Pages in category "Glycolysis"
The following 38 pages are in this category, out of 38 total. This list may not reflect recent changes (learn more).
The following 38 pages are in this category, out of 38 total. This list may not reflect recent changes (learn more).
1. Glycolysis – Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+. The free energy released in this process is used to form the high-energy molecules ATP, glycolysis is a determined sequence of ten enzyme-catalyzed reactions. The intermediates provide entry points to glycolysis, for example, most monosaccharides, such as fructose and galactose, can be converted to one of these intermediates. The intermediates may also be directly useful, for example, the intermediate dihydroxyacetone phosphate is a source of the glycerol that combines with fatty acids to form fat. Glycolysis is an oxygen independent metabolic pathway, meaning that it not use molecular oxygen for any of its reactions. However the products of glycolysis are sometimes metabolized using atmospheric oxygen, when molecular oxygen is used for the metabolism of the products of glycolysis the process is usually referred to as aerobic, whereas if no oxygen is used the process is said to be anaerobic. Thus, glycolysis occurs, with variations, in all organisms. The wide occurrence of glycolysis indicates that it is one of the most ancient metabolic pathways, glycolysis could thus have originated from chemical constraints of the prebiotic world. Glycolysis occurs in most organisms in the cytosol of the cell, the most common type of glycolysis is the Embden–Meyerhof–Parnas, which was discovered by Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas. Glycolysis also refers to other pathways, such as the Entner–Doudoroff pathway, however, the discussion here will be limited to the Embden–Meyerhof–Parnas pathway. The overall reaction of glycolysis is, The use of symbols in this equation makes it appear unbalanced with respect to oxygen atoms, hydrogen atoms, and charges. In the cellular environment, all three groups of ADP dissociate into −O− and H+, giving ADP3−, and this ion tends to exist in an ionic bond with Mg2+. ATP behaves identically except that it has four groups, giving ATPMg2−. When these differences along with the charges on the two phosphate groups are considered together, the net charges of −4 on each side are balanced. For simple fermentations, the metabolism of one molecule of glucose to two molecules of pyruvate has a net yield of two molecules of ATP, most cells will then carry out further reactions to repay the used NAD+ and produce a final product of ethanol or lactic acid. Many bacteria use inorganic compounds as hydrogen acceptors to regenerate the NAD+, cells performing aerobic respiration synthesize much more ATP, but not as part of glycolysis. These further aerobic reactions use pyruvate and NADH + H+ from glycolysis, the pathway of glycolysis as it is known today took almost 100 years to fully discover. The combined results of many experiments were required in order to understand the pathway as a whole
2. Enolase – The systematic name of this enzyme is 2-phospho-D-glycerate hydro-lyase. The reaction is reversible, depending on environmental concentrations of substrates, the optimum pH for the human enzyme is 6.5. Enolase is present in all tissues and organisms capable of glycolysis or fermentation, the enzyme was discovered by Lohmann and Meyerhof in 1934, and has since been isolated from a variety of sources including human muscle and erythrocytes. In humans, deficiency of ENO1 is linked to hereditary haemolytic anemia while ENO3 deficiency is linked to glycogen storage disease XIII. In humans there are three subunits of enolase, α, β, and γ, each encoded by a gene that can combine to form five different isoenzymes, αα, αβ, αγ, ββ. It is present at some level in all human cells. Also known as enolase 1 ββ or muscle specific enolase and this enzyme is largely restricted to muscle where it is present at very high levels in muscle γγ or neuron-specific enolase. Expressed at very high levels in neurons and neural tissues, where it can account for as much as 3% of total soluble protein and it is expressed at much lower levels in most mammalian cells. When present in the cell, different isozymes readily form heterodimers. Enolase is a member of the enolase superfamily. It has a weight of 82, 000-100,000 Daltons depending on the isoform. In human alpha enolase, the two subunits are antiparallel in orientation so that Glu20 of one forms an ionic bond with Arg414 of the other subunit. Each subunit has two distinct domains, the smaller N-terminal domain consists of three α-helices and four β-sheets. The enzyme’s compact, globular structure results from significant hydrophobic interactions between two domains. Enolase is a highly conserved enzyme with five active-site residues being especially important for activity, when compared to wild-type enolase, a mutant enolase that differs at either the Glu168, Glu211, Lys345, or Lys396 residue has an activity level that is cut by a factor of 105. Also, changes affecting His159 leave the mutant with only 0. 