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Aldehyde dehydrogenase 7A1 (ALDH7A1) catalyzes the terminal step of lysine catabolism, the NAD+-dependent oxidation of α-aminoadipate semialdehyde to α-aminoadipate. Structures of ALDH7A1 reveal the C-terminus is a gate that opens and closes in response to the binding of α-aminoadipate. In the closed state, the C-terminus of one protomer stabilizes the active site of the neighboring protomer in the dimer-of-dimers tetramer. Specifically, Ala505 and Gln506 interact with the conserved aldehyde anchor loop structure in the closed state. The apparent involvement of these residues in catalysis is significant because they are replaced by Pro505 and Lys506 in a genetic deletion (c.1512delG) that causes pyridoxine-dependent epilepsy. Inspired by the c.1512delG defect, we generated variant proteins harboring either A505P, Q506K, or both mutations (A505P/Q506K). Additionally, a C-terminal truncation mutant lacking the last eight residues was prepared. The catalytic behaviors of the variants were examined in steady-state kinetic assays, and their quaternary structures were examined by analytical ultracentrifugation. The mutant enzymes exhibit a profound kinetic defect characterized by markedly elevated Michaelis constants for α-aminoadipate semialdehyde, suggesting that the mutated residues are important for substrate binding. Furthermore, analyses of the in-solution oligomeric states revealed that the mutant enzymes are defective in tetramer formation. Overall, these results suggest that the C-terminus of ALDH7A1 is crucial for the maintenance of both the oligomeric state and the catalytic activity.

In humans, certain mutations in the gene encoding aldehyde dehydrogenase 7A1 are associated with pyridoxine‐dependent epilepsy (PDE). Understanding the impact of PDE‐causing mutations on the structure and activity of ALDH7A1 could allow for the prediction of symptom‐severity and aid the development of patient‐specific medical treatments. Herein, we investigate the biochemical and structural consequences of PDE missense mutations targeting residues in the aldehyde substrate binding site: N167S, P169S, A171V, G174V, and W175G. All but G174V could be purified for biochemical and X‐ray crystallographic analysis. W175G has a relatively mild kinetic defect, exhibiting a fivefold decrease in kcat with no change in Km. P169S and N167S have moderate defects, characterized by catalytic efficiencies of 20‐ and 100‐times lower than wild‐type, respectively. A171V has a profound functional defect, with catalytic efficiency 2000‐times lower than wild‐type. The crystal structures of the variants are the first for any PDE‐associated mutant of ALDH7A1. The structures show that missense mutations that decrease the steric bulk of the side chain tend to create a cavity in the active site. The protein responds by relaxing into the vacant space, and this structural perturbation appears to cause misalignment of the aldehyde substrate in W175G and N167S. The P169S structure is nearly identical to that of the wild‐type enzyme; however, analysis of B‐factors suggests the catalytic defect may result from altered protein dynamics. The A171V structure suggests that the potential for steric clash with Val171 prevents Glu121 from ion pairing with the amino group of the aldehyde substrate. Enzymes Aldehyde dehydrogenase 7A1 (http://www.chem.qmul.ac.uk/iubmb/enzyme/EC1/2/1/31.html). Databases Coordinates have been deposited in the Protein Data Bank under the following accession codes: http://www.rcsb.org/pdb/search/structidSearch.do?structureId=6O4B, http://www.rcsb.org/pdb/search/structidSearch.do?structureId=6O4C, http://www.rcsb.org/pdb/search/structidSearch.do?structureId=6O4D, http://www.rcsb.org/pdb/search/structidSearch.do?structureId=6O4E, http://www.rcsb.org/pdb/search/structidSearch.do?structureId=6O4F, http://www.rcsb.org/pdb/search/structidSearch.do?structureId=6O4G, http://www.rcsb.org/pdb/search/structidSearch.do?structureId=6O4H.

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