Definitions

glycolysis

glycolysis

[glahy-kol-uh-sis]
glycolysis, term given to the metabolic pathway utilized by most microorganisms (yeast and bacteria) and by all "higher" animals (including humans) for the degradation of glucose. Glycolysis means, literally, the dissolution of sugar. The process is a series of consecutive chemical conversions that require the participation of eleven different enzymes, most of which have been crystallized and thoroughly studied. Glycolysis begins with a single molecule of glucose and concludes with the production of two molecules of pyruvic acid. The pathway is seen to be degradative, or catabolic, in that the six-carbon glucose is reduced to two molecules of the three-carbon pyruvic acid. Much of the energy that is liberated upon degradation of glucose is conserved by the simultaneous formation of the so-called high-energy molecule adenosine triphosphate (ATP). Two reactions of the glycolytic sequence proceed with the concomitant production of ATP, thus ATP synthesis is said to be coupled to glycolysis. Hundreds of cellular reactions, particularly those involved in the synthesis of cellular components and those that allow the cell to perform mechanical work, require the participation of ATP as a source of chemical energy. While glycolysis is the primary fuel process for some organisms that do not require oxygen, such as yeast, aerobic organisms can only gain a small portion of their needed energy from this process. Glycolysis occurs in two major stages, the first of which is the conversion of the various sugars to a common intermediate, glucose-6-phosphate. The second major phase is the conversion of glucose-6-phosphate to pyruvate. The products of glycolysis are further metabolized to complete the breakdown of glucose. Their ultimate fate varies depending upon the organism. In certain microorganisms lactic acid is the final product produced from pyruvic acid, and the process is referred to as homolactic fermentation. In certain bacteria and in brewer's yeast, lactic acid is not produced in large quantities. Instead pyruvic acid, which is also the precursor of lactic acid, is converted to ethanol and carbon dioxide by an enzyme-catalyzed two-step process, termed alcoholic fermentation. In the tissues of many organisms, including mammals, glycolysis is a prelude to the complex metabolic machinery that ultimately converts pyruvic acid to carbon dioxide and water with the concomitant production of much ATP and the consumption of oxygen. See Krebs cycle; respiration.
See also: Gluconeogenesis, which carries out a process wherein glucose is synthesized rather than catabolized.
Glycolysis is the sequence of reactions that converts glucose into pyruvate with the concomitant production of a relatively small amount of adenosine triphosphate (ATP). The word is derived from Greek γλυκύς (sweet) and λύσις (letting loose).

It is the initial process of most carbohydrate catabolism, and it serves three principal functions:

  1. Generation of high-energy molecules (ATP and NADH) as cellular energy sources as part of aerobic respiration and anaerobic respiration; that is, in the former process, oxygen is present, and, in the latter, oxygen is not present.
  2. Production of pyruvate for the citric acid cycle as part of aerobic respiration.
  3. Production of a variety of six- and three-carbon intermediate compounds, which may be removed at various steps in the process for other cellular purposes.

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.

Overview

The overall reaction of glycolysis is:
D-Glucose Pyruvate
| align="center" | + 2 NAD+ + 2 ADP + 2 Pi | align="center" | 2 | align="center" | + 2 NADH + 2 H+ + 2 ATP + 2 H2O
The products all have vital cellular uses:

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.

Discovery

The first formal studies of the glycolytic process were initiated in 1860 when Louis Pasteur discovered that microorganisms are responsible for fermentation, and in 1897 when Eduard Buchner found certain cell extracts can cause fermentation. The next major contribution was from Arthur Harden and William Young in 1905 who determined that a heat-sensitive high-molecular-weight subcellular fraction (the enzymes) and a heat-insensitive low-molecular-weight cytoplasm fraction (ADP, ATP and NAD+ and other cofactors) are required together for fermentation to proceed. The details of the pathway itself were eventually determined by 1940, with a major input from Otto Meyerhof and some years later by Luis Leloir. The biggest difficulties in determining the intricacies of the pathway were due to the very short lifetime and low steady-state concentrations of the intermediates of the fast glycolytic reactions.

Sequence of reactions

These are the major reactions, through which most glucose will pass. There are additional alternative pathways and regulatory products, which are not seen here.