01% of its catalytic activity, an integral part of enolase are two Mg2+ cofactors in the active site, which serve to stabilize negative charges in the substrate. Recently, moonlighting functions of several enolases, such as interaction with plasminogen, have brought interest to the enzymes catalytic loops, using isotopic probes, the overall mechanism for converting 2-PG to PEP is proposed to be an E1cb elimination reaction involving a carbanion intermediate. The following detailed mechanism is based on studies of crystal structure, when the substrate, 2-phosphoglycerate, binds to α-enolase, its carboxyl group coordinates with two magnesium ion cofactors in the active site
3. Hexokinase – A hexokinase is an enzyme that phosphorylates hexoses, forming hexose phosphate. In most organisms, glucose is the most important substrate of hexokinases, scientists have discovered and demonstrated that Hexokinase posses the ability to transfer a inorganic phosphate group from ATP to a substrate. Hexokinases should not be confused with glucokinase, which is an isoform of hexokinase. While other hexokinases are capable of phosphorylating several hexoses, glucokinase acts with a 50-fold lower substrate affinity and its only hexose substrate is glucose. Genes that encode hexokinase have been discovered in every domain of life, and exist among a variety of species range from bacteria, yeast. They are categorized as actin fold proteins, sharing a common ATP binding site core that is surrounded by more variable sequences which determine substrate affinities, several hexokinase isoforms or isozymes that provide different functions can occur in a single species. Phosphorylation of a such as glucose often limits it to a number of intracellular metabolic processes. This is because phosphorylated hexoses are charged, and thus difficult to transport out of a cell. Most bacterial hexokinases are approximately 50 kD in size, multicellular organisms including plants and animals often have more than one hexokinase isoform. Most are about 100 kD in size and consist of two halves, which share much sequence homology and this suggests an evolutionary origin by duplication and fusion of a 50kD ancestral hexokinase similar to those of bacteria. There are four important mammalian hexokinase isozymes that vary in subcellular locations and kinetics with respect to different substrates and conditions, and physiological function. They are designated hexokinases I, II, III, and IV or hexokinases A, B, C, and D. Hexokinases I, II, Hexokinases I and II follow Michaelis-Menten kinetics at physiologic concentrations of substrates. All three are strongly inhibited by their product, glucose-6-phosphate, molecular weights are around 100 kD. Each consists of two similar 50kD halves, but only in hexokinase II do both halves have functional active sites, Hexokinase I/A is found in all mammalian tissues, and is considered a housekeeping enzyme, unaffected by most physiological, hormonal, and metabolic changes. Hexokinase II/B constitutes the principal regulated isoform in many types and is increased in many cancers. It is the hexokinase found in muscle and heart, Hexokinase II is also located at the mitochondria outer membrane so it can have direct access to ATP. Hexokinase III/C is substrate-inhibited by glucose at physiologic concentrations, little is known about the regulatory characteristics of this isoform. Mammalian hexokinase IV, also referred to as glucokinase, differs from other hexokinases in kinetics, the location of the phosphorylation on a subcellular level occurs when glucokinase translocates between the cytoplasm and nucleus of liver cells
4. Phosphoglycerate kinase – Phosphoglycerate kinase is an enzyme that catalyzes the reversible transfer of a phosphate group from 1, 3-bisphosphoglycerate to ADP producing 3-phosphoglycerate and ATP. Like all kinases it is a transferase, PGK is a major enzyme used in glycolysis, in the first ATP-generating step of the glycolytic pathway. In gluconeogenesis, the reaction catalyzed by PGK proceeds in the direction, generating ADP and 1. In humans, two isozymes of PGK have been so far identified, PGK1 and PGK2, PGK is present in all living organisms as one of the two ATP-generating enzymes in glycolysis. In the gluconeogenic pathway, PGK catalyzes the reverse reaction, under biochemical standard conditions, the glycolytic direction is favored. In the Calvin cycle in photosynthetic organisms, PGK catalyzes the phosphorylation of 3-PG, producing 1, 