Figure 6.

Pyridoxal 5′-phosphate is a competitive inhibitor of ACCO with respect to Asc [see Supporting Information]. The ACCO fusion protein was employed in the standard assay with Asc varied with four concentrations of PLP (0 mM, circles; 100 mM, squares; 200 mM, triangles; 300 mM, diamonds). Pyridoxal 5′-phosphate may form a Schiff base with the ε-amino group of lysine blocking a binding site for Asc. This is likely to be Lys292 shown to be essential for Asc activation as shown in Figure 7.