Electrochemistry is the branch of physical chemistry that studies the relationship between electricity, as a measurable and quantitative phenomenon, identifiable chemical change, with either electricity considered an outcome of a particular chemical change or vice versa. These reactions involve electric charges moving between an electrolyte, thus electrochemistry deals with the interaction between electrical energy and chemical change. When a chemical reaction is caused by an externally supplied current, as in electrolysis, or if an electric current is produced by a spontaneous chemical reaction as in a battery, it is called an electrochemical reaction. Chemical reactions where electrons are transferred directly between molecules and/or atoms are called oxidation-reduction or reactions. In general, electrochemistry describes the overall reactions when individual redox reactions are separate but connected by an external electric circuit and an intervening electrolyte. Understanding of electrical matters began in the sixteenth century.
During this century, the English scientist William Gilbert spent 17 years experimenting with magnetism and, to a lesser extent, electricity. For his work on magnets, Gilbert became known as the "Father of Magnetism." He discovered various methods for strengthening magnets. In 1663, the German physicist Otto von Guericke created the first electric generator, which produced static electricity by applying friction in the machine; the generator was made of a large sulfur ball cast inside a glass globe, mounted on a shaft. The ball was rotated by means of a crank and an electric spark was produced when a pad was rubbed against the ball as it rotated; the globe could be used as source for experiments with electricity. By the mid—18th century the French chemist Charles François de Cisternay du Fay had discovered two types of static electricity, that like charges repel each other whilst unlike charges attract. Du Fay announced that electricity consisted of two fluids: positive, electricity; this was the two-fluid theory of electricity, to be opposed by Benjamin Franklin's one-fluid theory in the century.
In 1785, Charles-Augustin de Coulomb developed the law of electrostatic attraction as an outgrowth of his attempt to investigate the law of electrical repulsions as stated by Joseph Priestley in England. In the late 18th century the Italian physician and anatomist Luigi Galvani marked the birth of electrochemistry by establishing a bridge between chemical reactions and electricity on his essay "De Viribus Electricitatis in Motu Musculari Commentarius" in 1791 where he proposed a "nerveo-electrical substance" on biological life forms. In his essay Galvani concluded that animal tissue contained a here-to-fore neglected innate, vital force, which he termed "animal electricity," which activated nerves and muscles spanned by metal probes, he believed that this new force was a form of electricity in addition to the "natural" form produced by lightning or by the electric eel and torpedo ray as well as the "artificial" form produced by friction. Galvani's scientific colleagues accepted his views, but Alessandro Volta rejected the idea of an "animal electric fluid," replying that the frog's legs responded to differences in metal temper and bulk.
Galvani refuted this by obtaining muscular action with two pieces of the same material. In 1800, William Nicholson and Johann Wilhelm Ritter succeeded in decomposing water into hydrogen and oxygen by electrolysis. Soon thereafter Ritter discovered the process of electroplating, he observed that the amount of metal deposited and the amount of oxygen produced during an electrolytic process depended on the distance between the electrodes. By 1801, Ritter observed thermoelectric currents and anticipated the discovery of thermoelectricity by Thomas Johann Seebeck. By the 1810s, William Hyde Wollaston made improvements to the galvanic cell. Sir Humphry Davy's work with electrolysis led to the conclusion that the production of electricity in simple electrolytic cells resulted from chemical action and that chemical combination occurred between substances of opposite charge; this work led directly to the isolation of sodium and potassium from their compounds and of the alkaline earth metals from theirs in 1808.
Hans Christian Ørsted's discovery of the magnetic effect of electric currents in 1820 was recognized as an epoch-making advance, although he left further work on electromagnetism to others. André-Marie Ampère repeated Ørsted's experiment, formulated them mathematically. In 1821, Estonian-German physicist Thomas Johann Seebeck demonstrated the electrical potential in the juncture points of two dissimilar metals when there is a heat difference between the joints. In 1827, the German scientist Georg Ohm expressed his law in this famous book "Die galvanische Kette, mathematisch bearbeitet" in which he gave his complete theory of electricity. In 1832, Michael Faraday's experiments led him to state his two laws of electrochemistry. In 1836, John Daniell invented a primary cell which solved the problem of polarization by eliminating hydrogen gas generation at the positive electrode. Results revealed that alloying the amalgamated zinc with mercury would produce a higher voltage. William Grove produced the first fuel cell in 1839.
