Editing Glycolysis (section)

== 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. 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.<ref>{{Cite journal |last1=Judge |first1=Ayesha |last2=Dodd |first2=Michael S. |date=2020-10-08 |title=Metabolism |url=https://portlandpress.com/essaysbiochem/article/64/4/607/226177/Metabolism |journal=Essays in Biochemistry |language=en |volume=64 |issue=4 |pages=607–647 |doi=10.1042/EBC20190041 |issn=0071-1365 |pmc=7545035 |pmid=32830223}}</ref>

The following metabolic pathways are all strongly reliant on glycolysis as a source of metabolites: and many more.
* [[Pentose phosphate pathway]], which begins with the dehydrogenation of [[glucose-6-phosphate]], the first intermediate to be produced by glycolysis, produces various pentose sugars, and  [[Nicotinamide adenine dinucleotide phosphate|NADPH]] for the synthesis of [[fatty acid]]s and [[cholesterol]].
* [[Glycogenesis|Glycogen synthesis]] also starts with glucose-6-phosphate at the beginning of the glycolytic pathway.
* [[Glycerol]], for the formation of [[triglyceride]]s and [[phospholipid]]s, is produced from the glycolytic intermediate [[glyceraldehyde-3-phosphate]].
* Various post-glycolytic pathways:
:* [[Fatty acid metabolism#Fatty acid Synthesis|Fatty acid synthesis]]
:* [[Cholesterol#Biosynthesis|Cholesterol synthesis]]
:* The [[citric acid cycle]] which in turn leads to:
::*[[Amino acid synthesis]]
::*[[Nucleotide#Synthesis|Nucleotide synthesis]]
::*[[Porphyrin#Biosynthesis|Tetrapyrrole synthesis]]

Although [[gluconeogenesis]] and glycolysis share many intermediates the one is not functionally a branch or tributary of the other. There are two regulatory steps in both pathways which, when active in the one pathway, are automatically inactive in the other. The two processes can therefore not be simultaneously active.<ref name=stryer0>{{cite book | vauthors = Stryer L | title=Biochemistry. |edition= Fourth |location= New York |publisher= W.H. Freeman and Company|date= 1995 |pages= 559–565, 574–576, 614–623|isbn= 0-7167-2009-4 }}</ref> Indeed, if both sets of reactions were highly active at the same time the net result would be the hydrolysis of four high energy phosphate bonds (two ATP and two GTP) per reaction cycle.<ref name=stryer0 />

[[Nicotinamide adenine dinucleotide|NAD<sup>+</sup>]] is the oxidizing agent in glycolysis, as it is in most other energy yielding metabolic reactions (e.g. [[beta-oxidation]] of fatty acids, and during the [[citric acid cycle]]). The NADH thus produced is primarily used to ultimately transfer electrons to {{chem2|O2}} to produce water, or, when {{chem2|O2}} is not available, to produce compounds such as [[Lactic acid|lactate]] or [[ethanol]] (see ''Anoxic regeneration of NAD<sup>+</sup>'' above). NADH is rarely used for synthetic processes, the notable exception being gluconeogenesis.  During [[Fatty acid metabolism#Fatty acid Synthesis|fatty acid]] and [[Cholesterol#Biosyntesis|cholesterol synthesis]] the reducing agent is [[Nicotinamide adenine dinucleotide phosphate|NADPH]]. This difference exemplifies a general principle that NADPH is consumed during biosynthetic reactions, whereas NADH is generated in energy-yielding reactions.<ref name=stryer0 /> The source of the NADPH is two-fold. When [[Malic acid|malate]] is oxidatively decarboxylated by "NADP<sup>+</sup>-linked malic enzyme" [[Pyruvic acid|pyruvate]], {{chem2|CO2}} and NADPH are formed. NADPH is also formed by the [[pentose phosphate pathway]] which converts glucose into ribose, which can be used in synthesis of [[nucleotides]] and [[nucleic acids]], or it can be catabolized to pyruvate.<ref name=stryer0 />