Alteration of the O 2 -Producing Mn 4 Ca Cluster in Photosystem II by the Mutation of a Metal Ligand.

The O 2 -evolving Mn 4 Ca cluster in photosystem II (PSII) is arranged as a distorted Mn 3 Ca cube that is linked to a fourth Mn ion (denoted as Mn4) by two oxo bridges. The Mn4 and Ca ions are bridged by residue D1-D170. This is also the only residue known to participate in the high-affinity Mn(II) site that participates in the light-driven assembly of the Mn 4 Ca cluster. In this study, we use Fourier transform infrared difference spectroscopy to characterize the impact of the D1-D170E mutation. On the basis of analyses of carboxylate and carbonyl stretching modes and the O-H stretching modes of hydrogen-bonded water molecules, we show that this mutation alters the extensive network of hydrogen bonds that surrounds the Mn 4 Ca cluster in the same manner as that of many other mutations. It also alters the equilibrium between conformers of the Mn 4 Ca cluster in the dark-stable S 1 state so that a high-spin form of the S 2 state is produced during the S 1 -to-S 2 transition instead of the low-spin form that gives rise to the S 2 state multiline electron paramagnetic resonance signal. The mutation may also change the coordination mode of the carboxylate group at position 170 to unidentate ligation of Mn4. This is the first mutation of a metal ligand in PSII that substantially impacts the spectroscopic signatures of the Mn 4 Ca cluster without substantially eliminating O 2 evolution. The results have significant implications for our understanding of the roles of alternate active/inactive conformers of the Mn 4 Ca cluster in the mechanism of O 2 formation.