Modified helicases

A kit for characterising a target polynucleotide comprising (a) a pore and (b) a helicase comprising a polynucleotide binding domain, wherein two amino acid residues that are located on a surface of the helicase are covalently connected via a linkage between the two amino acid residues to form a covalently-closed structure, wherein the pore is a transmembrane pore, wherein the transmembrane pore is:

• [i] derived from a hemolysin, leukocidin, Mycobacterium smegmatis porin A (MspA), MspB, MspC, MspD, outer membrane porin F (OmpF), outer membrane porin G (OmpG), outer membrane phospholipase A, Neisseria autotransporter lipoprotein (NalP) and WZA, or
• [ii] a β-barrel pore or an α-helix bundle pore, 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), and wherein the two amino acid residues that are located on the surface of the helicase are on one or more loop regions connecting α-helices and β-strands of the helicase and/or are spatially located proximal to the polynucleotide binding domain.