Pigments, like chlorophyll and carotenoids, absorb and reflect light at a certain region of the electromagnetic spectrum. Light energy initiates the process of photosynthesis when pigments absorb the light. Organic pigments have a narrow range of energy levels that they can absorb. Energy levels lower than those represented by red light are insufficient to raise an orbital electron to an excited, or quantum, state. Energy levels higher than those in blue light will physically tear the molecules apart, a process called bleaching.
For the same reasons, plant pigment molecules absorb only light in the wavelength range of nm to nm; plant physiologists refer to this range for plants as photosynthetically-active radiation. The visible light seen by humans as the color white light actually exists in a rainbow of colors in the electromagnetic spectrum, with violet and blue having shorter wavelengths and, thus, higher energy.
At the other end of the spectrum, toward red, the wavelengths are longer and have lower energy. Visible Light : The colors of visible light do not carry the same amount of energy.
Violet has the shortest wavelength and, therefore, carries the most energy, whereas red has the longest wavelength and carries the least amount of energy. Different kinds of pigments exist, each of which has evolved to absorb only certain wavelengths or colors of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear in the corresponding color.
Chlorophylls and carotenoids are the two major classes of photosynthetic pigments found in plants and algae; each class has multiple types of pigment molecules. There are five major chlorophylls: a , b , c and d, along with a related molecule found in prokaryotes called bacteriochlorophyll.
With dozens of different forms, carotenoids are a much larger group of pigments. In photosynthesis, carotenoids function as photosynthetic pigments that are very efficient molecules for the disposal of excess energy. When a leaf is exposed to full sun, the light-dependent reactions are required to process an enormous amount of energy; if that energy is not handled properly, it can do significant damage.
Therefore, many carotenoids are stored in the thylakoid membrane to absorb excess energy and safely release that energy as heat. Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light, which is the absorption spectrum. Chlorophyll a absorbs light in the blue-violet region, while chlorophyll b absorbs red-blue light.
Neither a or b absorb green light; because green is reflected or transmitted, chlorophyll appears green. Carotenoids absorb light in the blue-green and violet region and reflect the longer yellow, red, and orange wavelengths.
Chlorophyll a and b , which are identical except for the part indicated in the red box, are responsible for the green color of leaves. Each pigment has d a unique absorbance spectrum. It is easy to think of light as something that exists and allows living organisms, such as humans, to see, but light is a form of energy.
Like all energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is transformed into chemical energy, which autotrophs use to build carbohydrate molecules.
However, autotrophs only use a specific component of sunlight Figure 5. Visit this site and click through the animation to view the process of photosynthesis within a leaf. The sun emits an enormous amount of electromagnetic radiation solar energy.
Scientists can determine the amount of energy of a wave by measuring its wavelength, the distance between two consecutive, similar points in a series of waves, such as from crest to crest or trough to trough Figure 5.
Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun. The electromagnetic spectrum is the range of all possible wavelengths of radiation Figure 5. Each wavelength corresponds to a different amount of energy carried. Each type of electromagnetic radiation has a characteristic range of wavelengths. The longer the wavelength or the more stretched out it appears , the less energy is carried.
Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a piece of moving rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy.
The sun emits a broad range of electromagnetic radiation, including X-rays and ultraviolet UV rays. The higher-energy waves are dangerous to living things; for example, X-rays and UV rays can be harmful to humans. Light energy enters the process of photosynthesis when pigments absorb the light. In addition, SEP3. Thus, it is also possible that SEP3. Unlike classic SDR family proteins, atypical SDR family proteins have no known enzyme activity because they have an altered glycine-rich cofactor-binding site and partially or completely lack the signature catalytic tetrad Link et al.
HCF and HCF have the same subcellular localization: they are both predominantly associated with chloroplast membranes, with a small fraction located in the chloroplast stroma.
The hcf and hcf single mutants are able to grow on sucrose-supplemented media but they are pale green and much smaller than the wild type Link et al. Polysome association experiments demonstrated that these defects are caused by reduced translation initiation of the psbA transcript Schult et al.
