Figure 1.

(a) Superposition of ribbon diagrams of WT NMNAT complexed with NAD⁺ (cyan) compared with NADP⁺ (magenta). The largest deviation in WT NMNAT to accommodate NADP⁺ is seen in helices 4, 5 and 6. There is an overall opening of the active site to fit the larger NADP⁺ molecule. (b) Superposition of ribbon diagrams of His19Ala NMNAT complexed with NAD⁺ (green) compared with WT NMNAT with NADP⁺ (magenta). The NAD⁺ and NADP⁺ molecules are completely superposed in this alternative mode of dinucleotide binding to NMNAT. (c) Close-up of the superposition of NAD⁺ (cyan) and NADP⁺ (magenta) in WT NMNAT. The nicotinyl moiety is very closely aligned between NAD⁺ and NADP⁺, whereas the adenylyl moiety is completely misaligned. The adenylyl moiety has undergone a rotation to allow its adenine ring to interact with the side chain of Tyr126 of NMNAT. The ribose of the adenylyl moiety has also undergone a rotation to allow the 2′-phosphate to interact with His16 and His19 of NMNAT. (d) Close-up of the superposition of NAD⁺ in His19Ala NMNAT (green) and NADP⁺ in WT NMNAT (magenta). An identical rotation around the adenylyl moiety is observed between NAD⁺ and NADP⁺. A sulfate molecule mimicking the γ-phosphate of ATP was identified in the active sites of the WT and H19A NMNAT–NAD⁺ complexes and is shown for clarity.