Glycerol phosphate shuttle
{{Short description|NADH transport mechanism in mitochondria}}
File:Glycerin-3-phosphat-Shuttle.svg
The glycerol-3-phosphate shuttle is a mechanism used in skeletal muscle and the brain{{Citation |last1=Blanco |first1=Antonio |title=Chapter 9 - Biological Oxidations: Bioenergetics |date=2017-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780128035504000094 |work=Medical Biochemistry |pages=177–204 |editor-last=Blanco |editor-first=Antonio |access-date=2023-05-14 |publisher=Academic Press |language=en |doi=10.1016/b978-0-12-803550-4.00009-4 |isbn=978-0-12-803550-4 |last2=Blanco |first2=Gustavo |editor2-last=Blanco |editor2-first=Gustavo}} that regenerates NAD+ from NADH, a by-product of glycolysis. NADH is a reducing equivalent that stores electrons generated in the cytoplasm during glycolysis. NADH must be transported into the mitochondria to enter the oxidative phosphorylation pathway. However, the inner mitochondrial membrane is impermeable to NADH and only contains a transport system for NAD+. Depending on the type of tissue either the glycerol-3-phosphate shuttle pathway or the malate–aspartate shuttle pathway is used to transport electrons from cytoplasmic NADH into the mitochondria.{{Citation |last=Bhagavan |first=N. V. |title=CHAPTER 14 - Electron Transport and Oxidative Phosphorylation |date=2002-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780120954407500160 |work=Medical Biochemistry (Fourth Edition) |pages=247–274 |editor-last=Bhagavan |editor-first=N. V. |access-date=2023-05-14 |place=San Diego |publisher=Academic Press |language=en |doi=10.1016/b978-012095440-7/50016-0 |isbn=978-0-12-095440-7}}
The shuttle consists of two proteins acting in sequence. Cytoplasmic glycerol-3-phosphate dehydrogenase (cGPD) transfers an electron pair from NADH to dihydroxyacetone phosphate (DHAP), forming glycerol-3-phosphate (G3P) and regenerating the NAD+ needed to generate energy via glycolysis.{{Cite web|title=GPD1 glycerol-3-phosphate dehydrogenase 1 [Homo sapiens (human)] – Gene – NCBI|url=https://www.ncbi.nlm.nih.gov/gene/2819|access-date=2021-05-03|website=www.ncbi.nlm.nih.gov}} Mitochondrial glycerol-3-phosphate dehydrogenase (mGPD) then catalyzes the oxidation of G3P by FAD, regenerating DHAP in the cytosol and forming FADH2 in the mitochondrial matrix.{{Cite web|title=GPD2 glycerol-3-phosphate dehydrogenase 2 [Homo sapiens (human)] – Gene – NCBI|url=https://www.ncbi.nlm.nih.gov/gene/2820|access-date=2021-05-03|website=www.ncbi.nlm.nih.gov}} In mammals, its activity in transporting reducing equivalents across the mitochondrial membrane is secondary to the malate–aspartate shuttle.
History
The glycerol phosphate shuttle was first characterized as a major route of mitochondrial hydride transport in the flight muscles of blow flies.{{cite journal | vauthors = Estabrook RW, Sacktor B | title = alpha-Glycerophosphate oxidase of flight muscle mitochondria | journal = The Journal of Biological Chemistry | volume = 233 | issue = 4 | pages = 1014–9 | date = October 1958 | doi = 10.1016/S0021-9258(18)64696-4 | pmid = 13587533 | doi-access = free }}{{cite journal | vauthors = Sacktor B, Dick A | title = Pathways of hydrogen transport in the oxidation of extramitochondrial reduced diphosphopyridine nucleotide in flight muscle | journal = The Journal of Biological Chemistry | volume = 237 | issue = 10 | pages = 3259–63 | date = October 1962 | doi = 10.1016/S0021-9258(18)50156-3 | pmid = 13975951 | doi-access = free }} It was initially believed that the system would be inactive in mammals due to the predominance of lactate dehydrogenase activity over glycerol-3-phosphate dehydrogenase 1 (GPD1){{cite journal | vauthors = Boxer GE, Shonk CE | title = Low levels of soluble DPN-linked alpha glycerophosphate dehydrogenase in tumors | journal = Cancer Research | volume = 20 | pages = 85–91 | date = January 1960 | pmid = 13803504}} until high GPD1 and GPD2 activity were demonstrated in mammalian brown adipose tissue and pancreatic ß-islets.