PDK2

{{Short description|Protein-coding gene in the species Homo sapiens}}

Pyruvate dehydrogenase kinase isoform 2 (PDK2) also known as pyruvate dehydrogenase lipoamide kinase isozyme 2, mitochondrial is an enzyme that in humans is encoded by the PDK2 gene.{{cite journal | vauthors = Gudi R, Bowker-Kinley MM, Kedishvili NY, Zhao Y, Popov KM | title = Diversity of the pyruvate dehydrogenase kinase gene family in humans | journal = The Journal of Biological Chemistry | volume = 270 | issue = 48 | pages = 28989–28994 | date = Dec 1995 | pmid = 7499431 | doi = 10.1074/jbc.270.48.28989 | doi-access = free }}{{cite web | title = Entrez Gene: PDK2 pyruvate dehydrogenase kinase, isozyme 2 | url = https://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=5164 }} PDK2 is an isozyme of pyruvate dehydrogenase kinase.

Structure

The protein encoded by the PDK2 gene has two sites, an active site and an allosteric site that allow for the activity and regulation of this enzyme. There are many structural motifs that are important to the regulation of this enzyme. Nov3r and AZ12 inhibitors bind at the lipoamide binding site that is located at one end of the R domain. Pfz3 binds in an extended site at the other end of the R domain. One inhibitor, dicholoroacetate (DCA), binds at the center of the R domain.{{cite journal | vauthors = Knoechel TR, Tucker AD, Robinson CM, Phillips C, Taylor W, Bungay PJ, Kasten SA, Roche TE, Brown DG | title = Regulatory roles of the N-terminal domain based on crystal structures of human pyruvate dehydrogenase kinase 2 containing physiological and synthetic ligands | journal = Biochemistry | volume = 45 | issue = 2 | pages = 402–415 | date = Jan 2006 | pmid = 16401071 | doi = 10.1021/bi051402s }} Within the active site, there are three amino acid residues, R250, T302, and Y320, that make the kinase resistant to the inhibitor dichloroacetate, which uncouples the active site from the allosteric site. This supports the theory that R250, T302, and Y320 stabilize the "open" and "closed" conformations of the built-in lid that controls the access of a nucleotide into the nucleotide-binding cavity. This strongly suggests that the mobility of ATP lid is central to the allosteric regulation of PDHK2 activity serving as a conformational switch required for communication between the active site and allosteric sites in the kinase molecule.{{cite journal | vauthors = Klyuyeva A, Tuganova A, Popov KM | title = Allosteric coupling in pyruvate dehydrogenase kinase 2 | journal = Biochemistry | volume = 47 | issue = 32 | pages = 8358–8366 | date = Aug 2008 | pmid = 18627174 | pmc = 2568900 | doi = 10.1021/bi800631h }} There is also a DW-motif that is crucial in mediating DCA, nucleotide, and lipoyl domain binding site communication. This network is responsible for rendering PDK2 locked in the closed, or inactive conformation.{{cite journal | vauthors = Li J, Kato M, Chuang DT | title = Pivotal role of the C-terminal DW-motif in mediating inhibition of pyruvate dehydrogenase kinase 2 by dichloroacetate | journal = The Journal of Biological Chemistry | volume = 284 | issue = 49 | pages = 34458–34467 | date = Dec 2009 | pmid = 19833728 | pmc = 2797213 | doi = 10.1074/jbc.M109.065557 | doi-access = free }}

Function

The Pyruvate Dehydrogenase (PDH) complex must be tightly regulated due to its central role in general metabolism. Within the complex, there are three serine residues on the E1 component that are sites for phosphorylation; this phosphorylation inactivates the complex. In humans, there have been four isozymes of Pyruvate Dehydrogenase Kinase that have been shown to phosphorylate these three sites: PDK1, PDK2, PDK3, and PDK4.{{cite journal | vauthors = Kolobova E, Tuganova A, Boulatnikov I, Popov KM | title = Regulation of pyruvate dehydrogenase activity through phosphorylation at multiple sites | journal = The Biochemical Journal | volume = 358 | issue = Pt 1 | pages = 69–77 | date = Aug 2001 | pmid = 11485553 | pmc = 1222033 | doi = 10.1042/0264-6021:3580069 }} PDK2 has been identified as the most abundant isoform in human tissues. Through many studies, it has been made clear that the activity of this enzyme is essential, even at rest, to regulate glycolysis/carbodydrate oxidation and producing metabolites for oxidative phosphorylation and the electron transport chain. These studies have illustrated that the kinetics of the PDK isoform population, specifically PDK2, is more important in determining PDH activity than measuring PDK activity.{{cite journal | vauthors = Dunford EC, Herbst EA, Jeoung NH, Gittings W, Inglis JG, Vandenboom R, LeBlanc PJ, Harris RA, Peters SJ | title = PDH activation during in vitro muscle contractions in PDH kinase 2 knockout mice: effect of PDH kinase 1 compensation | journal = American Journal of Physiology. Regulatory, Integrative and Comparative Physiology | volume = 300 | issue = 6 | pages = R1487–R1493 | date = Jun 2011 | pmid = 21411764 | doi = 10.1152/ajpregu.00498.2010 }}

