Oxygen generation for Earth

Photosystem II is a protein that first appeared in cyanobacterial organisms some 2.5-2.7 billion year ago (GYr) before transitioning in to eukaryotic algae (~1.5 Gyr) and then finally into higher plants (~0.5 Gyr). The PSII protein is a photosynthetic reaction centre that contains chlorophyll a (Chl a, shown to the left) as the pigment responsible for primary photochemistry. The reaction centre upon absorption of light undergoes a perfect 100% efficient photochemical reaction that separates charge. This charge separation reaction involves the creation of an oxidised pigment (P) and a reduced acceptor (A) molecule. In photosystem II a cluster of 4 coupled Chl a molecules represents P, and a quinone represents A. The key point upon forming this charge separated state is that the oxidised P+ reaction centre pigment is sufficiently oxidising to be able to drive water oxidation chemistry. This unique capability left PSII containing organisms with potentially limitless sources of oxidisable substrate (Water) and was a runaway success on earth.

As a enzymatic solution the oxidation of water in Photosystem II is both complex is unique. The biosphere contains only one water oxidise enzyme and the enzyme shows minimal mechanistic diversity between the primitive cyanobacterial and advanced higher plant forms. This is a spectacular demonstration of the inherent difficulty of the water oxidation chemistry. Often enzymes come up with multiple families of proteins that perform the same reaction, however, this is not the case with water oxidation.

Irrespective of this apparent difficulty, PSII containing organisms have been incredibly successful on earth and the molecular oxygen byproduct of their chemistry is released into the atmosphere at a annual rate of 10¹⁵ tons per year. This oxygen byproduct has had profound implications for life on earth and a range of biochemical reactions have had to come in to being to deal with reactive oxygen intermediates. The appearance of PSII thus represents the greatest change for life on earth as O₂ concentrations increased the atmosphere became reductive and also harmful to many life forms.

Water Oxidase Catalyst

The mechanism of water oxidation has been a quest for many researches for many years. The details have been converging toward a solution and many opinions exist as to how this chemistry is best accomplished. The key difficulties are the inability to visualise directly the intermediates using a structural approach. Thus a range of spectroscopic techniques have been used to look at the initial and (transient) intermediate states that are involved in the cycle. The first key piece of understanding was made in the late 60's with experiments performed in France by Pierre Joliot. The experiments used a polarised platinum (Pt) electrode to electrochemically determine the O₂ pattern of photosynthesis following a flash train of very short (single turnover) light flashes. The figure below illustrates this observation that was recorded with Spinach thylakoids and a 7 ns laser flash. The figure shows the O₂ released from PSII was not constant.

Oxygen Flash Yield Measurement (Joliot Measurement)

The key observation was that the O₂ release peaked on the 3rd flash and then oscillated with every 4th subsequent flash. There were a number of interpretations to the significance of this observation and Bessel Kok (at Martin Marietta) and co-workers in 1970 developed a cycle that was and still is used to explain the pattern. Kok's notion was to introduce 5 "storage" or S-states. They were termed S₀, S₁, S₂, S₃ and S₄. Kok also introduced the terminology to account for the damping of the pattern. As seen above after ~20 flashes the oscillation begins to damp out. The terms were the miss parameter and the double hit parameter. These terms also referred to as the alpha and beta terms respectively.

Kok's Oxygen Flash Pattern

A representation of Kok's cycle is shown above. The upper green panel is the oxidised chlorophyll that drives the oxidation of the catalyst through the S-states. The middle green panel depicts the S-state cycle and the five intermediate states that are driven by light. Beginning in the S₀ a single turnover flash of light advances the cycle ultimately to the S₄ state where O₂ is released and the cycle deactivates back to S₀. Each forward flash has a finite probability to undergo a miss. The miss results in no forward S-state transition with a flash. A second option is a double hit can occur with a flash of µs duration. The miss and double hit parameters account for the damping of the flash pattern in the oxygen flash measurements. The precise reason for the misses is not understood.

Returning to the catalyst. The above observation then relate to changes in the catalytic site that involve oxidation of Mn ions, accumulation or neutralisation of charge, movement of substrate, release of product O₂ and protons. It is not known yet the precise mechanism by which this water splitting chemistry takes place. For more detail however see the following pages on the Structure of the OEC

Photosynthetic Reaction Centres

The capability for photosynthesis is widely distributed amongst organisms on earth. Oxygenic organisms contain both Photosystem I and II and and are referred to as Oxygenic Phototrophs. This group of photosynthetic organisms is widely distributed on land and in the aquatic world and includes the following organisms.

In addition there is a diverse range of photosynthetic microorganisms that are referred to as anoxygenic phototrophs. These organisms have one of four of the "other types" of reaction centres and live in oxygen free anaerobic environments.

Thus far on earth there are 4 different classes of anoxygenic reaction centres which divide into two types based on the cofactors used for the electron transfer on the acceptor side of the reaction centre.

The following figure depicts the an energy scale for the photochemistry and electron transfer reactions. Linear electron flow is only seen with PSII/PSI, the other RC's use cyclic electron transfer reactions and involve cytochrome intermediate electron carriers.

From Hillier and Babcock (2001) Plant Physiology 125, 33-37.

Adapted from Blankenship, R.E. (1992) Photosynthesis Research 33, 91-111.

The fundamental differences between the different reaction centres are summarised below and do not result in large variations in structure. In a functional sense the assembly of a reaction centre uses a protein scaffold that is formed from a dimerisation of two proteins. There is then a high degree of symmetry associated with placement of the cofactors involved in the electron transfer reactions.

• Purple Bacteria (Type II): contain bacteriochlorophyll a or b and can grow with or without sulfide.
• Green Filamentous Bacteria (Type II): contain bacteriochlorophyll c or d and chlorosomes.
• Green Sulfur Bacteria (Type I): contain bacteriochlorophyll a in combination with bacteriochlorophyll c, d, or e and require sulfide for growth.
• Heliobacteria (Type I): are unique in containing bacteriochlorophyll g.

Considerable diversity exists however in the means by which light energy is captured and transferred into the reaction centre