Photoassembly of the manganese cluster and oxygen evolution from monomeric and dimeric CP47 reaction center photosystem II complexes

Abstract

Isolated subcomplexes of photosystem II from spinach (CP47RC), composed of D1, D2, cytochrome b 559, CP47, and a number of hydrophobic small subunits but devoid of CP43 and the extrinsic proteins of the oxygen-evolving complex, were shown to reconstitute the Mn 4Ca1Clx cluster of the water-splitting system and to evolve oxygen. The photoactivation process in CP47RC dimers proceeds by the same two-step mechanism as observed in PSII membranes and exhibits the same stoichiometry for Mn 2؉ , but with a 10-fold lower affinity for Ca 2؉ and an increased susceptibility to photodamage. After the lower Ca 2؉ affinity and the 10-fold smaller absorption cross-section for photons in CP47 dimers is taken into account, the intrinsic rate constant for the ratelimiting calcium-dependent dark step is indistinguishable for the two systems. The monomeric form of CP47RC also showed capacity to photoactivate and catalyze water oxidation, but with lower activity than the dimeric form and increased susceptibility to photodamage. After optimization of the various parameters affecting the photoactivation process in dimeric CP47RC subcores, 18% of the complexes were functionally reconstituted and the quantum efficiency for oxygen production by reactivated centers approached 96% of that observed for reconstituted photosystem II-enriched membranes. G reen plants, algae, and cyanobacteria are unique in their ability to catalyze the oxidation of water to molecular oxygen by using light energy. This process is carried out by photosystem II (PSII), a protein complex embedded in the thylakoid membrane. It consists of more than 25 different subunits, binds a large number of pigments, and contains a photochemical reaction center for light-driven charge separation (see recent reviews in refs. 1-5). In this paper we show that oxygen evolution is possible from a subcomplex composed of a small number of these subunits after reconstituting the catalytic inorganic center.The innermost part of PSII consists of the reaction center (RC), which, when isolated, is composed of the D1 and D2 proteins, where the primary charge separation occurs, the psbI gene product, and the ␣-and ␤-subunits of cytochrome b 559 (6). Closely associated with the RC are the chlorophyll-and ␤-carotene-binding ''inner antenna'' proteins CP43 and CP47 and a range of small, hydrophobic polypeptides, which, together, comprise the PSII core complex. Several of these subunits have been proposed to contain residues essential for water oxidation.Water oxidation occurs at a catalytic site containing a tetramanganese cluster, one calcium ion, and an indeterminate number of chloride ions in association with tyrosyl residue Tyr161 (Y Z ) located in the D1 protein. Evidence to date indicates that the majority of ligands binding the manganese cluster are located in the D1 protein (7-10). The oxidized tyrosyl radical Y Z⅐ is reduced by electrons from the manganese cluster after being oxidized by the light-driven charge separation. This charge separation involves the oxidation of the primary ele...

Photoassembly of the water-oxidizing complex in photosystem II

“…This evidence then gives credence to the proposal that Mn 4 Ca could be photoassembled in the stromal region of the thylakoid membrane [33]. Observations on isolated PS II sub-complexes from which both CP43 and CP47 have been dissociated by detergents shows that these do not assemble a functional inorganic core during photoactivation, while the CP47RC sub-complex lacking only CP43 can be photoactivated, although the O 2 evolution rate is 20% that of photoactivated PSII membranes and highly unstable [23].…”

“…showed O 2 activity when the cluster was reconstituted), albeit with much lower photochemical quantum yield and to a lower level of activity [23]. The D1 polypeptide provides most of the amino acid residues comprising the active site surrounding the cluster, with a notable contribution by residues Arg357 and Glu354 from CP43 in the active site.…”

“…A stable CP47RC subcore complex has been prepared from spinach by removal of CP43 with detergents. Although the CP47RC subcore is devoid of Mn and inactive in O 2 evolution, it is capable of substantial activity (20% vs native control) after reconstitution of the Mn cluster by photoactivation in the presence of the free inorganic cofactors [23]. The aforementioned high sensitivity O 2 electrode/ photoactivation cell was absolutely essential for observing this reconstitution.…”

Manganese-oxidizing photosynthesis before the rise of cyanobacteria

“…For these reasons we do not favor O 2 as the oxidant responsible for creating the sedimentary Mn enrichments we observe. Oxidation rates catalyzed by a photosystem are much faster (59), and consequently we postulate that these Mn deposits were derived from a manganese-based photosynthetic process. Similar to photoferrotrophy proposed for the accumulation of iron in Archean iron formations (60), we interpret the original manganese oxides as products of anoxygenic photobiology, with Mn(II) donating electrons directly to an ancestral reaction center.…”

Manganese mineralogy and diagenesis in the sedimentary rock record

“…Low levels of manganese oxidation is possible from reactions with UV light, but this is suppressed in the presence of Fe 2+ or other reductants (Anbar and Holland, 1992). Manganese is oxidized phototrophically during the biosynthesis of the water-oxidizing complex of Photosystem II in Cyanobacteria, plants and algae (Tamura and Cheniae, 1987;Büchel et al, 1999). However, this phototrophic Mn oxidation is not thought to produce environmentallysignificant manganese oxides (Madison et al, 2013) and no solely manganese-oxidizing photosystem has been documented in modern phototrophs (White, 2007).…”