Abstract
Pradimicin A (PRM-A) is an antibiotic isolated from actinomycetes. This compound is unique as a non-peptidic natural product with the lectin-like property of being able to recognize D-mannopyranoside (Man) in the presence of Ca' ion. Recently, PRM-A has been attracting attention as a conceptually novel drug candidate for human immunodeficiency virus (HIV). Although the anti-HIV effects are explained by its specific binding to Man residues of glycans on the viral envelope, the molecular basis of Man recognition by PRM-A remains unclear. Conventional interaction analysis of PRM-A with Ca^<2+> ion and Man in solution has been hampered by aggregation of the ternary PRM-A/Ca^<2+>/Man complexes and complicated three-component equilibrium. In this report, we describe the interaction analysis of PRM-A with Ca^<2+> ion and methyl a-D-mannopyranoside (Man-OMe) in the solid state. The use of the solid PRM-A aggregate composed of exclusively the [PRM-A2/Ca^<2+>/Man-OMe_2] complex facilitated the analysis by eliminating the problem associated with the complicated equilibrium in solution. Solid-state ^<113>Cd-NMR experiments using ^<113>Cd^<2+> ion as a surrogate probe for Ca^<2+> ion led us to propose an unprecedented binding model, in which PRM-A binds Man-OMe in a Ca^<2+>"-mediated manner through the carboxylate group of D-alanine moiety. This binding model was further supported by two-dimensional dipolar assisted rotational resonance (2D-DARR) experiments using biosynthetically ^<13>C-enriched PRM-As, which confirmed that the D-alanine moiety and A-C rings of PRM-A constitute the binding site for Man-OMe. The present study provides a clue toward the full elucidation of the molecular basis of Man recognition by PRM-A.
