Nucleoside-diphosphate kinases (NDKs, also nucleoside diphosphate kinases and nucleoside diphosphokinases) are enzymes which catalyzes the exchange of phosphate groups between different nucleoside diphosphates. As the citric acid (Krebs) cycle can produce only ATP, NDKs serve to maintain an equilibrium between the concentrations of different nucleoside triphosphates.

Function

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The overall effect of NDKs is to transfer a phosphate group from a nucleoside triphosphate to a nucleoside diphosphate. Starting with adenosine triphosphate (ATP) and guanosine diphosphate (GDP), the action of NDK would produce adenosine diphosphate (ADP) and guanosine triphosphate (GTP).

ATP + GDP → ADP + GTP

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~ADP → NDK~GTP → NDK + GTP

Prokaryotic systems

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Prokaryotic NDK forms a functional homotetramer.

Eukaryotic systems

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There are two isoforms of NDK in human beings: NDK-A and NDK-B. Both have very similar structure, and can combine in any proportion to form functional NDK hexamers.

See also

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• Nucleoside
• Nucleotide