FEATURE ARTICLE
How Plants Produce Dioxygen
At its core, oxygen production comes down to the chemistry of a poorly understood manganese-containing complex in the membranes of plant chloroplasts
Veronika Szalai, Gary Brudvig
Models and Mechanisms
The structural model of the Mn4 cluster of the OEC shown in Figure 13 has initiated consideration of the discrete steps involved in water oxidation and O2 evolution. Most mechanistic proposals concentrate on the Mn4 cluster because S-state advance correlates with O2 production. However, a recent suggestion is that the tyrosine residue TyrZ, which becomes oxidized and reduced, may also be intimately involved in water oxidation. Initial measurements of the distance between TyrZ and the Mn4 cluster indicated that the two species were 10–20 angstroms apart. Consequently, it was believed that TyrZ? acquired just an electron from the Mn4 cluster: The proton required for TyrZ? reduction presumably originated from some other source. Now, studies conducted in our lab and others have demonstrated that TyrZ and the Mn4 cluster are in close proximity. Simulations of an EPR spectrum containing features from both TyrZ? and the Mn4 cluster indicate that the distance between the two cofactors is approximately 8 angstroms.
This means that TyrZ is close enough to the Mn4 cluster to participate directly in water-oxidation chemistry. Figure 13 illustrates a possible sequence of reactions that takes place between the S3 and S0 states during which O2 is released. Starting with the Mn4 cluster in the S3 state, removal of one electron and one proton (H+) from TyrZ forms the tyrosine radical, TyrZ?. The advance to the S4 state happens when TyrZ? removes an electron from the Mn4 cluster and a proton from a hydroxide (OH–) group bound to the Mn4 cluster. Once an electron and a proton have been removed from the Mn4 cluster by TyrZ?, a Mn=O species can form in the S4 state. Creation of a Mn=O species in the Mn4 cluster in PSII is based on analogy to the chemistry exhibited by small model complexes that generate Mn=O species.
In the O2 bond-forming step, a hydroxide (OH–) group attacks the Mn=O species. This hydroxide group is not expected to be diffusing freely in solution. Instead, it may be bound to a calcium ion before it is delivered to the Mn=O group. This may explain why calcium is required for maximal O2 production. The end result is collapse of the Mn4 cluster back to the S0 state and the concomitant release of O2.
Many factors have contributed to our increased understanding of photosynthetic O2 evolution. Continued spectroscopic scrutiny of the Mn4 cluster in PSII can be used to refine mechanistic proposals. Study of small-molecule model complexes that generate O2 provides insight into the feasibility of mechanistic proposals of water oxidation and O2 evolution in plants. The discovery of specific strains of cyanobacteria that do not require Photosystem II for growth means that genetic manipulation at discrete sites ("site-directed mutagenesis") is now used to probe the structure, location and function of many of the cofactors in PSII. And, in the not too distant future, advances in purification procedures of PSII may permit measurements that could eventually yield the entire structure of PSII. The question of how plants accomplish the incredible feat of converting water to O2 remains a challenging and rewarding problem.
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