Pyruvate kinase is the enzyme that catalyzes the final step of glycolysis. It catalyzes the transfer of a phosphate group from phosphoenolpyruvate to adenosine diphosphate, yielding one molecule of pyruvate and one molecule of ATP. Pyruvate kinase was inappropriately named before it was recognized that it did not directly catalyze phosphorylation of phosphoenolpyruvate, which does not occur under physiological conditions. Pyruvate kinase is present in four distinct, tissue-specific isozymes in animals, each consisting of particular kinetic properties necessary to accommodate the variations in metabolic requirements of diverse tissues. There are four isozymes of pyruvate kinase in vertebrates: L, R, M1 and M2. R and L isozymes differ from M1 and M2 in that they are both allosterically and reversibly regulated. From a kinetic standpoint, the R and L isozymes of pyruvate kinase have two key conformation states; the R-state, characterized by high substrate affinity, serves as the activated form of pyruvate kinase and is stabilized by PEP and FBP, promoting the glycolytic pathway.
The T-state, characterized by low substrate affinity, serves as the inactivated form of pyruvate kinase and stabilized by ATP and alanine, causing phosphorylation of pyruvate kinase and the inhibition of glycolysis. Gene expression varies between the different isozymes. M1 and M2 isozymes are regulated by the gene PKM and R and L isozymes are regulated by the gene PKLR. In terms of structure, there is both a dimeric form of pyruvate kinase; the tetrameric form is the pyruvate kinase structure in its R-state conformation, namely with high binding affinity to PEP. In contrast, the dimeric form is its structure in T-state conformation, namely with a low binding affinity to PEP; as a result, gene expression can be regulated by converting the active tetrameric form of PKM2, which yields high PEP concentrations, into an inactive dimeric form, which yields a PEP concentration of nearly zero. The PKM gene consists of 11 introns. PKM1 and PKM2 are different splicing products of the M-gene and differ in 23 amino acids within a 56-amino acid stretch at their carboxy terminus.
The PKM gene is regulated through heterogenous ribonucleotide proteins like hnRNPA1 and hnRNPA2. Human PKM2 monomer is a single chain divided into A, B and C domains; the difference in amino acid sequence between PKM1 and PKM2 allows PKM2 to be allosterically regulated by FBP and for it to form dimers and tetramers while PKM1 can only form tetramers. Many Enterobacteriaceae, including E. coli, have two isoforms of pyruvate kinase, PykA and PykF, which are 37% identical in E. coli. They catalyze the same reaction as in eukaryotes, namely the generation of ATP from ADP and PEP, the last step in glycolysis, a step, irreversible under physiological conditions. PykF is allosterically regulated by fructose 1,6-bisphosphate which reflects the central position of PykF in cellular metabolism. PykF transcription in E. coli is regulated by Cra. PfkB was shown to be inhibited by MgATP at low concentrations of Fru-6P, this regulation is important for gluconeogenesis. There are two steps in the pyruvate kinase reaction in glycolysis.
First, PEP transfers a phosphate group to ADP, producing the enolate of pyruvate. Secondly, a proton must be added to the enolate of pyruvate to produce the functional form of pyruvate that the cell requires; because the substrate for pyruvate kinase is a simple phospho-sugar, the product is an ATP, pyruvate kinase is a possible foundation enzyme for the evolution of the glycolysis cycle, may be one of the most ancient enzymes in all earth-based life. In Archaean oceans, phospho-enolpyruvate may have been present abiotically. In yeast cells, the interaction of yeast pyruvate kinase with PEP and its allosteric effector Fructose 1,6-bisphosphate was found to be enhanced by the presence of Mg2+. Therefore, Mg2+ was isolated as an important component in the successful catalysis of PEP into pyruvate by pyruvate kinase. Furthermore, the metal ion Mn2+ was shown to have a similar, but stronger effect on the coupling free energy of YPK than Mg2+; the binding of metal ions to the metal binding sites on pyruvate kinase enhance the rate of this glycolytic reaction.
