Editing Glycolysis (section)

=== Regulated Enzymes in Glycolysis ===
The three [[enzymes#Control of activity|regulatory enzymes]] are [[hexokinase]] (or [[glucokinase]] in the liver), [[phosphofructokinase 1|phosphofructokinase]], and [[pyruvate kinase]]. The [[flux (biochemistry)|flux]] through the glycolytic pathway is adjusted in response to conditions both inside and outside the cell. The internal factors that  regulate glycolysis do so primarily to provide [[adenosine triphosphate|ATP]] in adequate quantities for the cell's needs. The external factors act primarily on the [[liver]], [[Adipose tissue|fat tissue]], and [[muscle]]s, which can remove large quantities of glucose from the blood after meals (thus preventing [[hyperglycemia]] by storing the excess glucose as fat or glycogen, depending on the tissue type). The liver is also capable of releasing glucose into the blood between meals, during fasting, and exercise thus preventing [[hypoglycemia]] by means of [[glycogenolysis]] and [[gluconeogenesis]]. These latter reactions coincide with the halting of glycolysis in the liver.

In addition hexokinase and [[glucokinase]] act independently of the hormonal effects as controls at the entry points of glucose into the cells of different tissues.  Hexokinase responds to the [[glucose-6-phosphate]] (G6P) level in the cell, or, in the case of glucokinase, to the blood sugar level in the blood to impart entirely intracellular controls of the glycolytic pathway in different tissues (see [[#Hexokinase and glucokinase|below]]).<ref name="stryer" />

When glucose has been converted into G6P by hexokinase or glucokinase, it can either be converted to [[glucose-1-phosphate]] (G1P) for conversion to [[glycogen]], or it is alternatively converted by glycolysis to [[Pyruvic acid|pyruvate]], which enters the [[mitochondrion]] where it is converted into [[acetyl-CoA]] and then into [[citrate]]. Excess [[citrate]] is exported from the mitochondrion back into the cytosol, where [[ATP citrate lyase]] regenerates [[acetyl-CoA]] and [[oxaloacetic acid|oxaloacetate]] (OAA). The acetyl-CoA is then used for [[fatty acid synthesis]] and [[Cholesterol|cholesterol synthesis]], two important ways of utilizing excess glucose when its concentration is high in blood. The regulated enzymes catalyzing these reactions perform these functions when they have been dephosphorylated through the action of insulin on the liver cells.  Between meals, during [[fasting]], [[Physical exercise|exercise]] or hypoglycemia, glucagon and epinephrine are released into the blood. This causes liver glycogen to be converted back to G6P, and then converted to glucose by the liver-specific enzyme [[glucose 6-phosphatase]] and released into the blood. Glucagon and epinephrine also stimulate gluconeogenesis, which converts non-carbohydrate substrates into G6P, which joins the G6P derived from glycogen, or substitutes for it when the liver glycogen store have been depleted. This is critical for brain function, since the brain utilizes glucose as an energy source under most conditions.<ref name=stryer8>{{cite book | vauthors = Stryer L | title=Biochemistry. |edition= Fourth |location= New York |publisher= W.H. Freeman and Company|date= 1995 |pages= 773|isbn= 0-7167-2009-4 }}</ref> The simultaneously phosphorylation of, particularly, [[phosphofructokinase]], but also, to a certain extent pyruvate kinase, prevents glycolysis occurring at the same time as gluconeogenesis and glycogenolysis.