In 1846, Wilhelm Weber developed the electrodynamometer. In 1868, Georges Leclanché patented a new cell which became the forerunner to the world's first used battery, the zinc carbon cell. Svante Arrhenius published
Vacuolar-type H+-ATPase is a conserved evolutionarily ancient enzyme with remarkably diverse functions in eukaryotic organisms. V-ATPases acidify a wide array of intracellular organelles and pump protons across the plasma membranes of numerous cell types. V-ATPases couple the energy of ATP hydrolysis to proton transport across intracellular and plasma membranes of eukaryotic cells, it is seen as the polar opposite of ATP synthase because ATP synthase is a proton channel that uses the energy from a proton gradient to produce ATP. V-ATPase however, is a proton pump that uses the energy from ATP hydrolysis to produce a proton gradient. V-ATPases are found within the membranes of many organelles, such as endosomes and secretory vesicles, where they play a variety of roles crucial for the function of these organelles. For example, the proton gradient across the yeast vacuolar membrane generated by V-ATPases drives calcium uptake into the vacuole through an H+/Ca2+ antiporter system. In synaptic transmission in neuronal cells, V-ATPase acidifies synaptic vesicles.
Norepinephrine enters vesicles by V-ATPase. V-ATPases are found in the plasma membranes of a wide variety of cells such as intercalated cells of the kidney, macrophages, sperm, midgut cells of insects, certain tumor cells. Plasma membrane V-ATPases are involved in processes such as pH homeostasis, coupled transport, tumor metastasis. V-ATPases in the acrosomal membrane of sperm acidify the acrosome; this acidification activates proteases required to drill through the plasma membrane of the egg. V-ATPases in the osteoclast plasma membrane pump protons onto the bone surface, necessary for bone resorption. In the intercalated cells of the kidney, V-ATPases pump protons into the urine, allowing for bicarbonate reabsorption into the blood; the yeast V-ATPase is the best characterized. There are at least thirteen subunits identified to form a functional V-ATPase complex, which consists of two domains; the subunits belong to either the V1 domain. The V1 includes eight subunits, A-H, with three copies of the catalytic A and B subunits, three copies of the stator subunits E and G, one copy of the regulatory C and H subunits.
In addition, the V1 domain contains the subunits D and F, which form a central rotor axle. The V1 domain contains tissue-specific subunit isoforms including B, C, E, G. Mutations to the B1 isoform result in the human disease distal renal tubular acidosis and sensorineural deafness; the Vo domain contains six different subunits, a, d, c, c', c", e, with the stoichiometry of the c ring still a matter of debate with a decamer being postulated for the tobacco hornworm V-ATPase. The mammalian Vo domain contains tissue-specific isoforms for subunits a and d, while yeast V-ATPase contains two organelle-specific subunit isoforms of a, Vph1p, Stv1p. Mutations to the a3 isoform result in the human disease infantile malignant osteopetrosis, mutations to the a4 isoform result in distal renal tubular acidosis, in some cases with sensorineural deafness; the V1 domain is responsible for ATP hydrolysis, whereas the Vo domain is responsible for proton translocation. ATP hydrolysis at the catalytic nucleotide binding sites on subunit A drives rotation of a central stalk composed of subunits D and F, which in turn drives rotation of a barrel of c subunits relative to the a subunit.
