Solution structure of marinostatin, a natural ester-linked protein protease inhibitor

Marinostatin is a unique protein protease inhibitor containing two ester linkages. We have purified a 12-residue marinostatin [MST(1-12), (1)FATMRYPSDSDE(12)] and determined the residues involved in the formation of the ester linkages and the solution structure by H-1 NMR spectroscopy and restrained molecular dynamics calculation. The two ester linkages of MST(1 - 12) are formed between hydroxyl and carboxyl groups, Thr(3)-Asp(9) and Ser(8)-Asp(11), indicating that MST(1-12) has two cyclic regions which are fused at the residues of Set(8) and Asp(9). A strong, NOE cross-peak between Tyr(6) H-alpha and Pro(7) H-alpha was observed, indicating that the Pro7 residue takes a cis-conformation. Well-converged structures and hydrogen-deuterium experiments of MST(1-12) showed that the backbone NH proton of the P1', residue, Arg(5), is hydrogen-bonded to the carboryl oxygen of the ester linkage between Thr(3) and Asp(9). To reveal the significance of the ester linkages, a marinostatin analogue, MST-2SS ((1)FACMRYPCCSCE(12)) with two disulfide bridges of Cys(3)-Cys(9) and Cys(8)-Cys(11), was also synthesized. The inhibitory activity of MST-2SS was as strong as that of MST(1-12), and the Pro 7 residue of MST-2SS also takes a cisconformation. However, the exchange rate of the Arg5 NH proton of MST-2SS was about 100 times faster than that of MST(1-12), and the structure calculation of MST-2SS was not converged on account of the small number of NOEs, indicating that MST-2SS takes a more flexible structure. The hydrogen acceptability of the ester linkage formed by the P2 position residue, Thr(3), is crucial for suppressing the fluctuation of the reactive site and sustaining, the inhibitory activity, which enables marinostatin to be one of the smallest protease inhibitors in nature.