Abstract
Chlorophylls are a fundamental class of tetrapyrroles and function as the central reaction center, accessory and photoprotective pigments in photosynthesis. Their unique individual photochemical properties are a consequence of the tetrapyrrole macrocycle, the structural chemistry and coordination behavior of the phytochlorin system, and specific substituent pattern. They achieve their full potential in solar energy conversion by working in concert in highly complex, supramolecular structures such as the reaction centers and light-harvesting complexes of photobiology. The biochemical function of these structures depends on the controlled interplay of structural and functional principles of the apoprotein and pigment cofactors. Chlorophylls and bacteriochlorophylls are optically active molecules with several chiral centers, which are necessary for their natural biological function and the assembly of their supramolecular complexes. However, in many cases the exact role of chromophore stereochemistry in the biological context is unknown. This review gives an overview of chlorophyll research in terms of basic function, biosynthesis and their functional and structural role in photosynthesis. It highlights aspects of chirality and symmetry of chlorophylls to elicit further interest in their role in nature.
Highlights
In plants this is a light-dependent reaction catalyzed by single subunit nicotinamide adenine dinucleotide phosphate (NADPH): protochlorophyllide oxidoreductases, some organisms use a so-called dark variant of this process incorporating a light-independent enzyme with adenosine triphosphate (ATP) for this reduction [136,137]
The reaction center complex (RC)-LHCI complex is associated with a cytochrome bc complex which transfers electrons between a hydroquinone (QH, which differs from species to species) and a soluble electron transfer protein carrier [191]
Once P is excited to P*, an electron is transferred to the primary acceptor bacteriopheophytin (BPheo) (HA) in ~2 ps and ~3–5 ps, for B. viridis and R. sphaeroides, respectively, the beginning of the formation of the cross-membrane electron gradient that drives photosynthesis [9,202]
Summary
Variations in the chemical structure open access to a multitude of functions, catalyzing an array of reactions [2] Those variations include the mentioned changes in the oxidation state, different associated metals or substituents on the outer sphere of the tetrapyrroles. Associated with oxygen transport and storage as well as electron transfer are heme proteins [1,3] Other important enzymes with heme cofactors are the cytochromes which are associated to various reactions occurring in nature They facilitate electron transfer in photosynthesis, respiration, and cell metabolism. Coenzyme B12 (Cobalamin) is the only example of a corrin in nature [12,13] It is the chemically most complex tetrapyrrole and is solely synthesized by prokaryotes (single-celled organisms, e.g., bacteria). This chromophore, as a member of red/far-red photoreceptors, mediates signals from the cell environment to the nucleus
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