The complex structure of the V-ATPase has been revealed through the structure of the M. Sexta and Yeast complexes that were solved by single-particle cryo-EM and negative staining, respectively; these structures have revealed that the V-ATPase has a 3-stator network, linked by a collar of density formed by the C, H, a subunits, while dividing the V1 and V0 domains, make no interactions with the central rotor axle formed by the F, D, d subunits. Rotation of this central rotor axle caused by the hydrolysis of ATP within the catalytic AB domains results in the movement of the barrel of c subunits past the a subunit, which drives proton transport across the membrane. A stoichiometry of two protons translocated for each ATP hydrolyzed has been proposed by Johnson. In addition to the structural subunits of yeast V-ATPase, associated proteins that are necessary for assembly have been identified; these associated proteins are essential for Vo domain assembly and are termed Vma12p, Vma21p, Vma22p. Two of the three proteins, Vma12p and Vma22p, form a complex that binds transiently to Vph1p to aid its assembly and maturation.
Vma21p coordinates assembly of the Vo subunits as well as escorting the Vo domain into vesicles for transport to the Golgi. The V1 domain of the V-ATPase is the site of ATP hydrolysis; this soluble domain consists of a hexamer of alternating A and B subunits, a central rotor D, peripheral stators G and E, regulatory subunits C and H. Hydrolysis of ATP drives a conformational change in the six A|B interfaces and with it rotation of the central rotor D. Unlike with the ATP synthase, the V1 domain is not an active ATPase. In molecular biology, V-ATPase C represents the C terminal subunit, part of the V1 complex, is localised to the interface between the V1 and V0 complexes; the C subunit plays an essential role in controlling the assembly of V-ATPase, acting as a flexible stator that holds together the catalytic and membrane sectors of the enzyme. The release of subunit C from the ATPase complex results in the dissociation of the V1 and V0 subcomplexes, an important mechanism in controlling V-ATPase activity in cells.
By creating a high electroch
Plants are multicellular, predominantly photosynthetic eukaryotes of the kingdom Plantae. Plants were treated as one of two kingdoms including all living things that were not animals, all algae and fungi were treated as plants. However, all current definitions of Plantae exclude the fungi and some algae, as well as the prokaryotes. By one definition, plants form the clade Viridiplantae, a group that includes the flowering plants and other gymnosperms and their allies, liverworts and the green algae, but excludes the red and brown algae. Green plants obtain most of their energy from sunlight via photosynthesis by primary chloroplasts that are derived from endosymbiosis with cyanobacteria, their chloroplasts contain b, which gives them their green color. Some plants are parasitic or mycotrophic and have lost the ability to produce normal amounts of chlorophyll or to photosynthesize. Plants are characterized by sexual reproduction and alternation of generations, although asexual reproduction is common.
There are about 320 thousand species of plants, of which the great majority, some 260–290 thousand, are seed plants. Green plants provide a substantial proportion of the world's molecular oxygen and are the basis of most of Earth's ecosystems on land. Plants that produce grain and vegetables form humankind's basic foods, have been domesticated for millennia. Plants have many cultural and other uses, as ornaments, building materials, writing material and, in great variety, they have been the source of medicines and psychoactive drugs; the scientific study of plants is known as a branch of biology. All living things were traditionally placed into one of two groups and animals; this classification may date from Aristotle, who made the distincton between plants, which do not move, animals, which are mobile to catch their food. Much when Linnaeus created the basis of the modern system of scientific classification, these two groups became the kingdoms Vegetabilia and Animalia. Since it has become clear that the plant kingdom as defined included several unrelated groups, the fungi and several groups of algae were removed to new kingdoms.
However, these organisms are still considered plants in popular contexts. The term "plant" implies the possession of the following traits multicellularity, possession of cell walls containing cellulose and the ability to carry out photosynthesis with primary chloroplasts; when the name Plantae or plant is applied to a specific group of organisms or taxon, it refers to one of four concepts. From least to most inclusive, these four groupings are: Another way of looking at the relationships between the different groups that have been called "plants" is through a cladogram, which shows their evolutionary relationships; these are not yet settled, but one accepted relationship between the three groups described above is shown below. Those which have been called "plants" are in bold; the way in which the groups of green algae are combined and named varies between authors. Algae comprise several different groups of organisms which produce food by photosynthesis and thus have traditionally been included in the plant kingdom.