3-BPG and ADP, as part of the reactions that regenerate ribulose-1, 5-bisphosphate. PGK has been reported to exhibit thiol reductase activity on plasmin, leading to angiostatin formation, the enzyme was also shown to participate in DNA replication and repair in mammal cell nuclei. The human isozyme PGK2, which is expressed during spermatogenesis, was shown to be essential for sperm function in mice. Click on genes, proteins and metabolites below to link to respective articles, PGK is found in all living organisms and its sequence has been highly conserved throughout evolution. The enzyme exists as a 415-residue monomer containing two nearly equal-sized domains that correspond to the N- and C-termini of the protein, 3-phosphoglycerate binds to the N-terminal, while the nucleotide substrates, MgATP or MgADP, bind to the C-terminal domain of the enzyme. This extended two-domain structure is associated with large-scale hinge-bending conformational changes, the two domains of the protein are separated by a cleft and linked by two alpha-helices. At the core of each domain is a 6-stranded parallel beta-sheet surrounded by alpha helices, the two lobes are capable of folding independently, consistent with the presence of intermediates on the folding pathway with a single domain folded. Though the binding of either substrate triggers a change, only through the binding of both substrates does domain closure occur, leading to the transfer of the phosphate group. Magnesium ions are complexed to the phosphate groups the nucleotide substrates of PGK. It is known that in the absence of magnesium, no enzyme activity occurs and it is theorized that the ion may also encourage domain closure when PGK has bound both substrates. Without either substrate bound, PGK exists in an open conformation, then, in the case of the forward glycolytic reaction, the beta-phosphate of ADP initiates a nucleophilic attack on the 1-phosphate of 1, 3-BPG. The Lys219 on the enzyme guides the group to the substrate. In the glycolytic pathyway,1, 3-BPG is the donor and has a high phosphoryl-transfer potential
5. Glucose-6-phosphate isomerase – Glucose-6-phosphate isomerase, alternatively known as phosphoglucose isomerase or phosphohexose isomerase, is an enzyme that in humans is encoded by the GPI gene on chromosome 19. This gene encodes a member of the glucose phosphate isomerase protein family, the encoded protein has been identified as a moonlighting protein based on its ability to perform mechanistically distinct functions. In the cytoplasm, the gene product functions as an enzyme that interconverts glucose-6-phosphate and fructose-6-phosphate. The encoded protein is referred to as autocrine motility factor based on an additional function as a tumor-secreted cytokine. Defects in this gene are the cause of hemolytic anemia. Alternative splicing results in transcript variants. Functional GPI is a 64-kDa dimer composed of two identical monomers, the two monomers interact notably through the two protrusions in a hugging embrace. The active site of each monomer is formed by a cleft between the two domains and the dimer interface, GPI monomers are made of two domains, one made of two separate segments called the large domain and the other made of the segment in between called the small domain. The two domains are each αβα sandwiches, with the small domain containing a five-strand β-sheet surrounded by α-helices while the domain has a six-stranded β-sheet. The large domain, located at the N-terminal, and the C-terminal of each monomer also contain arm-like protrusions, since the isomerization activity occurs at the dimer interface, the dimer structure of this enzyme is critical to its catalytic function. It is hypothesized that serine phosphorylation of this protein induces a change to its secretory form. Compound C00668 at KEGG Pathway Database, enzyme 126.96.36.199 at KEGG Pathway Database. Compound C05345 at KEGG Pathway Database, reaction R00771 at KEGG Pathway Database. Glucose 6-phosphate binds to GPI in its pyranose form, the ring is opened in a push-pull mechanism by His388, which protonates the C5 oxygen, and Lys518, which deprotonates the C1 hydroxyl group. This creates an open chain aldose, then, the substrate is rotated about the C3-C4 bond to position it for isomerization. At this point, Glu357 deprotonates C2 to create a cis-enediolate intermediate stabilized by Arg272, to complete the isomerization, Glu357 donates its proton to C1, the C2 hydroxyl group loses its proton and the open-chain ketose fructose 6-phosphate is formed. Finally, the ring is closed by rotating the substrate about the C3-C4 bond again and this gene belongs to the GPI family. The protein encoded by this gene is an enzyme that catalyzes the reversible isomerization of G6P and F6P
