Table 3. Experimental Evidence for States of p-Hydroxybenzoate Hydroxylase Catalytic Cycle.

state	description of state	position of flavin	crystallographically observed	PDB ID	biochemical evidence
1	substrate-free	OPEN−IN equilibrium	yes, but as R220Q mutant	1K0L(13)	slow substrate binding but otherwise unchanged catalysis of E49N mutant suggests two interconverting states (29); R220Q mutant more stable in OPEN than IN state (25); single molecule studies on substrate-free pHBH (30)
2	substrate-bound	IN	yes	1PBE(26)
3	substrate-bound, NADPH-bound	transition to OUT	no, but R220Q mutant observed with NADPH (no substrate) and wild type with alternate substrate observed in OUT conformation	1K0J(13), 1DOD(8,28)	spectral shifts on flavin movement to the OUT position; R220K mutant stabilized in OUT conformation (31)
4	substrate-bound, FADH−, NADP+-bound	OUT	no, but R220Q mutant observed with NADPH (no substrate)	1K0J(13)
5	substrate-bound, FADH−	IN	yes	not deposited (27)	movement of reduced flavin to IN conformation driven by positive electrostatic field as shown by K297M mutant (15)
6	substrate-bound, flavin-C4a-O-OH	IN	no		observed spectroscopically (32)
7	product bound, flavin-C4a-OH	transition to OPEN	yes, but with product bound, FAD complex	1PHH(33)	observed spectroscopically (32)