=Regulation=

As the primary regulators of a crucial step in the central metabolic pathway, the pyruvate dehydrogenase family is tightly regulated itself by a myriad of factors. PDK2 activity is modulated by low levels of hydrogen peroxide; this happens because the compound temporarily oxidizes the cysteine residues 45 and 392 on the enzyme, resulting in an inactive PDK2 and greater PDH activity. These conditions also inactivate the TCA cycle, the next step in aerobic respiration. This alludes to the fact that when there is a high level of O₂ production in the mitochondria, which may occur because of nutrient excess, the increase in the products serve as a negative feedback that control mitochondria metabolism.{{cite journal | vauthors = Hurd TR, Collins Y, Abakumova I, Chouchani ET, Baranowski B, Fearnley IM, Prime TA, Murphy MP, James AM | title = Inactivation of pyruvate dehydrogenase kinase 2 by mitochondrial reactive oxygen species. | journal = The Journal of Biological Chemistry | volume = 287 | issue = 42 | pages = 35153–35160 | date = 12 October 2012 | pmid = 22910903 | pmc = 3471752 | doi = 10.1074/jbc.m112.400002 | doi-access = free }}

PDK2, in conjunction with PDK3 and PDK4, are primary targets of Peroxisome proliferator-activated receptor delta or beta, with PDK2 having two elements that respond to these receptors.{{cite journal | vauthors = Degenhardt T, Saramaki A, Malinen M, Rieck M, Vaisanen S, Huotari A, Herzig KH, Muller R, Carlberg C | title = Three members of the human pyruvate dehydrogenase kinase gene family are direct targets of the peroxisome proliferator-activated receptor beta/delta. | journal = Journal of Molecular Biology | volume = 372 | issue = 2 | pages = 341–355 | date = 14 September 2007 | pmid = 17669420 | doi = 10.1016/j.jmb.2007.06.091 }}

Clinical significance

All of the pyruvate dehydrogenase isozymes have been associated with various metabolic disorders, including diabetes. This is due to a mechanism by which consistently elevated free fatty acid levels stimulate the PDK enzymes, particularly, PDK2 and PDK4 in the liver. In stimulating this activity, there is less PDH activity, and therefore less glucose uptake.{{cite journal | vauthors = Bajotto G, Murakami T, Nagasaki M, Qin B, Matsuo Y, Maeda K, Ohashi M, Oshida Y, Sato Y, Shimomura Y | title = Increased expression of hepatic pyruvate dehydrogenase kinases 2 and 4 in young and middle-aged Otsuka Long-Evans Tokushima Fatty rats: induction by elevated levels of free fatty acids. | journal = Metabolism: Clinical and Experimental | volume = 55 | issue = 3 | pages = 317–323 | date = March 2006 | pmid = 16483874 | doi = 10.1016/j.metabol.2005.09.014 }}

=Cancer=

As the PDK enzymes are associated with central metabolism and growth, they are often associated with various mechanisms of cancer progression. Enhanced PDK2 activity leads to increased glycolysis and lactic acid production, known as the Warburg effect. In some studies, the wild-type form of tumor protein p53 prevents manifestation of tumorigenesis by regulating PDK2 activity.{{cite journal | vauthors = Contractor T, Harris CR | title = p53 negatively regulates transcription of the pyruvate dehydrogenase kinase Pdk2. | journal = Cancer Research | volume = 72 | issue = 2 | pages = 560–567 | date = 15 January 2012 | pmid = 22123926 | doi = 10.1158/0008-5472.can-11-1215 | doi-access = free }}

Additionally, inhibition of PDK2 subsequently inhibits HIF1A in cancer cells by both a prolyl-hydroxylase (PHD)-dependent mechanism and a PHD-independent mechanism. Therefore, mitochondria-targeting metabolic modulators increase pyruvate dehydrogenase activity, and suppress angiogenesis as well, normalizing the pseudo-hypoxic signals that lead to normoxic HIF1A activation in solid tumors.{{cite journal | vauthors = Sutendra G, Dromparis P, Kinnaird A, Stenson TH, Haromy A, Parker JM, McMurtry MS, Michelakis ED | title = Mitochondrial activation by inhibition of PDKII suppresses HIF1a signaling and angiogenesis in cancer. | journal = Oncogene | volume = 32 | issue = 13 | pages = 1638–1650 | date = 28 March 2013 | pmid = 22614004 | doi = 10.1038/onc.2012.198 | doi-access = free }}