Serine–threonineprotein phosphatase inhibitors derived from nodularin: role of the 2-methyl and 3-diene groups in the Adda residue and the effect of macrocyclic conformational restraint

Abstract

In order to probe the effect upon macrocycle conformation and PP1_cat enzyme inhibition of structural changes to nodularin, specific replacements for the Adda residue were introduced. Two new analogues, cyclo[-(3S,E)-3-phenylethenyl-3-aminopropanoyl-α-(R)-Glu-α-OH-γ-Sar-(R)-Asp-α-OH-β-(S)-Phe-] 19a and cyclo[-(2S,3S,E)-2-methyl-3-phenylethenyl-3-aminopropanoyl-β-(R)-Glu-α-OH-γ-Sar-(R)-Asp-α-OH-β-(S)-Phe-] 19b were prepared incorporating previously optimised preparative protocols [see previous article, K. L. Webster, A. B. Maude, M. E. O'Donnell, A. P. Mehrotra and D. Gani, J. Chem. Soc., Perkin Trans. 1 (DOI: 10.1039/b100401h)], and these differed only at C-2 of the Adda residue. The presence of a (2S)-methyl group in compound 19b stabilised the trans-rotameric form of the (2R)-Glu-γ-Sar amide bond in solution as determined by NMR spectroscopic analysis (trans–cis; 10:1), and enhanced efficacy as a PP1_cat inhibitor by 20-fold over compound 19a. The methyl homologue displayed a competitive mode of inhibition, with respect to the substrate Ac-Arg-Arg-Thr(P)-Val-Ala and displayed a K_i value of 206 ± 30 μmol dm⁻³. Substitution of the Sar residue in the methyl homologue by (2S)-Pro gave a competitive inhibitor of similar efficacy (K_i = 400 ± 75 μmol dm⁻³). The proline analogue 22 existed as a 6:1 mixture of trans–cis rotamers. Evidently the trans-rotamer of the (2S)-Pro-containing compound differed in conformational structure compared to the sarcosine-containing variant, only close to the site of the substitution. A structural model for the inhibition of PP1_cat and a strategy for the selective inhibition of PP1 over PP2A are discussed within the context of the results.