Fig. 4

Active site of canonical 2HADHs: (a), active site residues, reaction substrates/products (2-keto acids/2-hydroxy acids), and cofactors [NADP(H) or NAD (H))]; (b), structural support of the active site arginine. Shown are selected residues of five ternary complexes: S. meliloti GHRB with 2-keto-D-gluconic acid and NADP⁺ (PDB ID: 5v7n, shown in wider sticks), human GRHPR with 2,3-dihydroxypropanoic acid and NADP⁺ (PDB ID: 2gcg), human CTBP1 with 4-methylthio-2-oxobutyric acid and NAD⁺ (PDB ID: 4lce), human CTBP2 and 4-methylthio-2-oxobutyric acid and NAD⁺ (PDB ID: 4lcj), and A. aeolicus subfamily X9 member with lactic acid and NAD⁺ (PDB ID: 3 kb6). Oxygen and nitrogen atoms are shown in blue and red, respectively, with carbon atoms in green (for PDB ID: 5v7n) or gray (in other structures). Hydrogen bonds between protein residues and product are shown with gray dashed lines. Residues are labeled according to the sequence of PDB ID: 5v7n. Labels of highly conserved residues (i.e., present in > 90% of 2HADH sequences) are shown in bold and underlined