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ADP is the key allosteric inhibitor of ribose-phosphate diphosphokinase. It has been shown that at lower concentrations of the substrate ribose 5-phosphate, ADP may inhibit the enzyme competitively. Ribose-phosphate pyrophosphokinase is also inhibited by some of its downstream biosynthetic products. [2] [6]
A key regulatory step is the production of 5-phospho-α-D-ribosyl 1-pyrophosphate by ribose-phosphate diphosphokinase, which is activated by inorganic phosphate and inactivated by purine ribonucleotides. It is not the committed step to purine synthesis because PRPP is also used in pyrimidine synthesis and salvage pathways.
ATP + d-ribose ⇌ ADP + d-ribose 5-phosphate. Thus, the two substrates of this enzyme are ATP and d-ribose, whereas its two products are ADP and d-ribose 5-phosphate. The systematic name of this enzyme class is ATP: d-ribose 5-phosphotransferase. Other names in common use include deoxyribokinase, ribokinase (phosphorylating), and d-ribokinase.
Phosphoribosyl pyrophosphate (PRPP) is a pentose phosphate. It is a biochemical intermediate in the formation of purine nucleotides via inosine-5-monophosphate, as well as in pyrimidine nucleotide formation. Hence it is a building block for DNA and RNA.
The first three enzymes of the process are all coded by the same gene in CAD which consists of carbamoyl phosphate synthetase II, aspartate carbamoyltransferase and dihydroorotase. Dihydroorotate dehydrogenase (DHODH) unlike CAD and UMPS is a mono-functional enzyme and is localized in the mitochondria.
Beta-D-ribofuranose can then be converted to ribose-5-phosphate. In the images on the right, both structures have five carbons but differ in the amount of hydrogens by one, oxygens by three, and phosphorus by one. The key difference is the group on carbon four. From here, ribose 5-phosphate can enter into the pentose phosphate pathway (PPP). [4]
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This enzyme belongs to the family of transferases, specifically those transferring phosphorus-containing groups (phosphotransferases) with a phosphate group as acceptor. The systematic name of this enzyme class is ATP:ribose-1,5-bisphosphate phosphotransferase. Other names in common use include ribose 1,5-bisphosphokinase, and PhnN.