The glycolytic reaction catalyzed by pyruvate kinase is the final step of glycolysis. It is one of the three rate-affecting steps of the catabolic reaction cascade; the rate-affecting steps are the slower steps of a reaction and thus determines the rate of the overall reaction. In glycolysis, the rate-affecting steps are coupled with the hydrolysis of ATP or the phosphorylation of ADP to create the energetically favorable and irreversible reaction mechanism; this final step is regulated and deliberately irreversible because pyruvate is a crucial intermediate building block for further metabolic pathways. Once pyruvate kinase synthesizes pyruvate, pyruvate either enters the TCA cycle for further production of ATP under aerobic conditions, or is reduced to lactate under anaerobic conditions. Both of these secondary metabolic pathways are essential to the function of the metabolism. Pyruvate kinase serves as a regulatory enzyme for gluconeogenesis, a biochemical pathway in which the liver generates glucose from pyruvate and other substrates.
Gluconeogenesis utilizes noncarbohydrate sources to provide glucose to the brain and red blood cells in times of starvat
Biochemistry, sometimes called biological chemistry, is the study of chemical processes within and relating to living organisms. Biochemical processes give rise to the complexity of life. A sub-discipline of both biology and chemistry, biochemistry can be divided in three fields. Over the last decades of the 20th century, biochemistry has through these three disciplines become successful at explaining living processes. All areas of the life sciences are being uncovered and developed by biochemical methodology and research. Biochemistry focuses on understanding how biological molecules give rise to the processes that occur within living cells and between cells, which in turn relates to the study and understanding of tissues and organism structure and function. Biochemistry is related to molecular biology, the study of the molecular mechanisms by which genetic information encoded in DNA is able to result in the processes of life. Much of biochemistry deals with the structures and interactions of biological macromolecules, such as proteins, nucleic acids and lipids, which provide the structure of cells and perform many of the functions associated with life.
The chemistry of the cell depends on the reactions of smaller molecules and ions. These can be inorganic, for example water and metal ions, or organic, for example the amino acids, which are used to synthesize proteins; the mechanisms by which cells harness energy from their environment via chemical reactions are known as metabolism. The findings of biochemistry are applied in medicine and agriculture. In medicine, biochemists investigate the cures of diseases. In nutrition, they study how to maintain health wellness and study the effects of nutritional deficiencies. In agriculture, biochemists investigate soil and fertilizers, try to discover ways to improve crop cultivation, crop storage and pest control. At its broadest definition, biochemistry can be seen as a study of the components and composition of living things and how they come together to become life, in this sense the history of biochemistry may therefore go back as far as the ancient Greeks. However, biochemistry as a specific scientific discipline has its beginning sometime in the 19th century, or a little earlier, depending on which aspect of biochemistry is being focused on.
Some argued that the beginning of biochemistry may have been the discovery of the first enzyme, diastase, in 1833 by Anselme Payen, while others considered Eduard Buchner's first demonstration of a complex biochemical process alcoholic fermentation in cell-free extracts in 1897 to be the birth of biochemistry. Some might point as its beginning to the influential 1842 work by Justus von Liebig, Animal chemistry, or, Organic chemistry in its applications to physiology and pathology, which presented a chemical theory of metabolism, or earlier to the 18th century studies on fermentation and respiration by Antoine Lavoisier. Many other pioneers in the field who helped to uncover the layers of complexity of biochemistry have been proclaimed founders of modern biochemistry, for example Emil Fischer for his work on the chemistry of proteins, F. Gowland Hopkins on enzymes and the dynamic nature of biochemistry; the term "biochemistry" itself is derived from a combination of chemistry. In 1877, Felix Hoppe-Seyler used the term as a synonym for physiological chemistry in the foreword to the first issue of Zeitschrift für Physiologische Chemie where he argued for the setting up of institutes dedicated to this field of study.
