There is also a fingerprint motif (Gly-X-Gly-X-X-Ala/Val) that increases specificity of the enzyme to bind to the proper substrate which is highly conserved. The link between dIII and dII is less rigid, and consists of a scattering of smaller polar residues as well as a helix D/loop D region that perpendicularly extends from the Rossmann fold. This enzyme’s structure is tied to its function heavily. The transmembrane domain in dII formed by alpha helices lends itself well to its function as a proton channel across a membrane. The lid and fingerprint motifs used by dIII to increase affinity to NADP(H) are also examples of its structure being tied to function. In addition, the ridge of dIII fitting into the cleft of dI shows that even the structure of the dI/dIII dimer is helpful in moving their two substrates closer together to allow hydride transfer. Since this enzyme exists in multiple organisms, it is possible to look at multiple orthologs of this enzyme. Their structures are not identical, but most are highly similar, and all share the same architecture and domains. This all shows that the structure of this enzyme is closely connected to its role in reduction of NADP+. NADPH is used as a reducing agent in many anabolic reactions. Proton translocating NAD(P)+  transhydrogenase is one of the main ways that cells can regenerate NADPH after it is used. In E. coli, this pathway contribute equal amounts of NADPH as the pentose phosphate pathway, and both were the main producers of NADPH under standard growth conditions. NADP(H) and NAD(H) are equal and opposite contributers to metabolism, and this enzyme is necessary to keep a balance between the two. It can also function to generate a proton gradient across a membrane, as it is a reversible reaction, and is used as such in certain organisms. “It can contribute to NADPH formation for bio-synthesis and glutathione reduction, in the regulation of flux through the tricarboxylic acid cycle, and, operating in reverse in some invertebrates, it can transfer reducing power from NADPH to NAD+ to fuel an anaerobic respiratory chain.” – J. Baz Jackson, Sarah J. Peake, and Scott A. White on the role of proton translocating NAD(P)+ transhydrogenase in the cell.