A,B. Overview ODC cartoon structures with catalytic site zoom-in in insets. A. APA-bound ODC/PLP. The amino-oxy group of APA forms a Schiff base with the aldehyde group of PLP. B. Apo-ODC/PLP with bound citrate. Citrate stabilizes the Schiff base bond between PLP and K69 of ODC. PLP and K69 are shown as white stick model in the inset, overlaid with structurally aligned PLP and K69 from apo ODC PDB 1D7K in green. The two monomers of the ODC homodimer are shown in green and cyan cartoon representation, respectively. PLP, PLP-APA, and citrate are shown in sphere representation in the overview cartoon models. C. Close-up of PLP-APA in the catalytic center of ODC with 2mFo-DFc density map contoured at 1 σ. D. Close-up of PLP and citrate in the catalytic center of ODC with 2mFo-DFc density map contoured at 1 σ. Key residues directly binding to APA are shown in stick presentation.