Figure 6. NADP⁺ binding of E. coli SSADH.

Stereo view of the active site showing the NADP⁺ moiety (yellow), SSADH residues (green) involved in binding NADP⁺, water molecules can be seen as red spheres and all bonds are depicted with a black dashed line. The 2F₀–F_c omit electron density of the NADP⁺ moiety contoured at 1σ is also shown (light blue mesh). Interactions of monomer A and NADP⁺ can be seen, specifically both AN1 and AN6 of the adenine moiety (labelled adn) interacts with Q239(O^ε1), Q243(O^ε1) and N217(O^δ1) via water molecules. Adjacent to the adenine moiety, both AO2 and AO3 of the ribose (labelled rb2) hydrogen bonds with T153(O) and K179(N^Z, O) via a single water molecule. 3AOP of the 2'-phosphate interacts with K179(N^Z). AO2 of the pyrophosphate interacts with S233(N, O^G) and the NO1 hydrogen bonds directly with W155(N^ε1). Both NO2 and NO3 of the adjacent ribose moiety (labelled rb1) hydrogen bonds with K338(N^Z), while NO2 also interacts with E385(O^ε1). NN7 of the nicotinamide (labelled nt) moiety interacts with N156(N^δ2) and the catalytic C288(N) via the single water molecule. While NO7 interacts directly with G232(O) and L255(O), as well as with L255(N) and E254(O^ε1) via the same water molecule. Up to 13 SSADH residues make 24 van der Waals or hydrogen bonds interactions with NADP⁺ per monomer, 16 of which are mediated by water (Table S2). Notably, all of the residues involved directly NADP⁺ binding in E. coli SSADH [48] (Table S2) are also conserved in human SSADH.