Structure of a 129Xe-cryptophane biosensor complexed with human carbonic anhydrase II

Abstract

Cryptophanes represent an exciting class of xenon-encapsulating molecules that can be exploited as probes for nuclear magnetic resonance imaging. The 1.70 A resolution crystal structure of a cryptophane-derivatized benezenesulfonamide complexed with human carbonic anhydrase II shows how an encapsulated xenon atom can be directed to a specific biological target. The crystal structure confirms binding measurements indicating that the cryptophane cage does not strongly interact with the surface of the protein, which may enhance the sensitivity of 129Xe NMR spectroscopic measurements in solution.

Figure 1

(a) The M_oM_o and P_oP_o enantiomers of the cryptophane-A-derived CA biosensor. The benzenesulfonamide moiety serves as an affinity tag that targets the Zn²⁺ ion, and the R₁ substituents contain triazole propionate moieties that enhance aqueous solubility. (b) Stereoview of a simulated annealing omit map showing 1-M_oM_o (blue) and 1-P_oP_o (red) bound in the active site (1.9 σ contour, teal). A Bijvoet difference Fourier map (2.0 σ, black) confirms the encapsulation of Xe (yellow). Coordination interactions with Zn²⁺ (grey sphere) are indicated by dotted lines.

Figure 2

The unit cell of CAII crystals in space group C2 contains four molecules: I (x,y,z), II (x+1/2,y+1/2,z), III (-x,y,-z) and IV (-x+1/2,y+1/2,-z). The binding of 1 in the active site cleft of molecule I buries ∼500 Ų. Crystal contacts bury an additional 540 Ų of the surface of 1 as follows: 270 Ų with molecule III, and 240 Ų and 30 Ų with the front and back faces of molecule II, respectively. Molecule IV does not contact 1 bound to molecule I.