Nucleoside-diphosphate kinase
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'''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 → 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 ↔ NDK~NTP ↔ NDK-P~NDP ↔ NDK-P + NDP
For the transfer of a phosphate from ATP to GDP, the reaction would proceed as
:NDK + ATP → NDK~ATP → NDK-P~ADP → NDK-P + ADP →
:NDK-P + GDP → NDK-P~GDP → NDK~GTP → 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]]