<p>Alanine dehydrogenases (<db_xref db="EC" dbkey="1.4.1.1"/>) and pyridine nucleotide transhydrogenase (<db_xref db="EC" dbkey="1.6.1.1"/>) have beenshown to share regions of similarity [<cite idref="PUB00000218"/>]. Alanine dehydrogenase catalyzes the NAD-dependentreversible reductive amination of pyruvate into alanine. Pyridine nucleotide transhydrogenase catalyzesthe reduction of NADP<sup>+</sup> to NADPH with the concomitant oxidation of NADH to NAD<sup>+</sup>. This enzyme is locatedin the plasma membrane of prokaryotes and in the inner membrane of the mitochondria of eukaryotes. Thetranshydrogenation between NADH and NADP is coupled with the translocation of a proton across themembrane. In prokaryotes the enzyme is composed of two different subunits, an alpha chain (gene pntA)and a beta chain (gene pntB), while in eukaryotes it is a single chain protein. The sequence of alaninedehydrogenase from several bacterial species are related with those of the alpha subunit of bacterialpyridine nucleotide transhydrogenase and of the N-terminal half of the eukaryotic enzyme. The two mostconserved regions correspond respectively to the N-terminal extremity of these proteins and to a centralglycine-rich region which is part of the NAD(H)-binding site.</p><p> This signature corresponds to the central glycine-rich region which is part of the NAD(H)-binding site, it is found in alanine dehydrogenases (<db_xref db="EC" dbkey="1.4.1.1"/>) and also in the lysine 2-oxoglutarate reductases. </p>