The seaweeds range from large multicellular algae to single-celled organisms and are classified into three groups, the green algae, red algae and brown algae. There is good evidence that the brown algae evolved independently from the others, from non-photosynthetic ancestors that formed endosymbiotic relationships with red algae rather than from cyanobacteria, they are no longer classified as plants as defined here; the Viridiplantae, the green plants – green algae and land plants – form a clade, a group consisting of all the descendants of a common ancestor. With a few exceptions, the green plants have the following features in common, they undergo closed mitosis without centrioles, have mitochondria with flat cristae. The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria. Two additional groups, the Rhodophyta and Glaucophyta have primary chloroplasts that appear to be derived directly from endosymbiotic cyanobacteria, although they differ from Viridiplantae in the pigments which are used in photosynthesis and so are different in colour.
These groups differ from green plants in that the storage polysaccharide is floridean starch and is stored in the cytoplasm rather than in the plastids. They appear to have had a common origin with Viridiplantae and the three groups form the clade Archaeplastida, whose name implies that their chloroplasts were derived from a single ancient endosymbiotic event; this is the broadest modern definition of the term'plant'. In contrast, most other algae not only have different pigments but have chloroplasts with three or four surrounding membranes, they are not close relatives of the Archaeplastida having acquired chloroplasts separately from ingested or symbiotic green and red algae. They are thus not included in the broadest modern definition of the plant kingdom, although they were in the past; the green plants or Viridiplantae were traditionally divided into the green algae (including
Protein structure is the three-dimensional arrangement of atoms in an amino acid-chain molecule. Proteins are polymers – polypeptides – formed from sequences of amino acids, the monomers of the polymer. A single amino acid monomer may be called a residue indicating a repeating unit of a polymer. Proteins form by amino acids undergoing condensation reactions, in which the amino acids lose one water molecule per reaction in order to attach to one another with a peptide bond. By convention, a chain under 30 amino acids is identified as a peptide, rather than a protein. To be able to perform their biological function, proteins fold into one or more specific spatial conformations driven by a number of non-covalent interactions such as hydrogen bonding, ionic interactions, Van der Waals forces, hydrophobic packing. To understand the functions of proteins at a molecular level, it is necessary to determine their three-dimensional structure; this is the topic of the scientific field of structural biology, which employs techniques such as X-ray crystallography, NMR spectroscopy, dual polarisation interferometry to determine the structure of proteins.
Protein structures range in size from tens to several thousand amino acids. By physical size, proteins are classified as nanoparticles, between 1–100 nm. Large aggregates can be formed from protein subunits. For example, many thousands of actin molecules assemble into a microfilament. A protein undergoes reversible structural changes in performing its biological function; the alternative structures of the same protein are referred to as different conformational isomers, or conformations, transitions between them are called conformational changes. There are four distinct levels of protein structure; the primary structure of a protein refers to the sequence of amino acids in the polypeptide chain. The primary structure is held together by peptide bonds that are made during the process of protein biosynthesis; the two ends of the polypeptide chain are referred to as the carboxyl terminus and the amino terminus based on the nature of the free group on each extremity. Counting of residues always starts at the N-terminal end, the end where the amino group is not involved in a peptide bond.
The primary structure of a protein is determined by the gene corresponding to the protein. A specific sequence of nucleotides in DNA is transcribed into mRNA, read by the ribosome in a process called translation; the sequence of amino acids in insulin was discovered by Frederick Sanger, establishing that proteins have defining amino acid sequences. The sequence of a protein is unique to that protein, defines the structure and function of the protein; the sequence of a protein can be determined by methods such as Edman degradation or tandem mass spectrometry. However, it is read directly from the sequence of the gene using the genetic code, it is recommended to use the words "amino acid residues" when discussing proteins because when a peptide bond is formed, a water molecule is lost, therefore proteins are made up of amino acid residues. Post-translational modification such as phosphorylations and glycosylations are also considered a part of the primary structure, cannot be read from the gene.