The decrease in translation initiation is accompanied by a reduction in psbA mRNA stability. The hcf hcf double mutant grown on sucrose-supplemented media is smaller than the single mutants, suggesting that simultaneous loss of HCF and HCF has an additive effect Link et al. Some SDR family proteins, such as dihydrolipoamide acetyltransferases, glyceraldehydephosphate dehydrogenase, and lactate dehydrogenase, have evolved the capacity to bind RNA Hentze, ; Nagy et al.
Recombinant spinach Spinacia oleracea CtpA exhibited efficient proteolytic activity toward thylakoid membrane-embedded pD1 Yamamoto et al. Under high light, the ctpA1 mutant displays retarded growth, accelerated D1 turnover, as well as increased photosensitivity and delayed recovery of PSII activity. Unlike the ctpA1 mutant, the T-DNA mutant of the Arabidopsis CtpA2 gene is lethal under normal light but is viable in sucrose-supplemented media under low light Che et al.
The TPR is a amino acid repeated motif that ubiquitously exits among all organisms Ishikawa et al. The transcript levels of genes encoding PSII core subunits are unchanged in the mutants. MET1 is peripherally attached to thylakoid membranes on the stromal side and it is enriched in stroma lamellae Bhuiyan et al. T-DNA insertions in the Arabidopsis MET1 gene do not cause obvious changes to the accumulation and assembly state of the photosynthetic apparatus under normal light Bhuiyan et al.
Recombinant LQY1 is able to catalyze oxidative renaturation of reduced and denatured protein substrates and reductive renaturation of oxidized protein substrates.
T-DNA insertions in the Arabidopsis LQY1 gene cause reduced efficiency of PSII photochemistry, increased sensitivity to high light, and increased accumulation of reactive oxygen species under high light. The lqy1 mutant phenotype can be suppressed by complementation of lqy1 mutants with the wild-type LQY1 gene Lu, These hypotheses require further investigation. Compared to wild-type Arabidopsis, the pdi and pdi knockdown mutants display increased resistance to high light, reduced photoinhibition, and an accelerated rate of D1 synthesis Wittenberg et al.
As enzymes, thioredoxins are active in the reduced form and are able to reduce disulfide bonds in protein substrates Cain et al. PSII core proteins D1 and CP47 were found to be able to form redox-sensitive intermolecular disulfide bonds and concurrent loss of the three M-type thioredoxins interrupts the redox status of these PSII core subunits.
According to these results, Wang et al. One example is Lumen Thiol Oxidoreductase 1 LTO1 , a thylakoid membrane protein with an integral-membrane vitamin K epoxide reductase domain and a soluble disulfide-bond A oxidoreductase-like domain Feng et al.
Each of the two domains contains four conserved cysteine residues a pair of cysteine residues in the CXXC motif and another pair of separate cysteine residues , which are critical for the disulfide-bond-forming activity of LTO1 Feng et al. According to membrane topology analysis, Feng et al. This led to the hypothesis that LTO1 is involved in formation of the intramolecular disulfide bond in PsbO Figures 1 , 2 , which is located on the lumenal side of thylakoid membranes Karamoko et al.
In line with these observations, the amounts of PsbO, PsbQ, and PsbQ are substantially reduced in the LTO1-deficient Arabidopsis mutants and the mutants display reduced efficiency of PSII photochemistry, increased accumulation of reactive oxygen species, a smaller plant size, and delayed growth Karamoko et al. RBD1 rubredoxin 1 is a small iron-containing protein with a C-terminal transmembrane domain and a rubredoxin domain with two redox-active CXXC motifs Calderon et al.
Homologs of RBD1 have been found in thylakoid membranes but not plasma membranes of cyanobacteria and in thylakoid membranes of green algae and land plants Shen et al. The rbd1 knockout mutants in the cyanobacterium Synechocystis sp. PCC , the green alga Chlamydomonas reinhardtii , and the higher plant Arabidopsis display a substantial reduction or complete loss of PSII activity and photoautotrophy Calderon et al.
Based on these data, Calderon et al. Further studies are needed to dissect the precise function of RBD1. Peptide bonds to proline have cis and trans conformations Fischer et al. Under high light, D1 degradation is not affected in the mutant but repair and reassembly of photodamaged PSII is impaired. Under higher light, the difference in PSII activity between the mutant and the wild type is more pronounced. According to the mutant phenotype, Lima et al. Phosphorylation of PSII core proteins promotes unfolding of grana stacks and migration of photodamaged PSII complexes from grana stacks to stroma-exposed thylakoids.
