Summary
The flux of reducing equivalents out of Photosystem II (PS II) occurs through the two-electron gate function catalyzed by the iron-quinone complex on the acceptor side. The mechanism of the two-electron gate has been studied more completely in bacterial reaction centers, where an understanding of function has benefited from a structural context. However, the two-electron gate was discovered in green plants, and a large body of work had suggested that the mechanism and main structural features are similar in the two systems, and this is now confirmed by structures. In PS II a number of additional properties are found, which result from the redox activity of the non-heme iron of the acceptor complex, and from the lability of its ligands. Pending structures for PS II at a higher resolution, much of the discussion on the molecular architecture had borrowed the structural context from the bacterial homologue. One theme in this chapter is the justification for this borrowing that comes from the application of spectroscopic approaches to the PS II acceptor complex. This has been especially successful in studies of the QA-site semiquinone, the magnetic interaction between the semiquinone formed at the site and the iron, and the interaction of external ligands with the iron. A second theme, reflecting the poor stability of the semiquinone of the QB-site in isolated PS II preparations, is the use of indirect approaches, including kinetic studies and structural modeling, to understand the structure-function interface. The crystallographic structures now available provide a gratifying validation of these alternative approaches.