For example, insulin is composed of 51 amino acids in 2 chains. One chain has 31 amino acids, the other has 20 amino acids. Secondary structure refers to regular local sub-structures on the actual polypeptide backbone chain. Two main types of secondary structure, the α-helix and the β-strand or β-sheets, were suggested in 1951 by Linus Pauling et al; these secondary structures are defined by patterns of hydrogen bonds between the main-chain peptide groups. They have a regular geometry, being constrained to specific values of the dihedral angles ψ and φ on the Ramachandran plot. Both the α-helix and the β-sheet represent a way of saturating all the hydrogen bond donors and acceptors in the peptide backbone; some parts of the protein do not form any regular structures. They should not be confused with random coil, an unfolded polypeptide chain lacking any fixed three-dimensional structure. Several sequential secondary structures may form a "supersecondary unit". Tertiary structure refers to the three-dimensional structure of monomeric and multimeric protein molecules.
The α-helixes and β-pleated-sheets are folded into a compact globular structure. The folding is driven by the non-specific hydrophobic interactions, the burial of hydrophobic residues from water, but the structure is stable only when the parts of a protein domain are locked into place by specific tertiary interactions, such as salt bridges, hydrogen bonds, the tight packing of side chains and disulfide bonds; the disulfide bonds are rare in cytosolic proteins, since the cytosol is a reducing environment. Quaternary structure is the three-dimensional structure consisting of the aggregation of two or more individual polypeptide chains that operate as a single functional unit; the resulting multimer is stabilized by the same non-covalent interactions and disulfide bonds as in tertiary structure. There are many possible quaternary structure organisations. Complexes of two or more polypeptides are called multimers, it would be called a dimer if it contains two subunits, a trimer if it contains three subunits, a tetramer if it contains four subunits, a pentamer if it contains five subunits.
The subunits are related to one another by symmetry operations, such as a 2-fold axis in a dimer. Multimers made up of identical subunits are referred to with a prefix of "homo-" and those made up of different subuni
In chemistry, pH is a scale used to specify how acidic or basic a water-based solution is. Acidic solutions have a lower pH, while basic solutions have a higher pH. At room temperature, pure water is neither acidic nor basic and has a pH of 7; the pH scale is logarithmic and approximates the negative of the base 10 logarithm of the molar concentration of hydrogen ions in a solution. More it is the negative of the base 10 logarithm of the activity of the hydrogen ion. At 25 °C, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic; the neutral value of the pH depends on the temperature, being lower than 7 if the temperature increases. Contrary to popular belief, the pH value can be less than 0 or greater than 14 for strong acids and bases respectively; the pH scale is traceable to a set of standard solutions whose pH is established by international agreement. Primary pH standard values are determined using a concentration cell with transference, by measuring the potential difference between a hydrogen electrode and a standard electrode such as the silver chloride electrode.
The pH of aqueous solutions can be measured with a glass electrode and a pH meter, or a color-changing indicator. Measurements of pH are important in chemistry, medicine, water treatment, many other applications; the concept of pH was first introduced by the Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909 and revised to the modern pH in 1924 to accommodate definitions and measurements in terms of electrochemical cells. In the first papers, the notation had the "H" as a subscript to the lowercase "p", as so: pH; the exact meaning of the "p" in "pH" is disputed, but according to the Carlsberg Foundation, pH stands for "power of hydrogen". It has been suggested that the "p" stands for the German Potenz, others refer to French puissance. Another suggestion is that the "p" stands for the Latin terms pondus hydrogenii, potentia hydrogenii, or potential hydrogen, it is suggested that Sørensen used the letters "p" and "q" to label the test solution and the reference solution.
In chemistry, the p stands for "decimal cologarithm of", is used in the term pKa, used for acid dissociation constants. Bacteriologist Alice C. Evans, famed for her work's influence on dairying and food safety, credited William Mansfield Clark and colleagues with developing pH measuring methods in the 1910s, which had a wide influence on laboratory and industrial use thereafter. In her memoir, she does not mention how much, or how little and colleagues knew about Sørensen's work a few years prior, she said: In these studies Dr. Clark's attention was directed to the effect of acid on the growth of bacteria, he found that it is the intensity of the acid in terms of hydrogen-ion concentration that affects their growth. But existing methods of measuring acidity determined not the intensity, of the acid. Next, with his collaborators, Dr. Clark developed accurate methods for measuring hydrogen-ion concentration; these methods replaced the inaccurate titration method of determining acid content in use in biologic laboratories throughout the world.
