The Conundrums of 3'Phoaphoadenosine 5'Phosphosulfate (PAPS) Synthase (PAPSS)

3´-Phoaphoadenosine 5´-Phosphosulfate (PAPS) synthase (PAPSS) of human is comprised of two domains [ATP sulfurylase (ATPS)] and [APS kinase (APSK)]. ATP sulfurylase binds ATP and allows the sulfate anion to attack the alpha-phosphoryl by nucleophilic attack. This allows the elimination of pyrophosphate (PPi) and the formation of phospho-sulfate anhydride bond of APS which is energetically higher (~19 kcal/mol) compared to phospho-phosphate (~7.6 kcal/ mol) nucleotide. However, nature chose to have sulfur as well as phosphorous nucleotide, one serving as a universal cellular energy currency, as well as a donor for phosphorylation. In contrast, PAPS was chosen as a universal donor of sulfuryl group for molecule/ macromolecule modifications and not chosen as an energy source. PPi the eliminated product of ATPS is cleaved into two inorganic phosphates by the ubiquitous pyrophosphatase, a process that can drive the whole reaction of APS formation, to certain degree in the forward direction? with the help of substrate concentration gradient. The energy of ~4 kcal must be invested in balance, to at least reach the equilibrium on the ATPS reaction.