The German chemist Carl Neuberg however is cited to have coined the word in 1903, while some credited it to Franz Hofmeister. It was once believed that life and its materials had some essential property or substance distinct from any found in non-living matter, it was thought that only living beings could produce the molecules of life. In 1828, Friedrich Wöhler published a paper on the synthesis of urea, proving that organic compounds can be created artificially. Since biochemistry has advanced since the mid-20th century, with the development of new techniques such as chromatography, X-ray diffraction, dual polarisation interferometry, NMR spectroscopy, radioisotopic labeling, electron microscopy, molecular dynamics simulations; these techniques allowed for the discovery and detailed analysis of many molecules and metabolic pathways of the cell, such as glycolysis and the Krebs cycle, led to an understanding of biochemistry on a molecular level. Philip Randle is well known for his discovery in diabetes research is the glucose-fatty acid cycle in 1963.
He confirmed. High fat oxidation was responsible for the insulin resistance. Another significant historic event in biochemistry is the discovery of the gene, its role in the transfer of information in the cell; this part of biochemistry is called molecular biology. In the 1950s, James D. Watson, Francis Crick, Rosalind Franklin, Maurice Wilkins were instrumental in solving DNA structure and suggesting its relationship with genetic transfer of information. In 1958, George Beadle and Edward Tatum received the Nobel Prize for work in fungi showing that one gene produces one enzyme. In 1988, Colin Pitchfork was the first person convicted of murder with DNA evidence, which led to the growth of forensic science. More Andrew Z. Fire and Craig C. Mello received the 2006 Nobel Prize for discovering the role of RNA interference, in the silencing of gene expression. Around two dozen of the 92
Malic acid is an organic compound with the molecular formula C4H6O5. It is a dicarboxylic acid, made by all living organisms, contributes to the sour taste of fruits, is used as a food additive. Malic acid has two stereoisomeric forms; the salts and esters of malic acid are known as malates. The malate anion is an intermediate in the citric acid cycle; the word'malic' is derived from Latin'malus', meaning'apple'. It is the name of the genus Malus, which includes all apples and crabapples; this derivation is seen in the traditional German name for malic acid,'Äpfelsäure' meaning'apple acid'. L-Malic acid is the occurring form, whereas a mixture of L- and D-malic acid is produced synthetically. Malate plays an important role in biochemistry. In the C4 carbon fixation process, malate is a source of CO2 in the Calvin cycle. In the citric acid cycle, -malate is an intermediate, formed by the addition of an -OH group on the si face of fumarate, it can be formed from pyruvate via anaplerotic reactions. Malate is synthesized by the carboxylation of phosphoenolpyruvate in the guard cells of plant leaves.
Malate, as a double anion accompanies potassium cations during the uptake of solutes into the guard cells in order to maintain electrical balance in the cell. The accumulation of these solutes within the guard cell decreases the solute potential, allowing water to enter the cell and promote aperture of the stomata. Malic acid was first isolated from apple juice by Carl Wilhelm Scheele in 1785. Antoine Lavoisier in 1787 proposed the name acide malique, derived from the Latin word for apple, mālum—as is its genus name Malus. In German it is named Äpfelsäure after plural or singular of the fruit apple. Malic acid is the main acid in many fruits, including apricots, blueberries, grapes, peaches, pears and quince and is present in lower concentrations in other fruits, such as citrus, it contributes to the sourness of unripe apples. Sour apples contain high proportions of the acid, it is present in most wines with concentrations sometimes as high as 5 g/l. It confers a tart taste to wine; the taste of malic acid is clear and pure in rhubarb, a plant for which it is the primary flavor.
