Modified helicases

An NS3 helicase comprising a polynucleotide binding domain which comprises in at least one conformational state an opening through which a polynucleotide can unbind from the helicase, wherein the helicase is modified such that two or more parts of the helicase are connected using one or more linkers and wherein the helicase retains its ability to control the movement of the polynucleotide, wherein the linkers connect two amino acid residues that are located in different structural domains on the surface of the NS3 helicase surrounding the polynucleotide binding domain or wherein the linkers connect two amino acid residues on one or more loop regions connecting α-helices and β-strands of the NS3 helicase and wherein at least one amino acid of the two amino acid residues is substituted with cysteine, a non-natural amino acid or 4-azido-L-phenylalanine (Faz).