Environment of the dinucleotide analogue adenine ring. Interacting residues are highlighted in yellow and outlined by van der Waals surfaces. In the human type II complex, the adenine ring is tightly stacked between Phe-282 and His-253, and makes close edge-on contacts with Thr-45 (Right) and Thr-252 (Left, not labeled). In the human type I isoform, residues 45, 253 and 282 are substituted as indicated. Thr-45 and adjacent Gln-469 (right, not labeled) are contributed by the neighboring monomer.