They were found to be applicable in many industrial and other processes in which they came into wide usage. The first electronic method for measuring pH was invented by Arnold Orville Beckman, a professor at California Institute of Technology in 1934, it was in response to local citrus grower Sunkist that wanted a better method for testing the pH of lemons they were picking from their nearby orchards. PH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, aH+, in a solution. PH = − log 10 = log 10 For example, for a solution with a hydrogen ion activity of 5×10−6 we get 1/ = 2×105, thus such a solution has a pH of log10 = 5.3. For a commonplace example based on the facts that the masses of a mole of water, a mole of hydrogen ions, a mole of hydroxide ions are 18 g, 1 g, 17 g, a quantity of 107 moles of pure water, or 180 tonnes, contains close to 1 g of dissociated hydrogen ions and 17 g of hydroxide ions. Note that pH depends on temperature. For instance at 0 °C the pH of pure water is 7.47.
At 25 °C it's 7.00, at 100 °C it's 6.14. This definition was adopted because ion-selective electrodes, which are used to measure pH, respond to activity. Ideally, electrode potential, E, follows the Nernst equation, for the hydrogen ion can be written as E = E 0 + R T F ln = E 0 − 2.303 R T F pH where E is a measured potential, E0 is the standard electrode potential, R is the gas const
Coenzyme Q – cytochrome c reductase
The coenzyme Q: cytochrome c – oxidoreductase, sometimes called the cytochrome bc1 complex, at other times complex III, is the third complex in the electron transport chain, playing a critical role in biochemical generation of ATP. Complex III is a multisubunit transmembrane protein encoded by both the mitochondrial and the nuclear genomes. Complex III is present in the mitochondria of all animals and all aerobic eukaryotes and the inner membranes of most eubacteria. Mutations in Complex III cause exercise intolerance as well as multisystem disorders; the bc1 complex contains 11 subunits, 3 respiratory subunits, 2 core proteins and 6 low-molecular weight proteins. Ubiquinol—cytochrome-c reductase catalyzes the chemical reaction QH2 + 2 ferricytochrome c ⇌ Q + 2 ferrocytochrome c + 2 H+Thus, the two substrates of this enzyme are quinol and ferri- cytochrome c, whereas its 3 products are quinone, ferro- cytochrome c, H+; this enzyme belongs to the family of oxidoreductases those acting on diphenols and related substances as donor with a cytochrome as acceptor.
This enzyme participates in oxidative phosphorylation. It has four cofactors: cytochrome c1, cytochrome b-562, cytochrome b-566, a 2-Iron ferredoxin of the Rieske type; the systematic name of this enzyme class is ubiquinol:ferricytochrome-c oxidoreductase. Other names in common use include: Compared to the other major proton-pumping subunits of the electron transport chain, the number of subunits found can be small, as small as three polypeptide chains; this number does increase, eleven subunits are found in higher animals. Three subunits have prosthetic groups; the cytochrome b subunit has two b-type hemes, the cytochrome c subunit has one c-type heme, the Rieske Iron Sulfur Protein subunit has a two iron, two sulfur iron-sulfur cluster. Structures of complex III: PDB: 1KYO, PDB: 1L0L In vertebrates the bc1 complex, or Complex III, contains 11 subunits: 3 respiratory subunits, 2 core proteins and 6 low-molecular weight proteins. Proteobacterial complexes may contain as few as three subunits.
