Protein ligation of the photosynthetic oxygen-evolving center

Photosynthetic water oxidation is catalyzed by a unique Mn4Ca cluster in Photosystem II. The ligation environment of the Mn4Ca cluster optimizes the cluster's reactivity at each step in the catalytic cycle and minimizes the release of toxic, partly oxidized intermediates. However, our understanding of the cluster's ligation environment remains incomplete. Although the recent X-ray crystallographic structural models have provided great insight and are consistent with most conclusions derived from earlier site-directed mutagenesis studies, the ligation environments of the Mn4Ca cluster in the two available structural models differ in important respects. Furthermore, while these structural models and the earlier mutagenesis studies agree on the identity of most of the Mn4Ca cluster's amino acid ligands, they disagree on the identity of others. This review describes mutant characterizations that have been undertaken to probe the ligation environment of the Mn4Ca cluster, some of which have been inspired by the recent X-ray crystallographic structural models. Many of these characterizations have involved Fourier transform infrared (FTIR) difference spectroscopy because of the extreme sensitivity of this form of spectroscopy to the dynamic structural changes that occur during an enzyme's catalytic cycle.