This allows easier access of membrane or membrane-associated proteases and co-translational integration of D1 and therefore facilitates repair of photodamaged PSII complexes and proteins Bonardi et al. PBCP is a type 2C protein phosphatase predominantly found in the chloroplast stroma, with a minor fraction associated with thylakoid membranes Samol et al. Samol et al. It was originally identified as an auxiliary protein involved in dimerization of PSII monomers and degradation of photodamaged D1 Figures 1 , 2.
The TLP Compared to the wild type, the TLP Consistent with its dual roles in dimerization of PSII monomers, which occurs in grana stacks, and degradation of photodamaged D1, which takes place in stroma lamellae, TLP It was later found that the domain of unknown function in TLP However, how the acid phosphatase activity of TLP FtsHs typically consist of an N-terminal double-pass transmembrane domain, an ATPase domain, and a C-terminal zinc-binding site.
Crystal structures of the ATPase domain of bacterial FtsHs and single-particle electron cryo-microscopy analysis of cyanobacterial FtsHs showed that FtsHs exist as ringlike hexamers Krzywda et al. Bacteria contain one FtsH gene and the FtsH protein forms homohexamers while cyanobacteria and eukaryotes have multiple FtsH genes and the FtsH proteins form heterohexamers Mann et al. FtsH and Deg proteases have been known to be involved in degradation of photodamaged D1.
Early in vitro studies suggested that this is a two-step process including the initial cleavage at the stromal DE loop via Deg2 and the subsequent removal of the N-terminal fragment by FtsHs Lindahl et al.
It was later proposed that FtsHs play a more important role than Deg proteases in D1 turnover Silva et al. These observations are due to the proteolytic activity of FtsHs toward photodamaged D1 Bailey et al. Because the variegated phenotype complicates biochemical analyses, Kato et al. Using an in vitro degradation system i. However, in vivo degradation of LHCII proteins does not appear to be impaired in the ftsh6 knockout mutants Wagner et al.
Under various conditions, including high-light acclimation and dark-induced senescence, the abundances of LHCB1 and LHCB3 in the ftsh6 knockout mutants are not statistically different from those in the wild type. Further investigation is needed to understand the precise role of FtsH6. FtsH11 was reported to be critical in thermoprotection of the photosynthetic apparatus Chen et al.
Compared to the wild type under the same high-temperature treatment, the ftsh11 mutants have reduced levels of chlorophyll and reduced PSII activity. Consistent with the hypothesis that FtsH11 is involved in thermotolerance, the expression of the FtsH11 gene is up-regulated by high temperature Chen et al. The Arabidopsis genome encodes 16 Deg proteases, five of which are peripherally attached to thylakoid membranes: two Deg2 and Deg7 on the stroma side and three Deg1, Deg5, and Deg8 on the lumenal side Huesgen et al.
These five chloroplast-localized Deg proteases have been proposed to be involved in degradation of photodamaged D1 Schuhmann and Adamska, In addition to the trypsin-like protease domain, most Deg proteases, such as Deg1, Deg2, Deg7, and Deg8, have at least one PDZ domain for protein-protein interactions. It is conceivable that these chloroplast-localized PDZ domain-containing Deg proteases may act as chaperones and function in assembly of the photosynthetic apparatus Sun et al.
The RNAi lines display decreased accumulation of the and 5. The addition of recombinant Deg1 into inside-out thylakoid membranes isolated from the Deg1-deficient plants induces formation of the 5.
These data suggest that Deg1 is involved in degradation of photodamaged D1 specifically, the cleavage at the lumenal CD loop immediately downstream of the transmembrane helix E in PSII repair Figure 2 ; Kapri-Pardes et al.
Deg1 is capable of degrading lumenal proteins plastocyanin and PsbO, suggesting that Deg1 may also acts as a general purpose endopeptidase in the thylakoid lumen Chassin et al. Recombinant Deg1 has the ability to fold reduced and denatured protein substrates in the presence of both reduced and oxidized glutathione. Based on these results, Sun et al. However, these experimental data do not rule out the possibility that Deg1 is directly involved in degradation of these PSII subunits.