A In vertebrates, a cleavage product of 8 kDa from the N-terminus of the Rieske protein is retained in the complex as subunit 9. Thus subunits 10 and 11 correspond to fungal QCR9p and QCR10p, it catalyzes the reduction of cytochrome c by oxidation of coenzyme Q and the concomitant pumping of 4 protons from the mitochondrial matrix to the intermembrane space: QH2 + 2 cytochrome c + 2 H+in → Q + 2 cytochrome c + 4 H+outIn the process called Q cycle, two protons are consumed from the matrix, four protons are released into the inter membrane space and two electrons are passed to cytochrome c. The reaction mechanism for complex III is known as the ubiquinone cycle. In this cycle four protons get released into the positive "P" side, but only two protons get taken up from the negative "N" side; as a result, a proton gradient is formed across the membrane. In the overall reaction, two ubiquinols are oxidized to ubiquinones and one ubiquinone is reduced to ubiquinol. In the complete mechanism, two electrons are transferred from ubiquinol to ubiquinone, via two cytochrome c intermediates.
Overall: 2 x QH2 oxidised to Q 1 x Q reduced to QH2 2 x Cyt c reduced 4 x H+ released into intermembrane space 2 x H+ picked up from matrixThe reaction proceeds according to the following steps: Round 1: Cytochrome b binds a ubiquinol and a ubiquinone. The 2Fe/2S center and BL heme each pull an electron off the bound ubiquinol, releasing two hydrogens into the intermembrane space. One electron is transferred to cytochrome c1 from the 2Fe/2S centre, whilst another is transferred from the BL heme to the BH Heme. Cytochrome c1 transfers its electron to cytochrome c, the BH Heme transfers its electron to a nearby ubiquinone, resulting in the formation of a ubisemiquinone. Cytochrome c diffuses; the first ubiquinol is released, whilst the semiquinone remains bound. Round 2: A second ubiquinol is bound by cytochrome b; the 2Fe/2S center and BL heme each pull an electron off the bound ubiquinol, releasing two hydrogens into the intermembrane space. One electron is transferred to cytochrome c1 from the 2Fe/2S centre, whilst another is transferred from the BL heme to the BH Heme.
Cytocrome c1 transfers its electron to cytochrome c, whilst the nearby semiquinone produced from round 1 picks up a second electron from the BH heme, along with two protons from the matrix. The second ubiquinol, along with the newly formed ubiquinol are released. There are three distinct groups of Complex III inhibitors. Antimycin A binds to the Qi site and inhibits the transfer of electrons in Complex III from heme bH to oxidized Q. Myxothiazol and stigmatellin binds to the Qo site and inhibits the transfer of electrons from reduced QH2 to the Rieske Iron sulfur protein. Myxothiazol and stigmatellin bind to distinct but overlapping pockets within the Qo site. Myxothiazol binds nearer to cytochrome bL. Stigmatellin binds farther from heme bL and nearer the Rieske Iron sulfur protein, with which it interacts; some have been commercialized as anti-malaria agents. Propylhexedrine inhibits cytochrome c reductase. A small fraction of electrons leave the electron transport chain before reaching complex IV.
Cytochrome b6f complex
The cytochrome b6f complex is an enzyme found in the thylakoid membrane in chloroplasts of plants and green algae, that catalyzes the transfer of electrons from plastoquinol to plastocyanin. The reaction is analogous to the reaction catalyzed by cytochrome bc1 of the mitochondrial electron transport chain. During photosynthesis, the cytochrome b6f complex is one step along the chain that transfers electrons from Photosystem II to Photosystem I, at the same time pumps protons into the thylakoid space that contribute to create an electrochemical gradient, used to synthesize ATP from ADP; the cytochrome b6f complex is a dimer, with each monomer composed of eight subunits. These consist of four large subunits: a 32 kDa cytochrome f with a c-type cytochrome, a 25 kDa cytochrome b6 with a low- and high-potential heme group, a 19 kDa Rieske iron-sulfur protein containing a cluster, a 17 kDa subunit IV; the total molecular weight is 217 kDa. The crystal structure of cytochrome b6f complexes from Chlamydomonas reinhardtii, Mastigocladus laminosus, Nostoc sp.
