Nucleoside-diphosphate kinase 2281317 197885267 2008-03-13T03:26:13Z Shenme 101696 Spelling "coverted" -> "converted" '''Nucleoside-diphosphate kinase'''s ('''NDK'''s, also ('''poly''')'''nucleotide kinase'''s and '''nucleoside diphosphokinase'''s) are [[enzyme]]s that [[catalyst|catalyze]] the exchange of phosphate groups between different [[nucleotide|nucleoside diphosphate]]s. NDK activities maintain an equilibrium between the concentrations of different nucleoside triphosphates such as, for example, when [[Guanosine triphosphate|GTP]] produced in the [[Citric acid cycle|citric acid (Krebs) cycle]] is converted to [[adenosine triphosphate|ATP]]. <ref name=[[Stryer]]>{{cite book |last= Berg |first=JM |coauthors= JL Tymoczko, L Stryer |title= Biochemistry - 5th Edition |pages =476|publisher= WH Freeman and Company |year=2002 |isbn= 0-7167-4684-0}}</ref> ==Function== The overall effect of NDKs is to transfer a phosphate group from a nucleoside triphosphate to a nucleoside diphosphate. Starting with [[adenosine diphosphate]] GTP and ADP, the activity of NDK produces GDP and ATP. <ref name=[[Stryer]]/> :GTP + ADP &rarr; GDP + ATP Behind this apparently simple reaction is a multistep mechanism. The key steps are *NDK binds a nucleoside triphosphate (NTP) *NDK transfers a phosphate to itself, leaving a bound nucleoside diphosphate (NDP) *NDK releases the bound nucloside diphosphate *NDK binds another nucleoside diphosphate *NDK transfers the phosphate to the diphosphate, creating a bound nucleoside triphosphate *NDK releases the new nucleoside triphosphate Each step is part of a reversible process, such that the multistep equilibrium is of the following form. :NDK + NTP &harr; NDK~NTP &harr; NDK-P~NDP &harr; NDK-P + NDP For the transfer of a phosphate from ATP to GDP, the reaction would proceed as :NDK + ATP &rarr; NDK~ATP &rarr; NDK-P~ADP &rarr; NDK-P + ADP &rarr; :NDK-P + GDP &rarr; NDK-P~GDP &rarr; NDK~GTP &rarr; NDK + GTP ==Prokaryotic systems== Prokaryotic NDK forms a functional homotetramer. ==Eukaryotic systems== There are two [[isoform]]s of NDK in humans: [[NDPKA|NDK-A]] and NDK-B. Both have very similar structure, and can combine in any proportion to form functional NDK hexamers. ==See also== *[[Nucleoside]] *[[Nucleotide]] == References == <references/> ==External links== * {{MeshName|Nucleoside-Diphosphate+Kinase}} * {{EC number|2.7.4.6}} {{Enzyme-stub}} {{Kinases}} {{Nucleotide metabolism}} [[fr:Nucléoside diphosphate kinases]]