Nucleoside-diphosphate kinase

Nucleoside-diphosphate kinases (NDKs, also (poly)nucleotide kinases and nucleoside diphosphokinases) are enzymes that catalyze the exchange of phosphate groups between different nucleoside diphosphates. NDK activities maintain an equilibrium between the concentrations of different nucleoside triphosphates such as, for example, when GTP produced in the citric acid (Krebs) cycle is coverted to ATP.

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.


 * 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 isoforms of NDK in humans: NDK-A and NDK-B. Both have very similar structure, and can combine in any proportion to form functional NDK hexamers.