It is a component of some artificial vinegar flavors, such as "salt and vinegar" flavored potato chips. In citrus, fruits produced in organic farming contain higher levels of malic acid than fruits produced in conventional agriculture; the process of malolactic fermentation converts malic acid to much milder lactic acid. Malic acid occurs in all fruits and many vegetables, is generated in fruit metabolism. Malic acid, when added to food products, is denoted by E number E296. Malic acid is the source of extreme tartness in United States-produced confectionery, the so-called extreme candy, it is used with or in place of the less sour citric acid in sour sweets. These sweets are sometimes labeled with a warning stating that excessive consumption can cause irritation of the mouth, it is approved for use as a food additive in the New Zealand. Malic acid provides 10 kJ of energy per gram during digestion. Racemic malic acid is produced industrially by the double hydration of maleic anhydride. In 2000, American production capacity was 5000 tons per year.
Both enantiomers may be separated by chiral resolution of the racemic mixture, the - enantiomer may be obtained by fermentation of fumaric acid. Self-condensation of malic acid with fuming sulfuric acid gives the pyrone coumalic acid: Malic acid was important in the discovery of the Walden inversion and the Walden cycle, in which -malic acid first is converted into -chlorosuccinic acid by action of phosphorus pentachloride. Wet silver oxide converts the chlorine compound to -malic acid, which reacts with PCl5 to the -chlorosuccinic acid; the cycle is completed. Click on genes and metabolites below to link to respective articles. Acids in wine Crassulacean acid metabolism Malate-aspartate shuttle Malic acid MS Spectrum Calculator: Water and solute activities in aqueous malic acid
Simplified molecular-input line-entry system
The simplified molecular-input line-entry system is a specification in the form of a line notation for describing the structure of chemical species using short ASCII strings. SMILES strings can be imported by most molecule editors for conversion back into two-dimensional drawings or three-dimensional models of the molecules; the original SMILES specification was initiated in the 1980s. It has since been extended. In 2007, an open standard called. Other linear notations include the Wiswesser line notation, ROSDAL, SYBYL Line Notation; the original SMILES specification was initiated by David Weininger at the USEPA Mid-Continent Ecology Division Laboratory in Duluth in the 1980s. Acknowledged for their parts in the early development were "Gilman Veith and Rose Russo and Albert Leo and Corwin Hansch for supporting the work, Arthur Weininger and Jeremy Scofield for assistance in programming the system." The Environmental Protection Agency funded the initial project to develop SMILES. It has since been modified and extended by others, most notably by Daylight Chemical Information Systems.
In 2007, an open standard called "OpenSMILES" was developed by the Blue Obelisk open-source chemistry community. Other'linear' notations include the Wiswesser Line Notation, ROSDAL and SLN. In July 2006, the IUPAC introduced the InChI as a standard for formula representation. SMILES is considered to have the advantage of being more human-readable than InChI; the term SMILES refers to a line notation for encoding molecular structures and specific instances should be called SMILES strings. However, the term SMILES is commonly used to refer to both a single SMILES string and a number of SMILES strings; the terms "canonical" and "isomeric" can lead to some confusion when applied to SMILES. The terms are not mutually exclusive. A number of valid SMILES strings can be written for a molecule. For example, CCO, OCC and CC all specify the structure of ethanol. Algorithms have been developed to generate the same SMILES string for a given molecule; this SMILES is unique for each structure, although dependent on the canonicalization algorithm used to generate it, is termed the canonical SMILES.
These algorithms first convert the SMILES to an internal representation of the molecular structure. Various algorithms for generating canonical SMILES have been developed and include those by Daylight Chemical Information Systems, OpenEye Scientific Software, MEDIT, Chemical Computing Group, MolSoft LLC, the Chemistry Development Kit. A common application of canonical SMILES is indexing and ensuring uniqueness of molecules in a database; the original paper that described the CANGEN algorithm claimed to generate unique SMILES strings for graphs representing molecules, but the algorithm fails for a number of simple cases and cannot be considered a correct method for representing a graph canonically. There is no systematic comparison across commercial software to test if such flaws exist in those packages. SMILES notation allows the specification of configuration at tetrahedral centers, double bond geometry; these are structural features that cannot be specified by connectivity alone and SMILES which encode this information are termed isomeric SMILES.