6. Glyceraldehyde 3-phosphate dehydrogenase – Glyceraldehyde 3-phosphate dehydrogenase is an enzyme of ~37kDa that catalyzes the sixth step of glycolysis and thus serves to break down glucose for energy and carbon molecules. In sperm, a testis-specific isoenzyme GAPDHS is expressed, under normal cellular conditions, cytoplasmic GAPDH exists primarily as a tetramer. This form is composed of four identical 37-kDa subunits containing a single catalytic thiol group each, nuclear GAPDH has increased isoelectric point of pH8. 3–8.7. Of note, the cysteine residue C152 in the active site is required for the induction of apoptosis by oxidative stress. Notably, post-translational modifications of cytoplasmic GAPDH contribute to its functions outside of glycolysis, compound C00118 at KEGG Pathway Database. Enzyme 188.8.131.52 at KEGG Pathway Database, reaction R01063 at KEGG Pathway Database. Compound C00236 at KEGG Pathway Database and this is an example of phosphorylation coupled to oxidation, and the overall reaction is somewhat endergonic. Energy coupling here is possible by GAPDH. GAPDH uses covalent catalysis and general base catalysis to decrease the very large, first, a cysteine residue in the active site of GAPDH attacks the carbonyl group of GAP, creating a hemithioacetal intermediate. Next, an adjacent, tightly bound molecule of NAD+ accepts a hydride ion from GAP, forming NADH and this thioester species is much higher in energy than the carboxylic acid species that would result in the absence of GAPDH. Donation of the ion by the hemithioacetal is facilitated by its deprotonation by a histidine residue in the enzymes active site. Deprotonation encourages the reformation of the group in the thioester intermediate. This protein may use the model of allosteric regulation. As its name indicates, glyceraldehyde 3-phosphate dehydrogenase catalyses the conversion of glyceraldehyde 3-phosphate to D-glycerate 1 and this is the 6th step in the glycolytic breakdown of glucose, an important pathway of energy and carbon molecule supply which takes place in the cytosol of eukaryotic cells. The conversion occurs in two coupled steps, the first is favourable and allows the second unfavourable step to occur. The OCA-S transcriptional coactivator complex contains GAPDH and lactate dehydrogenase, two proteins previously thought to be involved in metabolism. GAPDH moves between the cytosol and the nucleus and may link the metabolic state to gene transcription. In 2005, Hara et al. showed that GAPDH initiates apoptosis and this is not a third function, but can be seen as an activity mediated by GAPDH binding to DNA like in transcription activation, discussed above
7. Dihydrolipoyl transacetylase – Dihydrolipoyl transacetylase is an enzyme component of the multienzyme pyruvate dehydrogenase complex. The pyruvate dehydrogenase complex is responsible for the decarboxylation step that links glycolysis to the citric acid cycle. This involves the transformation of pyruvate from glycolysis into acetyl-CoA which is used in the citric acid cycle to carry out cellular respiration. There are three different enzyme components in the pyruvate dehydrogenase complex, pyruvate dehydrogenase is responsible for the oxidation of pyruvate, dihydrolipoyl transacetylase transfers the acetyl group to coenzyme A, and dihydrolipoyl dehydrogenase regenerates the lipoamide. In humans, dihydrolipoyl transacetylase enzymatic activity resides in the pyruvate dehydrogenase complex component E2 that is encoded by the DLAT gene, the systematic name of this enzyme class is acetyl-CoA, enzyme N6-lysine S-acetyltransferase. Other names in use include, All dihydrolipoyl transacetylases have a unique multidomain structure consisting of,3 lipoyl domains, an interaction domain. Interestingly all the domains are connected by disordered, low complexity linker regions, depending on the species, multiple subunits of dihydrolipoyl transacetylase enzymes can arrange together into either a cubic or dodecahedral shape. The cubic core structure, found in such as Azotobacter vinelandii, is made up of 24 subunits total. The catalytic domains are assembled into trimers with the site located at the subunit interface. The topology of this active site is identical to that of chloramphenicol acetyltransferase. Eight of these trimers are then arranged into a truncated cube. The two main substrates, CoA and the lipoamide, are found at two opposite entrances of a 30 Å long channel which runs between the subunits and forms the catalytic center, CoA enters from the inside of the cube, and the lipoamide enters from the outside. The subunits are arranged in sets of three, similar to the trimers in the cubic shape, with each set making up one of the 20 dodecahedral vertices. Dihydrolipoyl transacetylase participates in the decarboxylation reaction that links glycolysis to the citric acid