Abstract
Oxygenic photosynthesis is indispensable both for the development and maintenance of life on earth by converting light energy into chemical energy and by producing molecular oxygen and consuming carbon dioxide. This latter process has been responsible for reducing the CO2 from its very high levels in the primitive atmosphere to the present low levels and thus reducing global temperatures to levels conducive to the development of life. Photosystem I and photosystem II are the two multi-protein complexes that contain the pigments necessary to harvest photons and use light energy to catalyse the primary photosynthetic endergonic reactions producing high energy compounds. Both photosystems are highly organised membrane supercomplexes composed of a core complex, containing the reaction centre where electron transport is initiated, and of a peripheral antenna system, which is important for light harvesting and photosynthetic activity regulation. If on the one hand both the chemical reactions catalysed by the two photosystems and their detailed structure are different, on the other hand they share many similarities. In this review we discuss and compare various aspects of the organisation, functioning and regulation of plant photosystems by comparing them for similarities and differences as obtained by structural, biochemical and spectroscopic investigations.
Highlights
Oxygenic photosynthesis is thought to have begun around 2.4 billion years ago [1] and drastically changed life on earth due to the accumulation of molecular oxygen in the atmosphere and an equivalent reduction in carbon dioxide levels
The large PsaA and PsaB subunits (MW ~80 kDa), which contain 11 trans-membrane helices (TMH) each, form a hetero-dimer that binds the vast majority of cofactors for light harvesting (~80 Chls a and ~20 carotenes) as well as cofactors involved in the electron transfer reactions (6 Chls a, 2 phylloquinones and a 4Fe-4S cluster, known as FX), with the exception of terminal electron acceptors (Fe-S clusters FA and FB), which are bound by the PsaC subunit (Fig. 1B, 1C)
From the case of probably operate at the most oxidising (PSII), where two electrons are accumulated by the terminal acceptor QB, and four oxidising equivalent are accumulated at the donor side at the level of the Oxygen Evolving Complex (OEC), so that the system is effectively reset after four charge separation events, in PSI there is no accumulation of reducing/oxidising equivalents within the reaction centre
Summary
Oxygenic photosynthesis is thought to have begun around 2.4 billion years ago [1] and drastically changed life on earth due to the accumulation of molecular oxygen in the atmosphere and an equivalent reduction in carbon dioxide levels. Due to the large difference in the redox potential between the electron donor (oxygen in a water molecule) and final electron acceptor during the light phase of photosynthesis (NADP+), the ancestor cyanobacteria had to evolve the capability to use two photosystems working in series in order to be able to accumulate the energy of two photons. If excitation energy cannot be used for photochemistry, for example when the light intensity exceeds the photosynthetic capacity, 3Chl* formation can lead to photo-oxidative stress of the photosynthetic apparatus All these factors (a rapidly fluctuating environment and a high reactivity of excited Chls with oxygen) were important for the evolution of the photosynthetic process and its regulations before and during land colonisation. We will discuss about the functioning, organisation, regulation of photosystems under different environmental conditions, by analysing common and specific aspects of each photosystem and by presenting open questions that requires further investigation in order to better understand their functioning
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