AbstractFollowing the introduction and widespread application of immobilized glucose isomerase for the industrial production of isosyrup (HFCS), the most significant challenge facing experts is to solve the problem of increasing the fructose content. In addition to well‐known procedures another approach may be the changing of the equilibrium of the enzymatic reaction by modifying the active centre of the enzyme. Histidyl‐, ϵ‐amino‐, arginyl‐, tyrosyl‐ and tryptophyl‐ amino acid side chains in the active centre have been modified by means of selective protein reagents. The results were evaluated by kinetic methods and the role of functional groups of amino side chains has been determined. A new hypothesis is proposed for the mechanism of enzym