Preparatory phase

The first five steps are regarded as the preparatory (or investment) phase since they consume energy to convert the glucose into two three-carbon sugar phosphates (G3P).

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.

Cofactors: Mg2+


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.

Cofactors: Mg2+


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.

Pay-off phase

The second half of glycolysis is known as the pay-off phase, characterised by a net gain of the energy-rich molecules ATP and NADH. Since glucose leads to two triose sugars in the preparatory phase, each reaction in the pay-off phase occurs twice per glucose molecule. This yields 2 NADH molecules and 4 ATP molecules, leading to a net gain of 2 NADH molecules and 2 ATP molecules from the glycolytic pathway per glucose.

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+

Oxidative decarboxylation

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.

Regulation

See also: Gluconeogenesis

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

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

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

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.

Post-glycolysis processes

The ultimate fate of pyruvate and NADH produced in glycolysis depends upon the organism and the conditions, most notably the presence or absence of oxygen and other external electron acceptors. In addition, not all carbon entering the pathway leaves as pyruvate and may be extracted at earlier stages to provide carbon compounds for other pathways.

Aerobic respiration

Main article: Aerobic respiration

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.

Anaerobic respiration

Main article: Anaerobic respiration
In animals, including humans, metabolism is primarily aerobic. However, under hypoxic (or partially-anaerobic) conditions, for example, in overworked muscles that are starved of oxygen or in infarcted heart muscle cells, pyruvate is converted to lactate by anaerobic respiration (also known as fermentation). This is a solution to maintaining the metabolic flux through glycolysis in response to an anaerobic or severely-hypoxic environment. In many tissues, this is a cellular last resort for energy, and most animal tissue cannot maintain anaerobic respiration for an extended length of time. Many single cellular organisms use anaerobic respiration as their only energy source.

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.

Intermediates for other pathways

This article concentrates on the catabolic role of glycolysis with regard to converting potential chemical energy to usable chemical energy during the oxidation of glucose to pyruvate. However, many of the metabolites in the glycolytic pathway are also used by anabolic pathways, and, as a consequence, flux through the pathway is critical to maintain a supply of carbon skeletons for biosynthesis.

These metabolic pathways are all strongly reliant on glycolysis as a source of metabolites:

*Amino acid synthesis
*Nucleotide synthesis
*Tetrapyrrole synthesis

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.

Glycolysis in disease

Genetic diseases

Glycolytic mutations are generally rare due to importance of the metabolic pathway, however some mutations are seen.

Cancer

Malignant rapidly-growing tumor cells typically have glycolytic rates that are up to 200 times higher than those of their normal tissues of origin. There are two common explanations. The classical explanation is that there is poor blood supply to tumors causing local depletion of oxygen. There is also evidence that attributes some of these high aerobic glycolytic rates to an overexpressed form of mitochondrially-bound hexokinase responsible for driving the high glycolytic activity. This phenomenon was first described in 1930 by Otto Warburg, and hence it is referred to as the Warburg effect. Warburg hypothesis claims that cancer is primarily caused by dysfunctionality in mitochondrial metabolism, rather than because of uncontrolled growth of cells. There is ongoing research to affect mitochondrial metabolism and treat cancer by starving cancerous cells in various new ways, including a ketogenic diet.

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).

Alzheimer's disease

Disfunctioning glycolysis or glucose metablism in fronto-temporo-parietal and cingulate cortices has been associated with the Alzheimer's disease , probably due to the decreased amyloid β (1-42) (Aβ42) and increased tau, phosphorylated tau in cerebrospinal fluid (CSF)

Alternative nomenclature

Some of the metabolites in glycolysis have alternative names and nomenclature. In part, this is because some of them are common to other pathways, such as the Calvin cycle.

This article Alternative names Alternative nomenclature
1 glucose Glc dextrose
3 fructose 6-phosphate F6P
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
7 1,3-bisphosphoglycerate 1,3BPG glycerate 1,3-bisphosphate,
glycerate 1,3-diphosphate,
1,3-diphosphoglycerate
PGAP, BPG, DPG
8 3-phosphoglycerate 3PG glycerate 3-phosphate PGA, GP
9 2-phosphoglycerate 2PG glycerate 2-phosphate
10 phosphoenolpyruvate PEP
11 pyruvate Pyr pyruvic acid

See also

External links

References

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