A survey of NADH, α-Kg, and lysine analogues has been undertaken to define the substrate specificity of saccharopine dehydrogenase, and to identify functional groups on all substrates and dinucleotides important for substrate binding. A number of NAD analogues, including NADP, 3-acetylpyridine adenine dinucleotide (3-APAD), 3-pyridinealdehyde adenine dinucleotide (3-PAAD), and thionicotinamide adenine dinucleotide (thio-NAD), can serve as a substrate in the oxidative deamination reaction, as can a number of α-keto analogues, including glyoxylate, pyruvate, α-ketobutyrate, α-ketovalerate, α-ketomalonate, and α-ketoadipate. Inhibition studies using nucleotide analogues suggest that the majority of the binding energy of the dinucleotides comes from the AMP portion, and that distinctly different conformations are generated upon binding of the oxidized and reduced dinucleotides. Addition of the 2'-phosphate as in NADPH causes poor binding of subsequent substrates, but has little effect on coenzyme binding and catalysis. In addition, the 10-fold decrease in affinity of 3-APAD in comparison to NAD suggests that the nicotinamide ring binding pocket is hydrophilic. Extensive inhibition studies using aliphatic and aromatic keto acid analogues have been carried out to gain insight into the keto acid binding pocket. Data suggest that a side chain with 3 carbons (from the α-keto group up to and including the side chain carboxylate) is optimal. In addition, the distance between the C1-C2 unit and the C5 carboxylate of the α-keto acid is also important for binding; the α-oxo group contributes a factor of 10 in affinity. The keto acid binding pocket is relatively large and flexible, can accommodate the bulky aromatic ring of a pyridine dicarboxylic acid, and a negative charge at the C3 but not the C4 position. However, the amino acid binding site is hydrophobic and the optimal length of the hydrophobic portion of amino acid carbon side chain is 3 or 4 carbons. In addition, the amino acid binding pocket can accommodate a branch at the γ-carbon, but not at the β-carbon.Saccharopine dehydrogenase (N6-(glutaryl-2)-L-lysine: NAD oxidoreductase (L-lysine forming); (EC 1.5.1.7)) (SDH 1 ) catalyzes the last step of the α-aminoadipate (AAA) pathway for the de novo synthesis of L-lysine in fungi. The reaction involves the reversible pyridine † This work is supported by the Grayce B. Kerr Endowment to the University of Oklahoma (to P. F. C.), and a grant (GM 071417) from the National Institutes of Health (to P. F. C. and A. H. W.). *Corresponding author: E-mail: pcook@chemdept.chem.ou.edu Tel: 405−325−4581 Fax: 405−325−7182. 1 Abbreviations: SDH, saccharopine dehydrogenase; AAA, α-aminoadipate pathway; NAD(P), β-nicotinamide adenine dinucleotide (phosphate) (the + charge is omitted for convenience); NADH(P), reduced β-nicotinamide adenine dinucleotide (phosphate); NADD, reduced nicotinamide adenine dinucleotide with deuterium in the 4R position; AMP, adenosine 5'-monophosphate; ADP, adenosine 5'-