Nucleoside-diphosphate kinases ( NDKs, also NDP Kinase, ( 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 converted to ATP. [1 ]
Function [ edit ]
The overall effect of NDKs is to transfer a phosphate group from a nucleoside triphosphate to a nucleoside diphosphate. Starting with
ADP and GTP, the activity of NDK produces GDP and ATP. [1 ]
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 [ edit ]
Prokaryotic NDK forms a functional homotetramer.
Eukaryotic systems [ edit ]
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.
See also [ edit ]
References [ edit ]
^ a b Berg, JM; JL Tymoczko; L Stryer (2002). Biochemistry - 5th Edition. WH Freeman and Company. p. 476. ISBN 0-7167-4684-0.
External links [ edit ]