Figure 6.

Mechanistic model of acceptor-side maturation in PSII. The cartoon shows the PsbA(3) (D1) and PsbD(3) (D2) subunits and whether their acceptor-side configuration is immature (red) or mature (green). The jagged white feature denotes an immature Q_B-binding site. The orange sphere denotes the NH-Fe. The “Glu” denotes PsbD(3)-Glu241. The triangle labeled “BCT” denotes bicarbonate. The yellow hexagon labeled “Q” denotes Q_B. The states shown are consistent with various published structures as noted except for the transient state (dashed box), where bicarbonate binds in the presence of Psb28, replacing PsbD(3)-Glu241 ligation to the NH-Fe and triggering the release of Psb28. NH-Fe, non-heme Fe(II). FRL, far-red light; PSII, photosystem II.