It is the initial process of most carbohydrate catabolism, and it serves three principal functions:
As the foundation of both aerobic and anaerobic respiration, glycolysis is the archetype of universal metabolic processes known and occurring (with variations) in many types of cells in nearly all organisms. Glycolysis, through anaerobic respiration, is the main energy source in many prokaryotes, eukaryotic cells devoid of mitochondria (e.g., mature erythrocytes) and eukaryotic cells under low-oxygen conditions (e.g., heavily-exercising muscle or fermenting yeast).
Glycolysis takes place in the cytoplasm. In plant cells, some of the glycolytic reactions are also found in the Calvin-Benson cycle, which functions inside the chloroplasts. The wide conservation includes the most phylogenetically deep-rooted extant organisms, and thus it is considered to be one of the most ancient metabolic pathways.
The most common and well-known type of glycolysis is the Embden-Meyerhof pathway, initially explained by Gustav Embden and Otto Meyerhof. The term can be taken to include alternative pathways, such as the Entner-Doudoroff Pathway. However, glycolysis will be used here as a synonym for the Embden-Meyerhof pathway.
|| align="center" | + 2 NAD+ + 2 ADP + 2 Pi||| align="center" | 2||| align="center" | + 2 NADH + 2 H+ + 2 ATP + 2 H2O|
For simple anaerobic 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. Eukaryotic aerobic respiration produces approximately 34 additional molecules of ATP for each glucose molecule, however most of these are produced by a vastly different mechanism to the substrate-level phosphorylation in glycolysis.
The lower energy production, per glucose, of anaerobic respiration relative to aerobic respiration, results in greater flux through the pathway under hypoxic (low-oxygen) conditions, unless alternative sources of anaerobically-oxidizable substrates, such as fatty acids, are found.
| The first step in glycolysis is phosphorylation of glucose by a family of enzymes called hexokinases to form glucose 6-phosphate (G6P). This reaction consumes ATP, but it acts to keep the glucose concentration low, promoting continuous transport of glucose into the cell through the plasma membrane transporters. In addition, it blocks the glucose from leaking out - the cell lacks transporters for G6P. Glucose may alternatively be from the phosphorolysis or hydrolysis of intracellular starch or glycogen. In animals, an isozyme of hexokinase called glucokinase is also used in the liver, which has a much lower affinity for glucose (Km in the vicinity of normal glycemia), and differs in regulatory properties. The different substrate affinity and alternate regulation of this enzyme are a reflection of the role of the liver in maintaining blood sugar levels.|
|G6P is then rearranged into fructose 6-phosphate (F6P) by glucose phosphate isomerase. Fructose can also enter the glycolytic pathway by phosphorylation at this point. The change in structure is an isomerization, in which the G6P has been converted to F6P. The reaction requires an enzyme, phosphohexose isomerase, to proceed. This reaction is freely reversible under normal cell conditions. However, it is often driven forward because of a low concentration of F6P, which is constantly consumed during the next step of glycolysis. Under conditions of high F6P concentration this reaction readily runs in reverse. This phenomenon can be explained through Le Chatelier's Principle.|
| The energy expenditure of another ATP in this step is justified in 2 ways: The glycolytic process (up to this step) is now irreversible, and the energy supplied destabilizes the molecule. Because the reaction catalyzed by Phosphofructokinase 1 (PFK-1) is energetically very favorable, it is essentially irreversible, and a different pathway must be used to do the reverse conversion during gluconeogenesis. This makes the reaction a key regulatory point (see below). The same reaction can also be catalysed by pyrophosphate dependent phosphofructokinase (PFP or PPi-PFK), which is found in most plants, some bacteria, archea and protists but not in animals. This enzyme uses pyrophosphate (PPi) as a phosphate donor instead of ATP. It is a reversible reaction, increasing the flexibility of glycolytic metabolism. A rarer ADP-dependent PFK enzyme variant has been identified in archaean species.|
|Destabilizing the molecule in the previous reaction allows the hexose ring to be split by aldolase into two triose sugars, dihydroxyacetone phosphate, a ketone, and glyceraldehyde 3-phosphate, an aldehyde. There are two classes of aldolases: class I aldolases, present in animals and plants, and class II aldolases which present in fungi and bacteria; the two classes use different mechanisms in cleaving the hexose ring.|
|Triosephosphate isomerase rapidly interconverts dihydroxyacetone phosphate with glyceraldehyde 3-phosphate (GADP) that proceeds further into glycolysis. This is advantageous, as it directs dihydroxyacetone phosphate down the same pathway as glyceraldehyde 3-phosphate, simplifying regulation.|
|The triose sugars are dehydrogenated and inorganic phosphate is added to them, forming 1,3-bisphosphoglycerate. The hydrogen is used to reduce two molecules of NAD+, a hydrogen carrier, to give NADH + H+.|
