Photosynthetic Oxygen Evolution: The Champagne of Biological Reactions

Natural photosynthesis dominates the biosphere as the most widespread and successful metabolism on Earth. Among photosynthetic organisms the oxygenic phototrophs are the most prolific, comprising all known species of cyanobacteria, algae, and higher plants. The early ancestors of these organisms transformed the surface of Earth beginning circa 3 billion years ago from a drab alumino-silicate composite to a lush green carpet visible from Outer Space. These organisms power the planet using the Photosystem II (PSII) enzyme to split water, yielding O2, hydrogen reductants, and proton gradients (energy). Remarkably, only one PSII enzyme has evolved on Earth.

 

The resulting electrons and protons are used to make chemical bonds and a membrane potential, while O2 is utilized for additional energy production by respiration.

 

Current Research Projects on PSII:

Engineer PSII D1 subunit in Nicotiana tabacum to optimize photochemistry at extreme solar intensities.

Personnel: Yuan Zhang

The D1 protein of photosystem II (PSII) is one of six core polypeptides that make up the unit performing the primary photochemistry of light-driven charge separation and water oxidation in all oxygenic phototrophs. Cyanobacteria encode at least two isoforms of the D1 protein, whereas algae and higher plants only one. Under low light conditions many cyanobacteria express a standard low-light inducible D1:1 isoform that has greater photochemical efficiency at low light intensity. However, when the cells are exposed to a stress such as even moderate light intensity, the expression of a more robust light-tolerant D1:2 isoform is upregulated and preferentially incorporated into PSII. Following the cyanobacterial design, we collaborate with Dr. Pal Maliga and modified the tobacco native D1 isoform by introducing point mutations that confer the greatest WOC efficiency at low light and photoprotection at high light, respectively. Both biophysical properties and growth were characterized in tobacco variants expressing exclusively the native D1, the light-tolerant D1 variant or the low light D1 variant.

 

 

 

Photosynthetic Water Oxidation.

Personnel: Colin Gates

Our goal is to reveal the range of kinetic and energetic performance by photosynthetic water oxidation enzymes in vivo, selected from diverse microbial phototrophs, cyanobacteria and microalgae. The outcome is a fundamental understanding of the principles of light energy conversion to chemical energy and the mechanisms used to oxidize water in nature. This project details alternative functionality of photosystem II, novel techniques for the study thereof, and applications across the tree of life.

 

PSII-WOC Photo-assembly & Inorganic Mutants.

Personnel: Colin Gates

This project aims to understand the biogenesis of the oxygenic reaction center (photosystem II) and the functions of the inorganic components comprising its catalytic site (WOC).  We do so by substitution of the inorganic cofactors (Mn2+, Ca2+, Cl-, CO3H-, H2O) and examination of the consequences using  multiple novel tools designed by our lab staff.  This project clarifies the role of calcium in the WOC and the effects of strontium substitution at this site.

 

 

Projects are supported by Department of Energy, Office of Basic Energy Sciences, and National Science Foundation, Chemistry of Life Processes.

 

 

 

Previous Research Projects on PSII:

  1. Diversity among oxygenic photosystems 
  2. Improving the efficiency of natural photosynthesis: Expression and function of natural variants (isoforms) of the D1 reaction center core protein encoded in cyanobacterial genomes.
  3. Biogenesis and repair of the water oxidizing complex: Photo-assembly of the inorganic cofactors
  4. The chemical mechanism of water oxidation by photosystem IIs
  5. Artificial photosynthesis: synthetic catalysts for water splitting and CO2 reduction

 

What Controls the Efficiency of Photosystem II?

 Personnel: David Vinyard, Graduate Student & Gennady Ananyev, Research Professor

 Cyanobacteria contain multiple D1 isoforms which are differentially expressed based on environmental conditions. However, the mechanisms by which changes in protein matrix affect the efficiency of photosynthetic water oxidation are poorly understood.

 Highlights:

  1. Natural and engineered D1 sequences can be expressed in a robust Chlamydomonas reinhardtii model system. (Collaboration with Mayfield Lab, UCSD < http://labs.biology.ucsd.edu/mayfield/index.html>
  2. New mathematical tools have been developed to fit flash oxygen yield and variable fluorescence data to analyze water-oxidizing complex cycling efficiency
     
  3. We have revealed for the first time a functional advantage for reaction centers containing the cyanobacterial "low light" D1:1 isoform

 Sponsored by the Department of Energy, Basic Energy Sciences, Grant DE-FG02-10ER16195

 

Biogenesis and Repair of the Water Oxidizing Complex: 
Photo-assembly of the Inorganic Cofactors

Personnel: Jennifer Sun, Undergraduate Researcher, David Vinyard, Graduate Student & Gennady Ananyev, Research Professor

The D1 subunit of Photosystem II is subject to frequent damage by reactive oxygen species and must be turned over frequently during ambient or high light conditions. After the damaged D1 protein is removed and replaced by a new D1 polypeptide, the central inorganic core (Mn4CaO5) must be assembled and activated within the apo-WOC-PSII through the individual binding and oxidation of Mn2+, Ca2+, Cl-, water, and electron acceptors. Previous work in our laboratory has elucidated the major photo-assembly pathway of spinach PSII membranes.

 The sequence of kinetic intermediates (top) and proposed chemical formulation (bottom) of intermediates formed during photo-assembly of the PSII-WOC. Adapted from Dasgupta et al. 2008.

 Highlights:

  1. Photo-assembly kinetics and yields are being studied in isolated thylakoid membrane fragments from Chlamydomonas reinhardtii strains expressing cyanobacterial D1 isoforms
     
  2. An "inorganic mutant" in which Cd2+ replaced Ca2+ during photo-assembly showed that the binding of Ca2+ controls the rate limiting formation of IM1* (Bartlett et al. 2008)
     
  3. Seven flashes (one-electron transfer steps) were required to produce the first significant evolution of O2 from apo-WOC-PSII in photo-assembly conditions. This suggests that the Mn oxidation states in the S4 state (which spontaneously decomposes to produce O2) are collectively seven holes higher than 4 Mn2+. Equivalently, this arithmetic corresponds to an S1 state with average oxidation state Mn3+.

 Sponsored by the National Science Foundation, Chemistry of Life Processes, Award Number 1213772

 

Photosynthesis and Starch accumulation
Photosynthesis is the fundamental process that converts solar energy into chemical energy. Starch, the major electron sink for photosynthetically fixed reductant is known to regulate photosynthetic activity. However, the exact effect of altered sink availability on photosynthesis has not been clearly delineated in microalgae where starch and not sucrose is the primary reductant sink.

 

Key Findings 

  1. Carbon sink affects acceptor side of PSII, with donor side remaining relatively unchanged 
  2. Complete inhibition of polymeric glucan synthesis, reduces the oxygen evolution rate more than 2-fold 
  3. Starchless mutants have an efficient photosynthetic electron transport chain however are limited by carbon uptake and fixation

Students Involved: Anagha Krishnan, David Vinyard
Collaborators: Matthew Posewitz (NREL)
Supported By: Air Force Office of Scientific Research (AFOSR)