====Hexokinase and glucokinase====
[[File:Hexokinase B 1IG8 wpmp.png|thumb|right|[[Yeast]] [[hexokinase]] B ({{PDB|1IG8}})]]
All cells contain the enzyme [[hexokinase]], which catalyzes the conversion of glucose that has entered the cell into [[glucose-6-phosphate]] (G6P). Since the cell membrane is impervious to G6P, hexokinase essentially acts to transport glucose into the cells from which it can then no longer escape.  Hexokinase is inhibited by high levels of G6P in the cell. Thus the rate of entry of glucose into cells partially depends on how fast G6P can be disposed of by glycolysis, and by [[Glycogenesis|glycogen synthesis]] (in the cells which store glycogen, namely liver and muscles).<ref name=stryer /><ref name=voet>{{cite book | vauthors = Voet D, Voet JG, Pratt CW |title=Fundamentals of Biochemistry | edition = 2nd |publisher=John Wiley and Sons, Inc. |year=2006 |pages=[https://archive.org/details/fundamentalsofbi00voet_0/page/547 547, 556] |isbn=978-0-471-21495-3 |url=https://archive.org/details/fundamentalsofbi00voet_0/page/547 }}</ref>

[[Glucokinase]], unlike hexokinase, is not inhibited by G6P. It occurs in liver cells, and will only phosphorylate the glucose entering the cell to form G6P, when the glucose in the  blood is abundant. This being the first step in the glycolytic pathway in the liver, it therefore imparts an additional layer of control of the glycolytic pathway in this organ.<ref name=stryer />

==== Phosphofructokinase ====
[[File:Phosphofructokinase 6PFK wpmp.png|thumb|left|[[Bacillus stearothermophilus]] [[phosphofructokinase]] ({{PDB|6PFK}})]]
[[Phosphofructokinase 1|Phosphofructokinase]] is an important control point in the glycolytic pathway, since it is one of the irreversible steps and has key allosteric effectors, [[Adenosine monophosphate|AMP]] and [[fructose 2,6-bisphosphate]] (F2,6BP).

F2,6BP is a very potent activator of phosphofructokinase (PFK-1) that is synthesized when F6P is phosphorylated by a second phosphofructokinase ([[PFK2]]). In the 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 bisphosphatase-2]], 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 F2,6BP is a decrease in activity of [[phosphofructokinase]] and an increase in activity of [[fructose 1,6-bisphosphatase]], so that gluconeogenesis (in essence, "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.

[[Adenosine triphosphate|ATP]] competes with AMP for the allosteric effector site on the PFK enzyme. ATP concentrations in cells are much higher than those of AMP, typically 100-fold higher,<ref>{{cite journal | vauthors = Beis I, Newsholme EA | title = The contents of adenine nucleotides, phosphagens and some glycolytic intermediates in resting muscles from vertebrates and invertebrates | journal = The Biochemical Journal | volume = 152 | issue = 1 | pages = 23–32 | date = October 1975 | pmid = 1212224 | pmc = 1172435 | doi = 10.1042/bj1520023 }}</ref> 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.<ref>{{cite book | vauthors = Voet D, Voet JG | date = 2004 | title = Biochemistry | edition = 3rd | location = New York | publisher = John Wiley & Sons, Inc. }}</ref> 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 utilized mainly for conversion to [[acetyl-CoA]] for [[fatty acid]] and [[cholesterol]] synthesis.

