Enzyme method

1. A method of characterising a target polynucleotide, comprising:

(a) contacting the target polynucleotide with a transmembrane pore, wherein a first helicase is bound to the target polynucleotide such that the first helicase controls the movement of the target polynucleotide through the pore;

(b) further contacting the target polynucleotide with a second helicase as the first helicase controls the movement of the target polynucleotide through the transmembrane pore, wherein the second helicase binds to an internal polynucleotide of the target polynucleotide at a location on the polynucleotide different than where the first helicase is bound without concurrent binding to a terminal nucleotide of the target polynucleotide, and wherein the second helicase controls movement of the target polynucleotide through the pore after the first helicase disengages from the target polynucleotide; and

(c) measuring one or more characteristics of the target polynucleotide during one or more interactions as the target polynucleotide moves through the transmembrane pore, wherein the one or more interactions are measured along a length of the target polynucleotide that comprises at least 500 nucleotides and exceeds processivity of the first helicase, thereby characterising the target polynucleotide.

2. The method according to claim 1 , wherein the second helicase is a Hel308 helicase, Hel308 Tga, Hel308 Mhu or Hel308 Csy.

3. The method according to claim 2 , wherein the Hel308 helicase comprises the amino acid motif Q-X1-X2-G-R-A-G-R (SEQ ID NO: 8), wherein X1 is C, M or L and X2 is any amino acid residue, preferably wherein X2 is A, F, M, C, V, L, I, S, T or P.

4. The method according to claim 2 , wherein the Hel308 helicase is one of the helicases shown in Table 4 or 5 or wherein the Hel308 helicase comprises the sequence shown in any one of SEQ ID NOs: 10, 13, 16, 19, 22, 25, 28, 29, 32, 33, 34, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55; and 58.

5. The method according to claim 1 , wherein the one or more characteristics are selected from (i) the length of the target polynucleotide, (ii) the identity of the target polynucleotide, (iii) the sequence of the target polynucleotide, (iv) the secondary structure of the target polynucleotide, and (v) whether or not the target polynucleotide is modified by methylation, by oxidation, by damage, with one or more proteins or with one or more labels, tags or spacers.

6. The method according to claim 1 , wherein the one or more characteristics of the target polynucleotide are measured by electrical measurement and/or optical measurement.

7. The method according to claim 6 , wherein the electrical measurement is a current measurement, an impedance measurement, a tunnelling measurement, or a field effect transistor (FET) measurement.

8. The method according to claim 1 , wherein the method further comprises the step of applying a voltage across the pore to form a complex between the pore and the first helicase and wherein at least a portion of the polynucleotide is double stranded.

9. The method according to claim 1 , wherein the transmembrane pore is a protein pore or a solid state pore.

10. The method according to claim 9 , wherein the pore is a transmembrane protein pore selected from a hemolysin, leukocidin, Mycobacterium smegmatis porin A (MspA), outer membrane porin F (OmpF), outer membrane porin G (OmpG), outer membrane phospholipase A, Neisseria autotransporter lipoprotein (NalP) and WZA.

11. The method according to claim 9 , wherein the transmembrane protein is (a) formed of eight identical subunits as shown in SEQ ID NO: 2, or (b) α-hemolysin formed of seven identical subunits as shown in SEQ ID NO: 4.

12. The method according to claim 1 , wherein method is carried out using a salt concentration of at least 0.3 M or at least 1.0 M and the salt is optionally KCl.

13. The method according to claim 1 , wherein the one or more interactions are measured along a length of the target polynucleotide that comprises at least 1000 nucleotides.