Light-driven oxygen production from superoxide by Mn-binding bacterial reaction centers

Abstract

One of the outstanding questions concerning the early Earth is how ancient phototrophs made the evolutionary transition from anoxygenic to oxygenic photosynthesis, which resulted in a substantial increase in the amount of oxygen in the atmosphere. We have previously demonstrated that reaction centers from anoxygenic photosynthetic bacteria can be modified to bind a redoxactive Mn cofactor, thus gaining a key functional feature of photosystem II, which contains the site for water oxidation in cyanobacteria, algae, and plants [Thielges M, et al. (2005) Biochemistry 44:7389-7394]. In this paper, the Mn-binding reaction centers are shown to have a light-driven enzymatic function; namely, the ability to convert superoxide into molecular oxygen. This activity has a relatively high efficiency with a k cat of approximately 1 s −1 that is significantly larger than typically observed for designed enzymes, and a K m of 35-40 μM that is comparable to the value of 50 μM for Mn-superoxide dismutase, which catalyzes a similar reaction. Unlike wild-type reaction centers, the highly oxidizing reaction centers are not stable in the light unless they have a bound Mn. The stability and enzymatic ability of this type of Mn-binding reaction centers would have provided primitive phototrophs with an environmental advantage before the evolution of organisms with a more complex Mn 4 Ca cluster needed to perform the multielectron reactions required to oxidize water. I n photosynthesis, the primary conversion of light energy into chemical energy is performed by the evolutionarily related pigment-protein complexes, reaction centers in purple and green bacteria and photosystems I and II in cyanobacteria, algae, and plants (1, 2). In the purple bacterium Rhodobacter sphaeroides, the reaction center is composed of two core protein subunits, identified as the L and M subunits, which each have five transmembrane helices and are related by an approximate twofold symmetry axis. These protein subunits surround the cofactors, which are arranged in two branches with the same twofold symmetry axis. In addition, reaction centers have a third subunit, identified as the H subunit, which has a single transmembrane helix and a large extramembrane domain. Light excitation of the primary electron donor P, a bacteriochlorophyll dimer, is followed by electron transfer steps predominately along one branch of cofactors to Q A , the primary quinone, and then to Q B , the secondary quinone, forming the charge-separated state P þ· Q B −· . In the cell, the oxidized bacteriochlorophyll dimer is reduced by a water-soluble cytochrome c 2 allowing a second electron transfer to Q B in a proton-coupled process. The electrons and protons associated with the oxidized cytochrome c 2 and reduced quinone (quinol) are coupled to the cytochrome bc 1 complex resulting in a net proton transfer across the cell membrane that is used to create energy-rich compounds. Photosystem II has a significant structural homology to this type of reaction center, with a core of two protein subu...

Chlorophyll Biosynthesis Gene Evolution Indicates Photosystem Gene Duplication, Not Photosystem Merger, at the Origin of Oxygenic Photosynthesis

“…This suggests that no major evolutionary invention was required for photosystem II to tap environmental Mn as an electron source. In line with that, it has recently been shown (Allen et al 2012) that an engineered, Mn-binding RCII of R . sphaeroides will produce O 2 from {\rm O}_{\rm 2}^- in the presence of Mn in a light-dependent reaction in which photodamage is impeded in comparison with that in a wild-type, Mn-free RC.…”

“…sphaeroides will produce O 2 from {\rm O}_{\rm 2}^- in the presence of Mn in a light-dependent reaction in which photodamage is impeded in comparison with that in a wild-type, Mn-free RC. Allen et al (2012) interpret this observation as an important clue to the origin of oxygenic photosynthesis.…”

Plastid origin: who, when and why?

“…Recently, geochemical evidence was reported that is in agreement with both that scenario [70] and the model of Dismukes and colleagues [65,71], which also suggested a role for environmental Mn II while also pointing out a role for high CO 2 in the origin of water splitting. The Mn-oxidizing abilities of RCII from Rhodobacter [72] are also compatible with the models deriving water splitting complex from environmental Mn II . An understanding of the evolution of photosynthesis further highlights the uniqueness and importance of the origin of plastids.…”

Genomes of Stigonematalean Cyanobacteria (Subsection V) and the Evolution of Oxygenic Photosynthesis from Prokaryotes to Plastids

“…Attaining suitably high concentrations of Mn II/III as an environmentally available electron donor in the ocean would be problematic, but not in a freshwater setting. Allen et al (2012) have recently shown that an engineered, Mn-binding type II reaction center of Rhodobacter sphaeroides will produce O 2 from {\rm O}_{\rm 2}^{^- } in the presence of Mn in a light-dependent reaction in which photo-damage is impeded in comparison with that in a wild-type, Mn-free reaction center. Their observation (Allen et al 2012) is likely an important clue to the origin of oxygenic photosynthesis, at which time a protocyanobacterial type II reaction center acquired, via natural selection, the ability to (photo-)oxidize Mn II/III —itself ultimately rereduced by water—and then to reduce a newly constitutive type I reaction center.…”

“…Allen et al (2012) have recently shown that an engineered, Mn-binding type II reaction center of Rhodobacter sphaeroides will produce O 2 from {\rm O}_{\rm 2}^{^- } in the presence of Mn in a light-dependent reaction in which photo-damage is impeded in comparison with that in a wild-type, Mn-free reaction center. Their observation (Allen et al 2012) is likely an important clue to the origin of oxygenic photosynthesis, at which time a protocyanobacterial type II reaction center acquired, via natural selection, the ability to (photo-)oxidize Mn II/III —itself ultimately rereduced by water—and then to reduce a newly constitutive type I reaction center. Transition from environmental (substrate) Mn II/III ions to the catalytic Mn 4 Ca center of cyanobacterial RCII would then have permitted light-dependent CO 2 and/or nitrogen fixation, in the absence of electron donors other than water.…”