|
Exopolyphosphatase (PPX) is a phosphatase enzyme which catalyzes the hydrolysis of inorganic polyphosphate, a linear molecule composed of up to 1000 or more monomers linked by phospho-anhydride bonds. PPX is a processive exophosphatase, which means that it begins at the ends of the polyphosphate chain and cleaves the phospho-anhydride bonds to release orthophosphate as it moves along the polyphosphate molecule.〔 PPX has several characteristics which distinguish it from other known polyphosphatases, namely that it does not act on ATP, has a strong preference for long chain polyphosphate, and has a very low affinity for polyphosphate molecules with less than 15 phosphate monomers.〔 PPX plays an important role in the metabolism of phosphate and energy in all living organisms. It is especially important for maintenance of appropriate levels of intracellular polyphosphate, which has been implicated in a variety of cellular functions including response to stressors such as deficiencies in amino acids, orthophosphate, or nitrogen, changes in pH, nutrient downshift, and high salt, and as an inorganic molecular chaperone.〔 PPX is classified as a polyphosphatase, which are part of the large DHH phosphoesterase family. Both subfamilies within this super family share four N-terminus motifs but have different C-terminus moieties. PPX activity is quantified by measuring the loss of radioactively labeled 32P polyphosphate.〔 PPX is mixed with a known quantity of labeled polyphosphate, and the hydrolysis reaction is stopped with perchloric acid (HClO4).〔 The amount of remaining labeled polyphosphate is then measured by liquid scintillation counting.〔 ==History== PPX was discovered by the lab of Nobel laureate Arthur Kornberg in 1993 and is part of the polyphosphate operon along with polyphosphate kinase,〔 the enzyme which synthesizes polyphosphate. The Kornberg lab was very interested in polyphosphate and published a series of papers elucidating the metabolism and roles of polyphosphate ''in vivo''. Their interest in polyphosphate led them to identify and characterize the polyphosphate operon (which includes polyphosphate kinase () and PPX) and develop a wide variety of assays and techniques for quantification of polyphosphate production and degradation, ''in vitro'' and ''in vivo''. The results of these studies of polyphosphate by the Kornberg lab led Kornberg to speculate that due to its high energy and phosphate content and the degree to which it is conserved across species, polyphosphate may have been the precursor to RNA, DNA, and proteins.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Exopolyphosphatase」の詳細全文を読む スポンサード リンク
|