Editing Pyruvate kinase (section)

=== Covalent modifiers ===
Covalent modifiers serve as indirect regulators by controlling the phosphorylation, dephosphorylation, acetylation, succinylation and oxidation of enzymes, resulting in the activation and inhibition of enzymatic activity.<ref>{{cite journal | vauthors = Li YH, Li XF, Liu JT, Wang H, Fan LL, Li J, Sun GP | title = PKM2, a potential target for regulating cancer | journal = Gene | volume = 668 | pages = 48–53 | date = August 2018 | pmid = 29775756 | doi = 10.1016/j.gene.2018.05.038 | s2cid = 205030574 }}</ref> In the liver, [[glucagon]] and [[epinephrine]] activate [[protein kinase A]], which serves as a covalent modifier by phosphorylating and deactivating pyruvate kinase. In contrast, the secretion of insulin in response to blood sugar elevation activates phosphoprotein phosphatase I, causing the dephosphorylation and activation of pyruvate kinase to increase glycolysis. The same covalent modification has the opposite effect on gluconeogenesis enzymes. This regulation system is responsible for the avoidance of a futile cycle through the prevention of simultaneous activation of pyruvate kinase and enzymes that catalyze gluconeogenesis.<ref>{{cite journal | vauthors = Birnbaum MJ, Fain JN | title = Activation of protein kinase and glycogen phosphorylase in isolated rat liver cells by glucagon and catecholamines | journal = The Journal of Biological Chemistry | volume = 252 | issue = 2 | pages = 528–35 | date = January 1977 | doi = 10.1016/S0021-9258(17)32749-7 | pmid = 188818 | doi-access = free }}</ref>