[[TP53-inducible glycolysis and apoptosis regulator|TIGAR]], a p53 induced enzyme, is responsible for the regulation of [[phosphofructokinase 1|phosphofructokinase]] and acts to protect against oxidative stress.<ref>{{Cite book|title=TIGAR| vauthors = Lackie J |publisher=Oxford University Press|year=2010|isbn=978-0-19-954935-1|location=Oxford Reference Online}}</ref> TIGAR is a single enzyme with dual function that regulates F2,6BP. It can behave as a phosphatase (fructuose-2,6-bisphosphatase) which cleaves the phosphate at carbon-2 producing F6P. It can also behave as a kinase (PFK2) adding a phosphate onto carbon-2 of F6P which produces F2,6BP. In humans, the TIGAR protein is encoded by ''C12orf5'' gene. The TIGAR enzyme will hinder the forward progression of glycolysis, by creating a build up of fructose-6-phosphate (F6P) which is isomerized into glucose-6-phosphate (G6P). The accumulation of G6P will shunt carbons into the pentose phosphate pathway.<ref>{{cite journal | vauthors = Bensaad K, Tsuruta A, Selak MA, Vidal MN, Nakano K, Bartrons R, Gottlieb E, Vousden KH | title = TIGAR, a p53-inducible regulator of glycolysis and apoptosis | journal = Cell | volume = 126 | issue = 1 | pages = 107–120 | date = July 2006 | pmid = 16839880 | doi = 10.1016/j.cell.2006.05.036 | s2cid = 15006256 | doi-access = free }}</ref><ref>{{Cite web|url=https://www.ncbi.nlm.nih.gov/gene?Db=gene&Cmd=ShowDetailView&TermToSearch=57103|title=TIGAR TP53 induced glycolysis regulatory phosphatase [Homo sapiens (human)] - Gene - NCBI|website=www.ncbi.nlm.nih.gov|access-date=2018-05-17}}</ref>

==== Pyruvate kinase ====
[[File:Pyruvate Kinase 1A3W wpmp.png|thumb|right|[[Yeast]] [[pyruvate kinase]] ({{PDB|1A3W}})]]
{{Main|Pyruvate kinase}}
The final step of glycolysis is catalysed by pyruvate kinase to form pyruvate and another ATP. It is regulated by a range of different transcriptional, covalent and non-covalent regulation mechanisms, which can vary widely in different tissues.<ref>{{cite journal | vauthors = Carbonell J, Felíu JE, Marco R, Sols A | title = Pyruvate kinase. Classes of regulatory isoenzymes in mammalian tissues | journal = European Journal of Biochemistry | volume = 37 | issue = 1 | pages = 148–156 | date = August 1973 | pmid = 4729424 | doi = 10.1111/j.1432-1033.1973.tb02969.x | hdl = 10261/78345 | hdl-access = free }}</ref><ref>{{cite journal | vauthors = Valentini G, Chiarelli L, Fortin R, Speranza ML, Galizzi A, Mattevi A | title = The allosteric regulation of pyruvate kinase | journal = The Journal of Biological Chemistry | volume = 275 | issue = 24 | pages = 18145–18152 | date = June 2000 | pmid = 10751408 | doi = 10.1074/jbc.m001870200 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Israelsen WJ, Vander Heiden MG | title = Pyruvate kinase: Function, regulation and role in cancer | journal = Seminars in Cell & Developmental Biology | volume = 43 | pages = 43–51 | date = July 2015 | pmid = 26277545 | pmc = 4662905 | doi = 10.1016/j.semcdb.2015.08.004 }}</ref> For example, in the liver, pyruvate kinase is regulated based on glucose availability. During fasting (no glucose available), [[glucagon]] activates [[protein kinase A]] which phosphorylates pyruvate kinase to inhibit it.<ref name=":4">{{cite journal | vauthors = Engström L | title = The regulation of liver pyruvate kinase by phosphorylation--dephosphorylation | journal = Current Topics in Cellular Regulation | volume = 13 | pages = 28–51 | date = 1978 | pmid = 208818 | doi = 10.1016/b978-0-12-152813-3.50006-9 | publisher = Elsevier | isbn = 978-0-12-152813-3 }}</ref> An increase in blood sugar leads to secretion of [[insulin]], which activates [[protein phosphatase 1]], leading to dephosphorylation and re-activation of pyruvate kinase.<ref name=":4" /> These controls prevent pyruvate kinase from being active at the same time as the enzymes that catalyze the reverse reaction ([[pyruvate carboxylase]] and [[phosphoenolpyruvate carboxykinase]]), preventing a [[futile cycle]].<ref name=":4" /> Conversely, the isoform of pyruvate kinasein found in muscle is not affected by protein kinase A (which is activated by adrenaline in that tissue), so that glycolysis remains active in muscles even during fasting.<ref name=":4" />