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GlycolysisGlycolysis is the inital metabolic pathway of carbohydrate catabolism. The most common and well-known form of glycolysis is the Embden-Meyerhof pathway. 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.
Glycolysis proper is completely anaerobic; that is, oxygen is not required
Output
Glycolysis converts one molecule of glucose into two molecules of pyruvate, along with "reducing equivalents" in the form of the coenzyme NADH.
The global reaction of glycolysis is:
- Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 NADH + 2 pyruvate + 2 ATP + 2 H2O + 4 H+
So, for simple fermentations, the metabolism of 1 molecule of glucose has a net yield of 2 molecules of ATP. Cells performing respiration synthesize much more ATP but this is not considered part of glycolysis. Eukaryotic aerobic respiration produces an additional 34 molecules (approximately) of ATP for each glucose molecule oxidized.
Location
In eukaryotes glycolysis takes place within the cytosol of the cell (as opposed to the mitochondria, where reactions more closely connected to aerobic metabolism occur). Glucose gets into the cell through active transport.
Follow up
The ultimate fate of the pyruvate and NADH produced in glycolysis depends upon the organism and the conditions, most notably the presence or absence of oxygen or other external electron acceptors.
In fermentation, the pyruvate and NADH are anerobically metabolized to yield any of a variety of products. For example, the bacteria involved in making yogurt simply reduce the pyruvate to lactic acid, whereas yeast produce ethanol and carbon dioxide.
In aerobic organisms, the pyruvate typically enters the citric acid cycle, and the NADH is ultimately oxidized by oxygen during oxidative phosphorylation. Although human metabolism is primarily aerobic, under anerobic conditions, for example in over-worked muscles that are starved for oxygen, pyruvate is converted to lactate, as in many microorganisms.
Evolution
Glycolysis is the only metabolic pathway common to nearly all living organisms, suggesting great antiquity; it may have originated with the first prokaryotes, 3.5 billion years ago or more.
Pathway
The first step in glycolysis is phosphorylation of glucose by hexokinase (in liver the enzyme is glucokinase which has slightly different properties). This reaction consumes 1 ATP molecule, but the energy is well spent: although the cell membrane is freely permeable to glucose because of the presence of glucose transport proteins, it is impermeable to glucose 6-phosphate. Glucose 6-phosphate is then rearranged into fructose 6-phosphate by phosphoglucose isomerase. (Fructose can also enter the glycolytic pathway at this point.)
Phosphofructokinase-1 then consumes 1 ATP to form fructose 1,6-bisphosphate. The energy expenditure in this step is justified in 2 ways: the glycolytic process (up to this step) is now irreversible, and the energy supplied to the molecule allows the ring to be split by aldolase into 2 molecules - dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. (Triosephosphate isomerase converts the molecule of dihydroxyacetone phosphate into a molecule of glyceraldehyde 3-phosphate.) Each molecule of glyceraldehyde 3-phosphate is then oxidized by a molecule of NAD+ in the presence of glyceraldehyde 3-phosphate dehydrogenase, forming 1,3-bisphosphoglycerate.
In the next step, phosphoglycerate kinase generates a molecule of ATP while forming 3-phosphoglycerate. At this step glycolysis has reached the break-even point: 2 molecules of ATP were consumed, and 2 new molecules have 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.
Phosphoglyceromutase then forms 2-phosphoglycerate; enolase then forms phosphoenolpyruvate; and another substrate-level phosphorylation then forms a molecule of pyruvate and a molecule of ATP by means of the enzyme pyruvate kinase. This serves as an additional regulatory step.
After the formation of fructose 1,6 bisphosphate, many of the reactions are energetically unfavorable. The only reactions that are favorable are the 2 substrate-level phosphorylation steps that result in the formation of ATP. These two reactions pull the glycolytic pathway to completion.
Etymology
From Greek glyk meaning sweet and lysis meaning dissolving.
