The linear, four-step oxidation of water to molecular oxygen by photosystem lI requires cooperation between redox reactions driven by light and a set of redox reactions involving the S-states within the oxygen-evolving complex. The oxygenevolving complex is a highly ordered structure in which a number of polypeptides interact with one another to provide the appropriate environment for productive binding of cofactors such as manganese, chloride and calcium, as well as for productive electron transfer within the photoact. A number of recent advances in the knowledge of the polypeptide structure of photosystem 11 has revealed a correlation between primary photochemical events and a 'core' complex of five hydrophobic polypeptides which provide binding sites for chlorophyll a, pheophytin a, the reaction center chlorophyll (P680), and its immediate donor, denoted Z. Although the 'core' complex of photosystem II is photochemically active, it does not possess the capacity to evolve oxygen. A second set of polypeptides, which are water-soluble, have been discovered to be associated with photosystem II; these polypeptides are now proposed to be the structural elements of a special domain which promotes the activities of the loosely-bound cofactors (manganese, chloride, calcium) that participate in oxygen evolution activity. Two of these proteins (whose molecular weights are 23 and 17 kDa) can be released from photosystem II without concurrent loss of functional manganese; studies on these proteins and on the membranes from which they have been removed indicate that the 23 and 17 kDa species form part of the structure which promotes retention of chloride and calcium within the oxygen-evolving complex. A third water-soluble polypeptide of molecular weight 33 kDa is held to the photosystem II 'core' complex by a series of forces which in some circumstances may include ligation to manganese. The 33 kDa protein has been studied in some detail and appears to promote the formation of the environment which is required for optimal participation by manganese in the oxygen evolving reaction. This minireview describes the polypeptides of photosystem II, places an emphasis on the current state of knowledge concerning these species, and discusses current areas of uncertainty concerning these important polypeptides.
Abbreviations
A 23187, i o n o p h o r e that exchanges divalent cations with HΓ·; Chl, chlorophyll; cyt, c y t o c h r o m e ;DCPIP, d i c h l o r o p h e n o l i n d o p h e n o l ; DPC, diphenylcarbazide;
EGTA, ethyleneglycoltetraacetic acid; P680, the chlorophyll a reaction center of p h o t o s y s t e m II; pheo, p h e o p h y t i n ; PQ, p l a s t o q u i n o n e ; PS, p h o t o s y s t e m ; QA and QB, primary and secondary p l a s t o q u i n o n e electron acceptors of p h o t o s y s t e m II; Sn (n = 0, 1, 2, 3, 4), charge accumulating state of the *The survey of literature for this review ended in September, 1984.
oxygen evolving system; Signals live, IIf and IIs, epr-detectable free radicals associated with the oxidizing side of photosystem II; Z, primary electron donor to the photosystem II reaction center.