cycle. The various parts of cellular respiration take place in different parts of the cell, thus pyruvate dehydrogenase complexes are found in the mitochondria of eukaryotes. Pyruvate decarboxylation requires a few cofactors in addition to the enzymes that make up the complex, the first is thiamine pyrophosphate, which is used by pyruvate dehydrogenase to oxidize pyruvate and to form a hydroxyethyl-TPP intermediate. This intermediate is taken up by dihydrolipoyl transacetylase and reacted with a second lipoamide cofactor to generate an acetyl-dihydrolipoyl intermediate and this second intermediate can then be attacked by the nucleophilic sulfur attached to Coenzyme A, and the dihydrolipoamide is released. This results in the production of acetyl CoA, which is the end goal of pyruvate decarboxylation, the dihydrolipoamide is taken up by dihydrolipoyl dehydrogenase, and with the additional cofactors FAD and NAD+, regenerates the original lipoamide
8. Glycosome – The glycosome is a membrane-enclosed organelle that contains the glycolytic enzymes. The term was first used by Scott and Still in 1968 after they realized that the glycogen in the cell was not static, the organelle is bounded by a single membrane and contains a dense proteinaceous matrix. It is believed to have evolved from the peroxisome and this has been verified by work done on Leishmania genetics. The glycosome is currently being researched as a target for drug therapies. The term glycosome is also used for glycogen-containing structures found in hepatocytes responsible for storing sugar, glycosomes are composed of glycogen and proteins. The proteins are the enzymes that are associated with the metabolism of glycogen and these proteins and glycogen form a complex to make a distinct and separate organelle. The proteins for glycosomes are imported from free cytosolic ribosomes, the proteins imported into the organelle have a specific sequence, a PTS1 ending sequence to make sure they go to the right place. They are similar to alpha-granules in the cytosol of a cell that are filled with glycogen, glycosomes are typically round-to-oval shape with size varying in each cell. Although glycogen is found in the cytoplasm, that in the glycosome is separate, the membrane is a lipid bilayer. The glycogen that is found within the glycosome is identical to glycogen found freely in the cytosol, glycosomes can be associated or attached to many different types of organelles. They have been found to be attached to the sarcoplasmic reticulum, other glycosomes have been found to be attached to myofibrils and mitochondria, rough endoplasmic reticulum, sarcolemma, polyribosomes, or the Golgi apparatus. The glycosomes in the rough and smooth endoplasmic reticulum make use of its glycogen synthase and phosphorylase phosphatases, glycosomes function in many processes in the cell. These processes include glycolysis, purine salvage, beta oxidation of fatty acids, the main function that the glycosome serves is of the glycolytic pathway that is done inside its membrane. By compartmentalizing glycolysis inside of the glycosome, the cell can be more successful, in the cell, action in the cytosol, the mitochondria, and the glycosome are all completing the function of energy metabolism. This energy metabolism generates ATP through the process of glycolysis, the glycosome is a host of the main glycolytic enzymes in the pathway for glycolysis. This pathway is used to break down fatty acids for their carbon, the entire process of glycolysis does not take place in the glycosome however. Rather, only the Embden-Meyerhof segment where the glucose enters into the glycosome, importantly, the process in the organelle has no net ATP synthesis. This ATP comes later from processes outside of the glycosome, inside of the glycosome does need NAD+ for functioning and its regeneration
9. Pyruvate kinase – Pyruvate kinase is the enzyme that catalyzes the final step of glycolysis. It catalyzes the transfer of a group from phosphoenolpyruvate to adenosine diphosphate, yielding one molecule of pyruvate. There are four isozymes of pyruvate kinase in vertebrates, L, R, M1, R and L isozymes differ from M1 and M2 in that they are both exclusively allosterically and reversibly regulated. From a kinetic standpoint, the R and L isozymes of pyruvate kinase have two key conformation states, one with a high affinity and one with a low substrate affinity. The R-state, characterized by high affinity, serves as the activated form of pyruvate kinase and is stabilized by PEP and FBP. 