See also
External links
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ExPASy - Biochemical Pathways
Digitized version of wall charts courtesy Boehringer Mannheim et al, divided into Metabolic Pathways and Cellular and Molecular Processes, maintained by the Swiss Institute of Bioinformatics, Geneva, Switzerland.
http://www.expasy.ch/cgi-bin/search-biochem-index
Kyoto Encyclopedia of Genes and Genomes (KEGG)
Graphical and hypertext-based information on biochemical pathways, including metabolic and regulatory pathways (for instance, cell cycle and growth factor signaling).
http://www.genome.ad.jp/kegg/
Metabolic Pathways of Biochemistry
Graphical representation of metabolic pathways that are crucial to human metabolism, including 3D depictions from the George Washington University, Washington, DC.
http://www.gwu.edu/~mpb
Molecular Biology Databases - Enzymes and Pathways
Links to enzyme, metabolic and signaling pathways databases from the Bioinformatics Unit at Weizmann Institute of Science, Israel.
http://bioinformatics.weizmann.ac.il/mb/db/enzymes.html
Ubiquitin Index
The Ubiquitin System for protein modification and degradation.
http://www.ccc.nottingham.ac.uk/%7Embzmail/students/ub/ubindex.html
Metabolism Foundation
Non-profit organization dedicated to the study of plant anatomy and physiology, established by Dr. Jim Bidlack, Edmond, OK.
http://www.metabolism.net/
Citric Acid Cycle
Diagram of steps in Krebs' tricarboxylic acid cycle, with 3D presentation dependent on Chime plug-in, from student at University of North Texas.
http://people.unt.edu/~hds0006/tca/index.htm
EcoCyc Encyclopedia
Database that describes the genome and the biochemical machinery of E. coli, maintained by SRI International, Menlo Park, CA.
http://www.ecocyc.org/
DIY Glycolysis by John Maber
Discusses the conversion of glucose and other monosaccharids to lactate or other biproducts, depending on the organism. Includes an advertisement for the webmaster's book.
http://www.jonmaber.demon.co.uk/
UM-BBD: Biocatalysis/Biodegradation Database
Microbial enzyme-catalyzed reactions and compounds, FAQ and related resources at the University of Minnesota.
http://www.labmed.umn.edu/
MetaCyc - Metabolic Encyclopedia
Description of over 450 metabolic pathways and their associated enzymes, from over 150 organisms maintained by SRI International, Menlo Park, CA.
http://biocyc.org/metacyc/
Laboratory Research Projects
Studies on squalene monooxygenase dependent cholesterol biosynthesis, and the role of cytochrome P450 reductase as the electron transfer partner, by Todd D. Porter's group at the College of Pharmacy, Lexington, KY.
http://www.uky.edu/Pharmacy/ps/porter/
Glycolysis and the Krebs Cycle
Chapter from the biology hypertext book at MIT, Cambridge, MA.
http://web.mit.edu/esgbio/www/glycolysis/dir.html
Metabolic Pathways
IUBMB-Nicholson chart illustrating glycolytic pathway, Krebs Cycle, and over 500 enzyme reactions, with formulas, substrates, inhibitors and EC numbers. Available as PDF download, or to purchase from Sigma Chemical Co., St.Louis, MO.
http://www.sigmaaldrich.com/metabolicpathways/
Cytochrome P450 Reductase
Overview of the biology, enzymology, and structure of cytochrome P450 reductase, an unusual electron transfer flavoprotein.
http://www.uky.edu/Pharmacy/ps/porter/CPR.htm
Jonathan Dordick Research Group
Research and publications on biocatalylis, incorportaing polymers to accelerate biological processes. Includes staff profile and contacts at Rensselaer Polytechnic Institute, Troy, NY.
http://enzymes.che.rpi.edu/
General Overview of the Major Metabolic Pathways
A survey of human metabolism, written for undergraduates in the Health Sciences from a chemist's point of view. Includes profile of author at University of Oporto, Portugal.
http://www2.ufp.pt/~pedros/bq/integration.htm
Proteolysis for Fun and Profit
Protein degradation resource.
http://www.biochem.emory.edu/labs/genekdw/protdeg2000/home.html
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