A notable feature of these rules is. The term isomeric SMILES is applied to SMILES in which isotopes are specified. In terms of a graph-based computational procedure, SMILES is a string obtained by printing the symbol nodes encountered in a depth-first tree traversal of a chemical graph; the chemical graph is first trimmed to remove hydrogen atoms and cycles are broken to turn it into a spanning tree. Where cycles have been broken, numeric suffix labels are included to indicate the connected nodes. Parentheses are used to indicate points of branching on the tree; the resultant SMILES form depends on the choices: of the bonds chosen to break cycles, of the starting atom used for the depth-first traversal, of the order in which branches are listed when encountered. Atoms are represented by the standard abbreviation of the chemical elements, in square brackets, such as for gold. Brackets may be omitted in the common case of atoms which: are in the "organic subset" of B, C, N, O, P, S, F, Cl, Br, or I, have no formal charge, have the number of hydrogens attached implied by the SMILES valence model, are the normal isotopes, are not chiral centers.
All other elements must be enclosed in brackets, have charges and hydrogens shown explicitly. For instance, the SMILES for water may be written as either O or. Hydrogen may be written as a separate atom; when brackets are used, the symbol H is added if the atom in brackets is bonded to one or more hydrogen, followed by the number of hydrogen atoms if greater than 1 by the sign + for a positive charge or by - for a negative charge. For example, for ammonium. If there is more than one charge, it is written as digit.
Malate dehydrogenase is an enzyme that reversibly catalyzes the oxidation of malate to oxaloacetate using the reduction of NAD+ to NADH. This reaction is part including the citric acid cycle. Other malate dehydrogenases, which have other EC numbers and catalyze other reactions oxidizing malate, have qualified names like malate dehydrogenase. Several isozymes of malate dehydrogenase exist. There are two main isoforms in eukaryotic cells. One is found in the mitochondrial matrix, participating as a key enzyme in the citric acid cycle that catalyzes the oxidation of malate; the other is found in the cytoplasm, assisting the malate-aspartate shuttle with exchanging reducing equivalents so that malate can pass through the mitochondrial membrane to be transformed into oxaloacetate for further cellular processes. Humans and most other mammals express the following two malate dehydrogenases: The malate dehydrogenase family contains L-lactate dehydrogenase and L-2-hydroxyisocaproate dehydrogenases. L-lactate dehydrogenases catalyzes the conversion of L-lactate to pyruvate, the last step in anaerobic glycolysis.
The N-terminus is a Rossmann NAD-binding fold and the C-terminus is an unusual alpha+beta fold. In most organisms, malate dehydrogenase exists as a homodimeric molecule and is related to lactate dehydrogenase in structure, it is a large protein molecule with subunits weighing between 35 kDa. Based on the amino acid sequences, it seems that MDH has diverged into two main phylogenetic groups that resemble either mitochondrial isozymes or cytoplasmic/chloroplast isozymes; because the sequence identity of malate dehydrogenase in the mitochondria is more related to its prokaryotic ancestors in comparison to the cytoplasmic isozyme, the theory that mitochondria and chloroplasts were developed through endosymbiosis is plausible. The amino acid sequences of archaeal MDH are more similar to that of LDH than that of MDH of other organisms; this indicates that there is a possible evolutionary linkage between lactate dehydrogenase and malate dehydrogenase. Each subunit of the malate dehydrogenase dimer has two distinct domains that vary in structure and functionality.
A parallel β-sheet structure makes up the NAD+ binding domain, while four β-sheets and one α-helix comprise the central NAD+ binding site. The subunits are held together through extensive hydrophobic interactions. Malate dehydrogenase has been shown to have a mobile loop region that plays a crucial role in the enzyme's catalytic activity. Studies have shown that conformational change of this loop region from the open conformation to the closed conformation after binding of substrate enhances MDH catalysis through shielding of substrate and catalytic amino acids from solvent. Studies have indicated that this loop region is conserved in malate dehydrogenase; the active site of malate dehydrogenase is a hydrophobic cavity within the protein complex that has specific binding sites for the substrate and its coenzyme, NAD+. In its active state, MDH undergoes a conformational change that encloses the substrate to minimize solvent exposure and to position key residues in closer proximity to the substrate.