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 tetrameric and 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 form is its structure in T-state conformation. Many Enterobacteriaceae, including E. coli, have two isoforms of pyruvate kinase, PykA and PykF, which are 37% identical in E. coli. They catalyze the reaction as in eukaryotes, namely the generation of ATP from ADP and PEP, the last step in glycolysis. PykF is allosterically regulated by fructose 1, 6-bisphosphate which reflects the position of PykF in cellular metabolism. PykF transcription in E. coli is regulated by the transcriptional regulator. PfkB was shown to be inhibited by MgATP at low concentrations of Fru-6P, there are two steps in the pyruvate kinase reaction in glycolysis. First, PEP transfers a phosphate group to ADP, producing ATP, secondly, a proton must be added to the enolate of pyruvate to produce the functional form of pyruvate that the cell requires. In yeast cells, the interaction of yeast pyruvate kinase with PEP and its allosteric effector Fructose 1, 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, 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 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
10. Phosphofructokinase 1 – Phosphofructokinase-1 is one of the most important regulatory enzymes of glycolysis. It is an enzyme made of 4 subunits and controlled by many activators and inhibitors. PFK-1 catalyzes the important committed step of glycolysis, the conversion of fructose 6-phosphate and ATP to fructose 1, 6-bisphosphate, glycolysis is the foundation for respiration, both anaerobic and aerobic. PFK is able to regulate glycolysis through allosteric inhibition, and in this way, for example, a high ratio of ATP to ADP will inhibit PFK and glycolysis. The key difference between the regulation of PFK in eukaryotes and prokaryotes is that in eukaryotes PFK is activated by fructose 2, the purpose of fructose 2, 6-bisphosphate is to supersede ATP inhibition, thus allowing eukaryotes to have greater sensitivity to regulation by hormones like glucagon and insulin. Mammalian PFK1 is a 340kd tetramer composed of different combinations of three types of subunits, muscle, liver, and platelet, the composition of the PFK1 tetramer differs according to the tissue type it is present in. For example, mature muscle expresses only the M isozyme, therefore, the liver and kidneys express predominantly the L isoform. In erythrocytes, both M and L subunits randomly tetramerize to form M4, L4 and the three forms of the enzyme. As a result, the kinetic and regulatory properties of the various isoenzymes pools are dependent on subunit composition, tissue-specific changes in PFK activity and isoenzymic content contribute significantly to the diversities of glycolytic and gluconeogenic rates which have been observed for different tissues. PFK1 is an enzyme and has a structure similar to that of hemoglobin in so far as it is a dimer of a dimer. One half of each contains the ATP binding site whereas the other half the substrate binding site as well as a separate allosteric binding site. Each subunit of the tetramer is 319 amino acids and consists of two domain, one that binds the substrate ATP, and the other that binds fructose-6-phosphate, each domain is a b barrel, and has cylindrical b sheet surrounded by alpha helices. On the opposite side of the each subunit from each site is the allosteric site. ATP and AMP compete for this site, F6P binds with a high affinity to the R state but not the T state enzyme. For every molecule of F6P that binds to PFK1, the enzyme progressively shifts from T state to the R state, thus a graph plotting PFK1 activity against increasing F6P concentrations would adopt the sigmoidal curve shape traditionally associated with allosteric enzymes. PFK1 belongs to the family of phosphotransferases and it catalyzes the transfer of γ-phosphate from ATP to fructose-6-phosphate, the PFK1 active site comprises both the ATP-Mg2+ and the F6P binding sites. Some proposed residues involved with substrate binding in E. coli PFK1 include Asp127, in the T state, enzyme conformation shifts slightly such that the space previously taken up by the Arg162 is replaced with Glu161. This swap in positions between adjacent amino acid residues inhibits the ability of F6P to bind the enzyme, allosteric activators such as AMP and ADP bind to the allosteric site as to facilitate the formation of the R state by inducing structural changes in the enzyme
