Figure 4.

Relative positions of the purine rings in (a) structure 1rcn, (b) the present structure and (c) structure 1z6d after superposition of the C^α atoms of the active-site residues His12, Thr45, His119 and Phe120. The active-site residues in the background are displayed as line drawings to establish a point of reference for the positioning of the purine rings in each structure. The interactions for each purine ring are indicated by dashed lines. Residue Asn71 is fairly constantly disposed with respect to the active-site residues, while Gln69 and Glu111 move to accommodate the various base rings of the bound nucleotides at the B2 site. The displacement is approximately in the plane of the adenine ring with a magnitude of ∼2 Å for the guanine ring and of ∼4 Å for the hypoxanthine ring. This demonstrates how the enzyme can accommodate a variety of bases in the B2 site.