{{cite journal | vauthors = Ohkawa KI, Vogt MT, Farber E | title = Unusually high mitochondrial alpha glycerophosphate dehydrogenase activity in rat brown adipose tissue | journal = The Journal of Cell Biology | volume = 41 | issue = 2 | pages = 441–9 | date = May 1969 | pmid = 5783866 | pmc = 2107766 | doi = 10.1083/jcb.41.2.441}}{{cite journal | vauthors = Houstek J, Cannon B, Lindberg O | title = Glycerol-3-phosphate shuttle and its function in intermediate metabolism of hamster brown-adipose tissue | journal = European Journal of Biochemistry | volume = 54 | pages = 11–18 | date = 1975 | issue = 1 | doi = 10.1111/j.1432-1033.1975.tb04107.x | pmid = 168075 | doi-access = free }}{{cite journal | vauthors = Ratner PL, Fisher M, Burkart D, Cook JR, Kozak LP | title = The role of mRNA levels and cellular localization in controlling sn-glycerol-3-phosphate dehydrogenase expression in tissues of the mouse | journal = The Journal of Biological Chemistry | volume = 256 | issue = 7 | pages = 3576–9 | date = April 1981 | doi = 10.1016/S0021-9258(19)69647-X | pmid = 6782104 | doi-access = free }}{{cite journal | vauthors = Koza RA, Kozak UC, Brown LJ, Leiter EH, MacDonald MJ, Kozak LP | title = Sequence and tissue-dependent RNA expression of mouse FAD-linked glycerol-3-phosphate dehydrogenase | journal = Archives of Biochemistry and Biophysics | volume = 336 | issue = 1 | pages = 97–104 | date = December 1996 | pmid = 8951039 | doi =10.1006/abbi.1996.0536}}
Reaction
In this shuttle, the enzyme called cytoplasmic glycerol-3-phosphate dehydrogenase 1 (GPD1 or cGPD) converts dihydroxyacetone phosphate (2) to glycerol 3-phosphate (1) by oxidizing one molecule of NADH to NAD+ as in the following reaction:
File:Dihydroxyacetone phosphate to glycerol 3-phosphate en.svg
Glycerol-3-phosphate is converted back to dihydroxyacetone phosphate by an inner membrane-bound mitochondrial glycerol-3-phosphate dehydrogenase 2 (GPD2 or mGPD), this time reducing one molecule of enzyme-bound flavin adenine dinucleotide (FAD) to FADH2. FADH2 then reduces coenzyme Q (ubiquinone to ubiquinol) whose electrons enter into oxidative phosphorylation.{{cite book |author1=Stryer, Lubert |author2=Berg, Jeremy Mark |author3=Tymoczko, John L. |title=Biochemistry |publisher=W. H. Freeman |location=San Francisco |year=2007 |isbn=978-0-7167-8724-2 |url=http://bcs.whfreeman.com/biochem6/default.asp?s=&n=&i=&v=&o=&ns=0&uid=0&rau=0 |url-status=dead |archiveurl=https://web.archive.org/web/20070518142847/http://bcs.whfreeman.com/biochem6/default.asp?s=&n=&i=&v=&o=&ns=0&uid=0&rau=0 |archivedate=2007-05-18 }} This reaction is irreversible.{{cite journal |vauthors=Shen W, Wei Y, Dauk M, etal |title=Involvement of a glycerol-3-phosphate dehydrogenase in modulating the NADH/NAD+ ratio provides evidence of a mitochondrial glycerol-3-phosphate shuttle in Arabidopsis |journal=Plant Cell |volume=18 |issue=2 |pages=422–41 |date=February 2006 |pmid=16415206 |pmc=1356549 |doi=10.1105/tpc.105.039750}} These electrons bypass Complex I of the electron transport chain, making the glycerol-3-phosphate shuttle less energetically efficient compared to oxidation of NADH by Complex I.{{Cite journal |last1=Mráček |first1=Tomáš |last2=Drahota |first2=Zdeněk |last3=Houštěk |first3=Josef |date=2013-03-01 |title=The function and the role of the mitochondrial glycerol-3-phosphate dehydrogenase in mammalian tissues |journal=Biochimica et Biophysica Acta (BBA) - Bioenergetics |language=en |volume=1827 |issue=3 |pages=401–410 |doi=10.1016/j.bbabio.2012.11.014 |pmid=23220394 |issn=0005-2728|doi-access=free }}
See also
References
{{Reflist}}
External links
- http://chemistry.elmhurst.edu/vchembook/601glycolysissum.html {{Webarchive|url=https://web.archive.org/web/20200624183736/http://chemistry.elmhurst.edu/vchembook/601glycolysissum.html |date=2020-06-24 }} (describes the shuttle in the context of glycolysis)