Although recombinant Deg2 is proteolytically active, the deg2 knockout mutants have the same plant morphology, PSII activity, and D1 turnover rate as wild-type Arabidopsis, under normal or elevated light Huesgen et al. Therefore, it was proposed that Deg2 functions as a minor protease in in vivo degradation of photodamaged D1 Figure 2. Although Deg8 has a PDZ domain, it shows no chaperone activity i.
Recombinant Deg8 demonstrates proteolytic activity toward photodamaged D1 and is able to produce the kDa N-terminal and the kDa C-terminal degradation products, which correspond to the cleavage products at the lumenal CD loop Sun et al. Although only recombinant Deg8 is proteolytically active, the deg5 and deg8 single knockout mutants of Arabidopsis both display impaired degradation of newly synthesized D1 and the impairment is more pronounced in the deg5 deg8 double mutant Sun et al.
PSII in the deg5 and deg8 single mutants exhibits increased sensitivity to high light and the sensitivity is more obvious in the deg5 deg8 double mutant. Under normal light, the two single mutants and the double mutant have a normal phenotype. Based on these data, Sun et al. Deg7 amino acids at full length is twice as long as most Deg proteases; it has two trypsin-like protease domains one active and one degenerated and four PDZ domains three active and one degenerated Schuhmann et al.
The domain composition suggests that Deg7 is the result of a whole-gene duplication event followed by subsequent degeneration Schuhmann et al. Deg7 forms homotrimers and the oligomerization is mediated through the degenerated protease domain Schuhmann et al. However, under normal light, there is no apparent difference in plant growth or morphology between the deg7 mutant and the wild type. HCF High Chlorophyll Fluorescence is an intrinsic thylakoid membrane protein with no recognizable domain or motif Zhang et al.
In line with these observations, PSII activity is dramatically reduced in the hcf mutant and the mutant has pale-green leaves and a much smaller plant size than the wild type. Pulse-labeling experiments indicated that these defects are caused by the severely reduced synthesis of D1 and to a lesser degree of D2 Zhang et al. Indeed, the hcf mutant over-accumulates pD1, the D1 precursor with an unprocessed C-terminus.
HCF was also found to interact with D1 in vivo Zhang et al. These data suggest that HCF is involved in biogenesis, processing, and assembly of D1 and possible biogenesis of D2 as well Figures 1 , 2. Under high light, PSII activity and the amount of D1 decrease much faster in the psbH1 mutant than in the wild type.
HCF High Chlorophyll Fluorescence is a lumenal protein found in stroma lamellae; it contains no recognizable domain or motif Meurer et al. The cyanobacterial homolog of HCF, YCF48 hypothetical chloroplast reading frame number 48 , was found to interact with pD1, and to a lesser degree, partially processed and unassembled D1, but not with mature and unassembled D1 or D2, in a split-ubiquitin yeast-two-hybrid assay Komenda et al.
Pulse-labeling studies showed that the hcf mutant is defective in biogenesis of PSII minimal reaction-center complexes, not in biosynthesis of PSII proteins Meurer et al. PAM68 in vascular plants is an integral thylakoid membrane protein with an acidic domain and a double-pass transmembrane domain Armbruster et al.
The PAMdeficient Arabidopsis mutants have pale-green leaves, drastically reduced PSII activity, and severely retarded growth, under normal growth conditions. The pam68 mutants were also found to over-accumulate pD1 Armbruster et al.
PsbN is a small thylakoid membrane protein encoded by the plastid genome Krech et al. Thus, Krech et al. Pulse-labeling and two-dimensional gel electrophoresis showed that formation of PSII precomplexes, e. Consistent with this hypothesis, PsbN was found to be predominantly located in stroma lamellae Torabi et al.
In Synechocystis sp. Deletion of this protein in Synechocystis sp. Little is known about the role of PSB28 in photosynthetic eukaryotes except that PSB28 does exist in higher plants and that the absence of PSB28 results in a pale-green phenotype in rice Jung et al.
However, because PSB28 is evolutionary conserved, it is reasonable to predict that PSB28 in higher plants may also function in biogenesis and assembly of chlorophyll-containing proteins such as CP47 Figure 1. LPA2 is a small intrinsic thylakoid membrane protein with a C-terminal double-pass transmembrane domain Ma et al.