|This step is the enzymatic transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP by phosphoglycerate kinase, forming ATP and 3-phosphoglycerate. At this step, glycolysis has reached the break-even point: 2 molecules of ATP were consumed, and 2 new molecules have now been synthesized. This step, one of the two substrate-level phosphorylation steps, requires ADP; thus, when the cell has plenty of ATP (and little ADP), this reaction does not occur. Because ATP decays relatively quickly when it is not metabolized, this is an important regulatory point in the glycolytic pathway. Cofactors: Mg2+|
|Phosphoglycerate mutase now forms 2-phosphoglycerate. Notice that this enzyme is a mutase and not an isomerase. Whereas an isomerase changes the oxidation state of the carbons of the compound, a mutase does not.|
|Enolase next forms phosphoenolpyruvate from 2-phosphoglycerate. Cofactors: 2 Mg2+: one "conformational" ion to coordinate with the carboxylate group of the substrate, and one "catalytic" ion which participates in the dehydration.|
|A final substrate-level phosphorylation now forms a molecule of pyruvate and a molecule of ATP by means of the enzyme pyruvate kinase. This serves as an additional regulatory step, similar to the phosphoglycerate kinase step. Cofactors: Mg2+|
|This reaction is not technically a reaction of glycolysis, but is very common in most organisms as a link to the citric acid cycle. This reaction is carried out in the mitochondria, unlike the reactions of glycolysis which are cytosolic. The conversion of pyruvate to acetyl CoA by the pyruvate dehydrogenase complex is a key step in the liver in particular, as it removes any chance of conversion of pyruvate to glucose, or as a transmination substrate. It commits pyruvate to entering the citric acid cycle, where it is either used as a substrate for oxidative phosphorylation, or is converted to citrate for export to the cytosol to serve as a substrate for fatty acid and isoprenoid biosynthesis.|
The flux through the glycolytic pathway is adjusted in response to conditions both inside and outside the cell. The rate in liver is regulated to meet major cellular needs: (1) the production of ATP, (2) the provision of building blocks for biosynthetic reactions, and (3) to lower blood glucose, one of the major functions of the liver. When blood sugar falls, glycolysis is halted in liver to allow the reverse process, gluconeogenesis. In glycolysis, the reactions catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase are effectively irreversible in most organisms. In metabolic pathways, such enzymes are potential sites of control, and all three enzymes serve this purpose in glycolysis.
There are several different ways to regulate the activity of an enzyme. An immediate form of control is feedback via allosteric effectors or by covalent modification. A slower form of control is transcriptional regulation that controls the amounts of these important enzymes.
Hexokinase is inhibited by glucose-6-phosphate (G6P), the product it forms through the ATP-driven phosphorylation. This is necessary to prevent an accumulation of G6P in the cell when flux through the glycolytic pathway is low. Glucose will enter the cell, but, since the hexokinase has reduced activity, it can diffuse back into the blood through the glucose transporter in the plasma membrane.
In animals, regulation of blood glucose levels by the liver is a vital part of homeostasis. In liver cells, extra G6P may be converted to G1P for conversion to glycogen, or it is alternatively converted by glycolysis to acetyl-CoA and then citrate. Excess citrate is exported to the cytosol, where ATP citrate lyase will regenerate acetyl-CoA and OAA. The acetyl-CoA is then used for fatty acid and cholesterol synthesis, two important ways of utilizing excess glucose when its concentration is high in blood. Liver contains both hexokinase and glucokinase; the latter catalyses the phosphorylation of glucose to G6P and is not inhibited by G6P. Thus it allows glucose to be converted into glycogen, fatty acids, and cholesterol even when hexokinase activity is low. This is important when blood glucose levels are high. During hypoglycemia, the glycogen can be converted back to G6P and then converted to glucose by a liver-specific enzyme glucose 6-phosphatase. This reverse reaction is an important role of liver cells to maintain blood sugars levels during fasting. This is critical for brain function, since the brain utilizes glucose as an energy source under most conditions.
Phosphofructokinase has historically been regarded as an important control point in the glycolytic pathway, since it is one of the irreversible steps and has key allosteric effectors, AMP and fructose 1,6-bisphosphate (F1,6BP).
Fructose 2,6-bisphosphate (F2,6BP) is a very potent activator of phosphofructokinase (PFK-1) that is synthesised when F6P is phosphorylated by a second phosphofructokinase (PFK2). In liver, when blood sugar is low and glucagon elevates cAMP, PFK2 is phosphorylated by protein kinase A. The phosphorylation inactivates PFK2, and another domain on this protein becomes active as fructose 2,6-bisphosphatase, which converts F2,6BP back to F6P. Both glucagon and epinephrine cause high levels of cAMP in the liver. The result of lower levels of liver fructose-2,6-bisphosphate is a decrease in activity of phosphofructokinase and an increase in activity of fructose 1,6-bisphosphatase, so that gluconeogenesis (essentially "glycolysis in reverse") is favored. This is consistent with the role of the liver in such situations, since the response of the liver to these hormones is to release glucose to the blood.