11. Aldolase B – In mammals, aldolase B is preferentially expressed in the liver, while aldolase A is expressed in muscle and erythrocytes and aldolase C is expressed in the brain. Slight differences in isozyme structure result in different activities for the two molecules, FBP and fructose 1-phosphate. Aldolase B exhibits no preference and thus catalyzes both reactions, while aldolases A and C prefer FBP, in humans, aldolase B is encoded by the ALDOB gene located on chromosome 9. The gene is 14,500 base pairs long and contains 9 exons, defects in this gene have been identified as the cause of hereditary fructose intolerance. The generic fructose bisphosphate aldolase enzyme cleaves a 6-carbon fructose sugar into two 3-carbon products in an aldol reaction. After Schiff base formation, the hydroxyl group on the fructose backbone is then deprotonated by an aspartate residue. Schiff base hydrolysis yields two 3-carbon products, depending on the reactant, F1P or FBP, the products are DHAP and glyceraldehyde or glyceraldehyde 3-phosphate, respectively. The ΔG°’ of this reaction is +23.9 kJ/mol, though the reaction may seem too uphill to occur, it is of note that under physiological conditions, the ΔG of the reaction falls to close to or below zero. For example, the ΔG of this reaction under physiological conditions in erythrocytes is -0.23 kJ/mol, click on genes, proteins and metabolites below to link to respective articles. Aldolase B is an enzyme, composed of four subunits with molecular weights of 36 kDa with local 222 symmetry. Each subunit has a weight of 36 kDa and contains an eight-stranded α/β barrel. Such regions have been denoted isozyme-specific regions and these regions are thought to give isozymes their specificities and structural differences. ISRs 1-3 are all found in exon 3 of the ALDOB gene, ISR4 is the most variable of the four and is found at the c-terminal end of the protein. ISRs 1-3 are found predominantly in patches on the surface of the enzyme and these patches do not overlap with the active site, indicating that ISRs may change specific isozyme substrate specificity from a distance or cause the C-terminus interactions with the active site. A recent theory suggests that ISRs may allow for different conformational dynamics in the enzyme that account for its specificity. Aldolase B plays a key role in metabolism as it catalyzes one of the major steps of the glycolytic-gluconeogenic pathway. Though it does catalyze the breakdown of glucose, it plays an important role in fructose metabolism, which occurs mostly in the liver, renal cortex. When fructose is absorbed, it is phosphorylated by fructokinase to form fructose 1-phosphate, aldolase B then catalyzes F1P breakdown into glyceraldehyde and DHAP
12. Phosphofructokinase 2 – Phosphofructokinase 2 or fructose bisphosphatase 2, is an enzyme responsible for regulating the rates of glycolysis and gluconeogenesis in the human body. It is a homodimer of 55 kDa subunits arranged in a head-to-head fashion, when Ser-32 of the bifunctional protein is phosphorylated, the negative charge causes the conformation change of the enzyme to favor the FBPase2 activity, otherwise, PFK2 activity is favored. The monomers of the protein are clearly divided into two functional domains. The kinase domain is located on the N-terminal and it consists of a central six-stranded β sheet, with five parallel strands and an antiparallel edge strand, surrounded by seven α helices. The domain contains nucleotide-binding fold at the C-terminal end of the first β-strand, on the other hand, the phosphatase domain is located on the C-terminal. It resembles the family of proteins that include phosphoglycerate mutases and acid phosphatases, the domain has a mixed α/ β structure, with a six-stranded central β sheet, plus an additional α-helical subdomain that covers the presumed active site of the molecule. Finally, N-terminal region modulates PFK2 and FBPase2 activities, and stabilizes the dimer form of the enzyme, when glucose level is low, glucagon is released into the bloodstream, triggering a cAMP signal cascade. In the liver Protein kinase A inactivates the PFK-2 domain of the enzyme via phosphorylation. The F-2, 6-BPase domain is activated which lowers fructose 2. Because F-2, 6-BP normally stimulates phosphofructokinase-1, the decrease in its concentration leads to the inhibition of glycolysis, so PFK2 domain is activated and the kinase catalyzes the formation of F-2, 6-BP. Thus, glycolysis is stimulated and gluconeogenesis is inhibited, the allosteric regulation of PFK2 is very similar to the regulation of PFK1. High levels of AMP or phosphate group signifies a low energy state, on the other hand, a high concentration of phosphoenolpyruvate and citrate signifies that there is a high level of biosynthetic precursor and hence inhibits PFK2. However, unlike PFK1, PFK2 is not affected by the ATP concentration, yet, the formation of fructose 2, 6-bisphosphate could theoretically occur by a variety of mechanisms, including the intermediary formation of Fructose-6-phosphate 2-pyrophosphate. The breakdown of the state and the release of F6P. Histidine increases the nucleophilicity of water, which attacks phosphohistidine, generating phosphate, the Pfkfb2 gene encoding PFK2/FBPase2 protein is linked to the predisposition to schizophrenia. Furthermore, the control of PFK2/FBPase2 activity was found to be linked to heart functioning and the control against hypoxia