The three residues in particular that comprise a catalytic triad are histidine, both of which work together as a proton transfer system, arginines, which secure the substrate. Mechanistically, malate dehydrogenase catalyzes the oxidation of the hydroxyl group of malate by utilizing NAD+ as an electron acceptor; this oxidation step results in the elimination of a hydride ion from the substrate. NAD+ receives the hydride ion and becomes reduced to NADH while concomitantly, the His-195 residue on the enzyme accepts the proton; the positively charged His-195 residue, involved in base catalysis of the substrate, is stabilized by the adjacent, negatively charged Asp-168 residue. This electrostatic stabilization helps facilitate the transfer of the proton. Arg-102, Arg-109, Arg-171 participate in electrostatic catalysis and help to bind the negatively charged carboxylates on the substrate. Additionally, the Arginine residues on the enzyme provide additional substrate specificity and binding through hydrogen bonding between the guanidinium side chain of the Arginine amino acid residues and the carboxylates of the substrate.
Studies have identified a mobile loop in malate dehydrogenase that participates in the catalytic activity of the enzyme. The loop undergoes a conformational change to shield the substrate and catalytic amino acids from the solvent in response to the binding of the malate dehydrogenase:coenzyme complex to substrate; this flipping of the loop to the up position to cover the active site promotes enhanced interaction of the catalytically important amino residues on the enzyme with the substrate. Additionally, the movement of the loop has been shown to correlate with the rate determining step of the enzyme. Malate dehydrogenases catalyzes the interconversion of malate to oxaloacetate. In the citric acid cycle, malate dehydrogenase is responsible for catalyzing the regeneration of oxaloacetate This reaction occurs through the oxidation of hydroxyl group on malate and reduction of NAD+; the mechanism of the transfer of the hydride ion to NAD+ is carried out in a similar mechanism seen in lactate dehydrogenase and alcohol dehydrogenase.
The ΔG'° of malate dehydrogenase is +29.7 kJ/mol and the ΔG is 0 kJ/mol. Malate dehydrogenase is involved in gluconeogenesis, the synthesis of glucose from smaller molecules. Pyruvate in the mitochondria is acted upon by pyruvate
Carbon fixation or сarbon assimilation is the conversion process of inorganic carbon to organic compounds by living organisms. The most prominent example is photosynthesis, although chemosynthesis is another form of carbon fixation that can take place in the absence of sunlight. Organisms that grow by fixing carbon are called autotrophs. Autotrophs include photoautotrophs, which synthesize organic compounds using the energy of sunlight, lithoautotrophs, which synthesize organic compounds using the energy of inorganic oxidation. Heterotrophs are organisms; the organic compounds are used by heterotrophs to build body structures. "Fixed carbon", "reduced carbon", "organic carbon" are equivalent terms for various organic compounds. It is estimated that 258 billion tons of carbon dioxide are converted by photosynthesis annually; the majority of the fixation occurs in marine environments areas of high nutrients. The gross amount of carbon dioxide fixed is much larger since 40% is consumed by respiration following photosynthesis.
Given the scale of this process, it is understandable that RuBisCO is the most abundant protein on Earth. Six autotrophic carbon fixation pathways are known as of 2011; the Calvin cycle fixes carbon in the chloroplasts of plants and algae, in the cyanobacteria. It fixes carbon in the anoxygenic photosynthetic in one type of proteobacteria called purple bacteria, in some non-phototrophic proteobacteria. In photosynthesis, energy from sunlight drives the carbon fixation pathway. Oxygenic photosynthesis is used by the primary producers—plants and cyanobacteria, they contain the pigment chlorophyll, use the Calvin cycle to fix carbon autotrophically. The process works like this: 2H2O → 4e− + 4H+ + O2CO2 + 4e− + 4H+ → CH2O + H2OIn the first step, water is dissociated into electrons and free oxygen; this allows the use of water, one of the most abundant substances on Earth, as an electron donor—as a source of reducing power. The release of free oxygen is a side-effect of enormous consequence; the first step uses the energy of sunlight to oxidize water to O2, to produce ATP ADP + Pi ⇌ ATP + H2Oand the reductant, NADPH NADP+ + 2e− + 2H+ ⇌ NADPH + H+In the second step, called the Calvin cycle, the actual fixation of carbon dioxide is carried out.