ATP competes with AMP for the allosteric effector site on the PFK enzyme. ATP concentrations in cells are much higher than AMP, typically 100-fold higher, but the concentration of ATP does not change more than about 10% under physiological conditions, whereas a 10% drop in ATP results in a 6-fold increase in AMP. Thus, the relevance of ATP as an allosteric effector is questionable. An increase in AMP is a consequence of a decrease in energy charge in the cell.
Citrate inhibits phosphofructokinase when tested in vitro by enhancing the inhibitory effect of ATP. However, it is doubtful that this is a meaningful effect in vivo, because citrate in the cytosol is mainly utilized for conversion to acetyl-CoA for fatty acid and cholesterol synthesis.
Pyruvate kinase and phosphoglycerate kinase catalyze the two substrate-level phosphorylation steps, and produce ATP from ADP. While both of these reactions are exergonic, phosphoglycerate kinase is less exergonic (-18.8 kJ/mol) than pyruvate kinase. Phosphoglycerate kinase helps to "pull along" the endergonic glyceraldehyde phosphate dehydrogenase, and in fact, these enzymes are reversible and also function in gluconeogenesis. In contrast, the strongly exergonic pyruvate kinase is irreversible and thus a prime candidate for regulation.
In aerobic organisms, pyruvate is converted to acetyl-CoA, within the mitochondria, where it is fully oxidized to carbon dioxide and water by the pyruvate dehydrogenase complex (oxidative decarboxylation) and the set of enzymes of the citric acid cycle. There are five separate activities catalyzed by the pyruvate dehydrogenase complex, which is highly regulated because this step irreversibly converts a glucose precursor into acetyl-CoA. The NADH produced is ultimately oxidized by the electron transport chain, using oxygen as final electron acceptor to produce a large amount of ATP via the action of the ATP synthase complex, a process known as oxidative phosphorylation. A net of only two molecules of ATP per glucose are produced by substrate-level phosphorylation during the citric acid cycle.
Glycolysis is insufficient for anaerobic respiration, as it does not regenerate NAD+ from the NADH + H+ it produces. It is therefore critical for an anaerobic or hypoxic cell to carry out the additional steps of lactate or alcohol production to regenerate NAD+ that is required for glycolysis to proceed. This is important for normal cellular function, as glycolysis is the only source of ATP in anaerobic or severely-hypoxic conditions.
There are several types of anaerobic respiration wherein pyruvate and NADH are anaerobically metabolized to yield any of a variety of products with an organic molecule acting as the final hydrogen acceptor. For example, the bacteria involved in making yogurt simply reduce pyruvate to lactic acid, whereas yeast produces ethanol and carbon dioxide. Anaerobic bacteria are capable of using a wide variety of compounds, other than oxygen, as terminal electron acceptors in respiration: nitrogenous compounds (such as nitrates and nitrites), sulfur compounds (such as sulfates, sulfites, sulfur dioxide, and elemental sulfur), carbon dioxide, iron compounds, manganese compounds, cobalt compounds, and uranium compounds.
These metabolic pathways are all strongly reliant on glycolysis as a source of metabolites:
From an energy perspective, NADH is either recycled to NAD+ during anaerobic conditions, to maintain the flux through the glycolytic pathway, or used during aerobic conditions to produce more ATP by oxidative phosphorylation. From an anabolic metabolism perspective, the NADH has a role to drive synthetic reactions, doing so by directly or indirectly reducing the pool of NADP+ in the cell to NADPH, which is another important reducing agent for biosynthetic pathways in a cell.
This high glycolysis rate has important medical applications, as high aerobic glycolysis by malignant tumors is utilized clinically to diagnose and monitor treatment responses of cancers by imaging uptake of 2-18F-2-deoxyglucose (a radioactive modified hexokinase substrate) with positron emission tomography (PET).
|This article||Alternative names||Alternative nomenclature|
|4||fructose 1,6-bisphosphate||F1,6BP||fructose 1,6-diphosphate||FBP, FDP, F1,6DP|
|5||dihydroxyacetone phosphate||DHAP||glycerone phosphate|
|6||glyceraldehyde 3-phosphate||GADP||3-phosphoglyceraldehyde||PGAL, G3P, GALP,GAP,TP|
|PGAP, BPG, DPG|
|8||3-phosphoglycerate||3PG||glycerate 3-phosphate||PGA, GP|