The invention relates to an ADP binding molecule comprising a polypeptide, said polypeptide comprising amino acid sequence corresponding to at least amino acids 11 to 310 of SEQ ID NO:1, wherein said polypeptide comprises a substitution relative to SEQ ID NO:1 at amino acid C287, and wherein said polypeptide comprises a further cysteine residue for attachment of at least one reporter moiety, and wherein said polypeptide has at least 68% sequence identity to SEQ ID NO:1 at the amino acid residues corresponding to those shown in column III of table A.