PCC 7120 have been determined. The core of the complex is structurally similar to cytochrome bc1 core. Cytochrome b6 and subunit IV are homologous to cytochrome b and the Rieske iron-sulfur proteins of the two complexes are homologous. However, cytochrome f and cytochrome c1 are not homologous. Cytochrome b6f contains seven prosthetic groups. Four are found in both cytochrome b6f and bc1: the c-type heme of cytochrome c1 and f, the two b-type hemes in bc1 and b6f, the cluster of the Rieske protein. Three unique prosthetic groups are found in cytochrome b6f: chlorophyll a, β-carotene, heme cn; the inter-monomer space within the core of the cytochrome b6f complex dimer is occupied by lipids, which provides directionality to heme-heme electron transfer through modulation of the intra-protein dielectric environment. In photosynthesis, the cytochrome b6f complex functions to mediate the transfer of electrons between the two photosynthetic reaction center complexes, from Photosystem II to Photosystem I, while transferring protons from the chloroplast stroma across the thylakoid membrane into the lumen.
Electron transport via cytochrome b6f is responsible for creating the proton gradient that drives the synthesis of ATP in chloroplasts. In a separate reaction, the cytochrome b6f complex plays a central role in cyclic photophosphorylation, when NADP+ is not available to accept electrons from reduced ferredoxin; this cycle results in the creation of a proton gradient by cytochrome b6f, which can be used to drive ATP synthesis. It has been shown that this cycle is essential for photosynthesis, in which it is proposed to help maintain the proper ratio of ATP/NADPH production for carbon fixation; the p-side quinol deprotonation-oxidation reactions within the cytochrome b6f complex have been implicated in the generation of reactive oxygen species. An integral chlorophyll molecule located within the quinol oxidation site has been suggested to perform a structural, non-photochemical function in enhancing the rate of formation of the reactive oxygen species to provide a redox-pathway for intra-cellular communication.
The cytochrome b6f complex is responsible for "non-cyclic" and "cyclic" electron transfer between two mobile redox carriers and plastocyanin: Cytochrome b6f catalyzes the transfer of electrons from plastoquinol to plastocyanin, while pumping two protons from the stroma into the thylakoid lumen: QH2 + 2Pc + 2H+ → Q + 2Pc + 4H+ This reaction occurs through the Q cycle as in Complex III. Plastoquinone acts as the electron carrier, transferring its two electrons to high- and low-potential electron transport chains via a mechanism called electron bifurcation. First half of Q cycle QH2 binds to the positive'p' side of the complex, it is oxidized to a semiquinone by the iron-sulfur center and releases two protons to the thylakoid lumen. The reduced iron-sulfur center transfers its electron through cytochrome f to Pc. In the low-potential ETC, SQ transfers its electron to heme bp of cytochrome b6. Heme bp transfers the electron to heme bn. Heme bn reduces Q with one electron to form SQ. Second half of Q cycle A second QH2 binds to the complex.
In the high-potential ETC, one electron reduces another oxidized Pc. In the low-potential ETC, the electron from heme bn is transferred to SQ, the reduced Q2− takes up two protons from the stroma to form QH2; the oxidized Q and the reduced QH2, regenerated diffuse into the membrane. In contrast to Complex III, cytochrome b6f catalyzes another electron transfer reaction, central to cyclic photophosphorylation; the electron from ferredoxin is transferred to plastoquinone and the cytochrome b6f complex to reduce plastocyanin, reoxidized by P700 in Photosystem I. The exact mechanism for how plastoquinone is reduced by ferredoxin is still under investigation. One proposal is that there exists a ferredoxin: an NADP dehydrogenase. Since heme x does not appear to be required for the Q cycle and is not found in Complex III, it has been proposed that it is used for cyclic photophosphorylation by the following mechanism: Fd + heme x → Fd + heme x heme x + Fd + Q + 2H+ → heme x + Fd + QH2 Structure-Function Studies of the Cytochrome b6f Complex - Current research on cytochrome b6f in William Cramer's Lab at Purdue University, USA UMich Orientation of Proteins in Membranes families/superfamily-3 - Calculated positions of b6f and