Fig. 7.14 Superpostions of crystal structures of the enzyme thymidylate synthase with two tethered ligands, one bound to Cys143 (C atoms of ligand 7.4 are green) and the other to Cys146 (C atoms of ligand 7.4 are violet), both of which are N-tosyl-D-proline derivatives and which are covalently anchored through S―S bridges. Upon cleavage of the disulfide anchor, the free N-tosyl-D-proline (C atoms are gray, 7.12) proved to be a ligand with an affinity of 1.1 mM. Its binding geometry is very similar to both of the covalently anchored derivatives.