Mapping of the Primary Mannose Binding Site of Pradimicin A

Pradimicin A (PRM-A) is an actinomycete-derived antibiotic with the lectin-like property of being able to recognize d-mannopyranoside (Man) in the presence of Ca<sup>2+</sup> ion. PRM-A and its derivatives have been attracting a great deal of attention as the only family of natural carbohydrate receptors with nonpeptidic skeleton and, more recently, as conceptually novel drug candidates for human immunodeficiency virus (HIV). Despite its scientific interest and potential therapeutic importance, understanding how PRM-A recognizes Man has been severely limited. Conventional interaction analysis of PRM-A with Man in solution has been frustrated by aggregation of PRM-A and the three-component equilibrium consisting of the [PRM-A<sub>2</sub>/Ca<sup>2+</sup>], [PRM-A<sub>2</sub>/Ca<sup>2+</sup>/Man<sub>2</sub>], [PRM-A<sub>2</sub>/Ca<sup>2+</sup>/Man<sub>4</sub>] complexes, and their mixed oligomers. In this Article, we demonstrate the interaction analysis of PRM-A with methyl α-d-mannopyranoside (Man-OMe) in the solid state, which benefits from aggregate-forming propensity of PRM-A and eliminates the problem associated with the complicated equilibrium in solution. Isothermal titration calorimetry (ITC) analysis and coprecipitation experiments revealed that the primary Man binding of PRM-A is markedly tighter than the secondary one, leading to preparation of the solid aggregate solely composed of the [PRM-A<sub>2</sub>/Ca<sup>2+</sup>/Man-OMe<sub>2</sub>] complex. The simple 1:1 complexes of biosynthetically <sup>13</sup>C-enriched PRM-As and [<sup>13</sup>C<sub>6</sub>]Man-OMe facilitated the analysis of the primary Man binding of PRM-A by two-dimensional dipolar-assisted rotational resonance (2D-DARR), which clearly identified that the cavity consisted of d-alanine moiety and ABC rings of PRM-A is the Man binding site. Interestingly, the proposed Man binding site of PRM-A seems to resemble the typical architecture of artificial carbohydrate receptors.