reaction, it will make use primarily of unlabeled fructose. The net effect will be catalysis of a sucrose-fructose exchange ... 

Interestingly, in the absence of sucrose and fructose, sucrose phosphory-lase will catalyze the exchange of inorganic phosphate, P , into glucose-l-phos-phate. This reaction can be followed by using as a radioactive tracer and observing the appearance of P into glucose-l-phosphate ... 

Scheme 1 Sucrose is hydrolyzed into a mixture of glucose and fructose when exposed to an acid catalyst. Sulfuric acid has been used for this, but also heterogeneous solid acid catalysts have found use, especially acidic ion exchange resins.

Leucrose, 6-0-(a-D-glucopyranosyl)-p-D-fructopyranose [7158-70-5], is synthesized from sucrose using a dextranase enzyme from Leuconostoc mesenteriodes and a small proportion of fructose (2%). Pfeifer Langen of Germany have developed a production process for leucrose that involves extraction of the enzyme, treatment with 65% aqueous solution of sucrose and fructose (1 2 wt/wt) at 25°C, separation of the product from fructose by ion-exchange column chromatography, and crystallization. The product has not yet been launched on the market as of this writing (1996). 

Exchange reactions. In a double-displacement mechanism sucrose containing 14C in the fructose portion of the molecule should react with free enzyme E to form glycosyl enzyme and free radioactive fructose (Eq. 12-8). The 14C-containing groups are designated here by the asterisks. 

Later it was found that other monoses could be exchanged directly for D-fructose in the sucrose molecule. First, sucrose labelled with C14 in the D-fructose portion was prepared by the action of the Pseudomonas sac-charophila enzyme on ordinary sucrose and C14 labelled D-fructose.44 Subsequently D-fructose has been exchanged in the same manner for other monoses, for example, L-sorbose.48 Thus new oligosaccharides may be prepared by exchanging one monose for another through the action of this enzyme without the use of the phosphate intermediate. 

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