This process consumes ATP and NADPH. The Calvin cycle in plants accounts for the preponderance of carbon fixation on land. In algae and cyanobacteria, it accounts for the preponderance of carbon fixation in the oceans; the Calvin cycle converts carbon dioxide into sugar, as triose phosphate, glyceraldehyde 3-phosphate together with dihydroxyacetone phosphate: 3 CO2 + 12 e− + 12 H+ + Pi → TP + 4 H2OAn alternative perspective accounts for NADPH and ATP: 3 CO2 + 6 NADPH + 6 H+ + 9 ATP + 5 H2O → TP + 6 NADP+ + 9 ADP + 8 PiThe formula for inorganic phosphate is HOPO32− + 2H+. Formulas for triose and TP are C2H3O2-CH2OH and C2H3O2-CH2OPO32− + 2H+ Somewhere between 3.8 and 2.3 billion years ago, the ancestors of cyanobacteria evolved oxygenic photosynthesis, enabling the use of the abundant yet oxidized molecule H2O as an electron donor to the electron transport chain of light-catalyzed proton-pumping responsible for efficient ATP synthesis. When this evolutionary breakthrough occurred, autotrophy is believed to have been developed.
However, the proliferation of cyanobacteria, due to their novel ability to exploit water as a source of electrons, radically altered the global environment by oxygenating the atmosphere and by achieving large fluxes of CO2 consumption. Many photosynthetic organisms have not acquired CO2 concentrating mechanisms, which increase the concentration of CO2 available to the initial carboxylase of the Calvin cycle, the enzyme RuBisCO; the benefits of a CCM include increased tolerance to low external concentrations of inorganic carbon, reduced losses to photorespiration. CCMs can make plants more tolerant of water stress. CO2 concentrating mechanisms use the enzyme carbonic anhydrase, which catalyze both the dehydration of bicarbonate to CO2 and the hydration of CO2 to bicarbonate HCO3− + H+ ⇌ CO2 + H2OLipid membranes are much less permeable to bicarbonate than to CO2. To capture inorganic carbon more some plants have adapted the anaplerotic reactions HCO3− + H+ + PEP → OAA + Picatalyzed by PEP carboxylase, to carboxylate phosphoenolpyruvate to oxaloacetate, a C4 dicarboxylic acid.
CAM plants that use Crassulacean acid metabolism as an adaptation for arid conditions. CO2 enters through the stomata during the night and is converted into the 4-carbon compound, malic acid, which releases CO2 for use in the Calvin cycle during the day, when the stomata are closed; the dung jade plant and cacti are typical of CAM plants. Sixteen thousand species of plants use CAM; these plants have a carbon isotope signature of −20 to −10 ‰. C4 plants preface the Calvin cycle with reactions that incorporate CO2 into one of the 4-carbon compounds, malic acid or aspartic acid. C4 plants have a distinctive internal leaf anatomy. Tropical grasses, such as sugar cane and maize are C4 plants, but there are many broadleaf plants that are C4. Overall, 7600 species of terrestrial plants use C4 carbon fixation, representing around 3% of all species; these plants have a carbon isotope signature of −16 to −10 ‰. The large majority of plants are C3 plants, they are so-called to distinguish them from the CAM and C4 plants, because the carboxylation products of the Calvin cycle are 3-carbon compounds.
They lack C4 dicarboxylic a