Glycolysis

:"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:
# 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.
# Production of pyruvate for the citric acid cycle as part of aerobic respiration.
# 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. [Romano AH, Conway T. (1996) Evolution of carbohydrate metabolic pathways. "Res Microbiol." 147(6-7):448-55 PMID 9084754]

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:The products all have vital cellular uses:
* ATP provides an energy source for many cellular functions.
* NADH + H⁺ provides reducing power for other metabolic pathways or further ATP synthesis.
* Pyruvate is used in the citric acid cycle in aerobic respiration to produce more ATP, or is converted to other small carbon molecules in anaerobic respiration.

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.

equence 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).

ee also

* Pentose phosphate pathway
* Gluconeogenesis
* Fermentation (biochemistry)
* Pyruvate decarboxylation
* Citric acid cycle
* Triose kinase

External links

* [http://nist.rcsb.org/pdb/molecules/pdb50_1.html The Glycolytic enzymes in Glycolysis] at Protein Data Bank
* [http://www.wdv.com/CellWorld/Biochemistry/Glycolytic Glycolytic cycle with animations] at wdv.com
* [http://www.biochemweb.org/metabolism.shtml Metabolism, Cellular Respiration and Photosynthesis - The Virtual Library of Biochemistry and Cell Biology] at biochemweb.org
* [http://www.rahulgladwin.com/blog/2007/01/notes-on-glycolysis.html notes on glycolysis] at rahulgladwin.com
* [http://www2.ufp.pt/~pedros/bq/glycolysis.htm The chemical logic behind glycolysis] at ufp.pt
* [http://www.expasy.org/tools/pathways/boehringer_legends.html Expasy biochemical pathways poster] at ExPASy
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References