Publication Date:
2012-02-15
Description:
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 redox-active 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 kcat of approximately 1 s-1 that is significantly larger than typically observed for designed enzymes, and a Km 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 Mn4Ca cluster needed to perform the multielectron reactions required to oxidize water.
Print ISSN:
0027-8424
Electronic ISSN:
1091-6490
Topics:
Biology
,
Medicine
,
Natural Sciences in General
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