Nucleoside-diphosphate kinases (NDPKs, also NDP kinase, (poly)nucleotide kinases and nucleoside diphosphokinases) are enzymes that catalyze the exchange of terminal phosphate between different nucleoside diphosphates (NDP) and triphosphates (NTP) in a reversible manner to produce nucleotide triphosphates. Many NDP serve as acceptor while NTP are donors of phosphate group. The general reaction via ping-pong mechanism is as follows: XDP + YTP ←→ XTP + YDP (X and Y each represent different nitrogenous base). NDPK activities maintain an equilibrium between the concentrations of different nucleoside triphosphates such as, for example, when guanosine triphosphate (GTP) produced in the citric acid (Krebs) cycle is converted to adenosine triphosphate (ATP). Other activities include cell proliferation, differentiation and development, signal transduction, G protein-coupled receptor, endocytosis, and gene expression.

NDPK are homo hexameric proteins consisting of a nucleobase linked to a deoxyribose or ribose sugar esterified with diphosphate on the sugar. It is approximately 152 amino acids long with a theoretical weight of 17.17KDa. It is found in and in the soluble cytoplasm of cells.

NDPK are found in all cells, displaying not much specificity towards the types of nucleoside bases and are capable of accepting nucleotides and deoxyribonucleotides as substrates or donors. Therefore, NDPK is the source of RNA and DNA precursors, except ATP. NDPK utilize specific enzyme kinetics for multi-substrate reaction, namely ping-pong mechanism. A ping-pong mechanism integrates phosphorylation of a histidine residue by transferring terminal phosphate group (γ-phosphate) from ATP to NDP β-phosphate in order to produce a NTP, and NDPK catalyzes such reversible reactions. NTP phosphorylates a histidine, which in turn phosphorylates NDP. NDPK are involved in the synthesis of nucleoside triphosphates (NTP), such as guanosine triphosphate (GTP), cytidine triphosphate (CTP) and uridine triphosphate (UTP).

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