Fig. 1.

Schematic overview of the metabolism of l-lysine via the saccharopine and pipecolic acid (PA) pathways and biochemical pathophysiology of ATQ deficiency. The two pathways converge where L-Δ1-piperideine 6-carboxylate (P6C), produced via the pipecolic acid pathway, and α-aminoadipic semialdehyde (α-AASA), produced via the saccharopine pathway, are in equilibrium. α-AASA is then converted to α-aminoadipic acid (α-AAA) by ATQ. In ATQ deficiency, P6C and α-AASA accumulate due to a block in α-aminoadipic semialdehyde dehydrogenase (Antiquitin, ALDH7A1). P6C undergoes chemical condensation with pyridoxal phosphate (PLP) resulting in PLP deficiency. PA accumulates due to backpressure from the enzymatic block