Enzymatic Polyreactions

The enzymatic synthesis of certain polysaccharides is often considered to be by a polyinsertion mechanism. An example of this is the formation of the dextran polyglucoside (see Chapter 31) from saccharose under the influence of the dextran saccharase enzyme. Fructose is eliminated in this reaction. It is assumed that the polymer chain P formed in previous steps is absorbed onto the enzyme. The substrate S (in this case, saccharose) is also absorbed onto the enzyme. A substrate-enzyme complex SEP is formed from the enzyme-polymer complex EP with the substrate S. This is then converted into an enzyme/polymer complex EP +i by insertion of the glucose residue of saccharose with liberation of fructose  

If neither the equilibrium constant K nor the rate constant fe, depends on the degree of polymerization, and if the insertion step (19-41) is the rate- 

The rate is proportional to the substrate concentration and to the concentration of the EP complex. The proportionality constant consists of a rate constant and an equilibrium constant 

The enzyme is continuously bound to the polymer chain in a one-chain mechanism. Multichain mechanisms are much more common. In these, the enzyme disengages after every linking step and thus wanders from chain to chain (see also polycondensation. Chapter 17). In the multichain mechanism, the enzyme and the substrate form an enzyme-substrate complex ES in the first step  

In the second step, the substrate-enzyme complex separates into the product P and the enzyme E  

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It is frequently observed that Vmax and Km in enzymatic polyreactions are dependent on the degree of polymerization. This can be explained as follows 1. The binding strength of the nonreducing end of the sugar depends on the degree of polymerization, i.e., the principle of equal chemical reactivity... 

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