However, sub-chloroplast fractionation revealed that LPA3 could be located in chloroplast stroma or associated with thylakoid membranes Cai et al. Synthesis of CP43 is greatly reduced in the lpa2 and lpa3 single mutants while synthesis of other PSII core subunits D1, D2, and CP47 is comparable between the single mutants and the wild type Ma et al. PSB33 PSII protein 33 is a thylakoid membrane protein with an N-terminal Rieske-type domain exposed to the stroma side, a double-pass transmembrane domain, and a C-terminal partial chlorophyll-binding domain Fristedt et al.
According to these data, Fristedt et al. HHL1 Hypersensitive to High Light 1 is a thylakoid membrane protein with a single-pass transmembrane domain, and a C-terminal partial von Willebrand factor type A domain, which is known to mediate protein-protein interactions Jin et al. Many of these defects become milder under normal light. Consistent with this hypothesis, HHL1 was found in both grana stacks and stroma lamellae, and PSII core subunits in thylakoid membranes isolated from HHL1-deficient plants were found to be less stable than those isolated from wild-type plants Jin et al.
Jin et al. In line with this hypothesis, the hhl1 lqy1 double mutant is more sensitive to high light than the single mutants Jin et al.
MPH1 Maintenance of PSII under High light 1 is a proline-rich intrinsic thylakoid membrane protein with a single-pass transmembrane domain; it is present in grana stacks, grana margins, and stroma lamellae Liu and Last, a , b. Photosynthesis directly or indirectly provides chemical energy for nearly all life forms on earth.
Due to the importance of photosynthesis, the structure, biogenesis, and maintenance of the photosynthetic apparatus have long been one of the major focuses of research. The combination of proteomics, X-ray crystallography, and single-particle electron cryo-microscopy approaches has led to a comprehensive understanding of the structure and subunit composition of PSII. The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Adamska, I. Plant Mol. Albiniak, A. Targeting of lumenal proteins across the thylakoid membrane. Albrecht, V. Snowy cotyledon 2 : the identification of a zinc finger domain protein essential for chloroplast development in cotyledons but not in true leaves.
Anbudurai, P. The ctpA gene encodes the C-terminal processing protease for the D1 protein of the Photosystem II reaction center complex. Andersson, U. Plant Physiol. Of course, this leaves the original chlorophyll without an electron. The upper half of the reaction center has the job of replacing this electron with a low-energy electron from water. The oxygen-evolving center strips an electron from water and passes it to a tyrosine amino acid, which then delivers it to the chlorophyll, making it ready to absorb another photon.
Antenna proteins small triangular proteins at top and bottom associated with photosystem II. The central chlorophyll molecule of the reaction center is shown with an arrow. Of course, this whole process wouldn't be very efficient if plants had to wait for photons to hit that one special chlorophyll in the reaction center. Fortunately, the energy from a light-excited electron is easily transferred through the process of resonance energy transfer.
Thanks to the mysteries of quantum mechanics, the energy can jump from molecule to molecule, as long they are close enough to each other. To take advantage of this property, photosystems have large antennas of light-absorbing molecules that harvest light and transfer their energy inwards to the reaction center. Plants even build special light-harvesting proteins that sit next to the photosystems and assist with light collection.
The picture shows a top view of photosystem II PDB entry 1s5l , showing all of the light-absorbing molecules inside. The central chlorophyll molecule of the reaction center is shown with the arrow notice the second reaction center in the bottom half--photosystem II is composed of two identical halves. The little triangular molecules at top and bottom, stuffed full of chlorophyll and carotenoids, are light-harvesting proteins PDB entry 1rwt. The oxygen-evolving center of photosystem II is a complicated cluster of manganese ions magenta , calcium blue green and oxygen atoms red.
It grips two water molecules and removes four electrons, forming oxygen gas and four hydrogen ions. The actual binding site of the two water molecules is not known for certain, but in the PDB structure 1s5l a bicarbonate ion is bound to the cluster, providing a clue for location of the active site. The picture shows two oxygen atoms from this ion colored blue : one is bound to a manganese ion, the other is bound to the calcium ion.
Notice that the oxygen-evolving center is surrounded by histidines, aspartates and glutamates, which hold it in place.
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