The invention relates to a method for catalysing complex reactions or large molecules, more specifically, to enzyme-catalysed reactions during which undesirable consecutive or subsidiary reactions usually occur; using enzymes which are bonded to a polymer support. According to the invention, undesirable consecutive or subsidiary reactions are to a large extent avoided by selecting a non-porous or almost non-porous support material. In particular, the invention relates to a method for the enzymatic extraction of biomolecules, preferably peptides, proteins, oligosaccharides or polysaccharides from their biologically inactive precursors using enzymes which are bonded to a polymer support, especially a method for extracting insulins or their analogs from the corresponding precursors using enzymes which are bonded to a polymer support. As a result of selecting a non-porous almost non-porous support material, this method leads to a selective formation of biomolecules, especially of insulins or insulin analogs and corresponding valuable substances which can be split into said insulins or their analogs, undesirable consecutive or subsidiary reactions being to a large extent avoided.