= Adenosine 3',5'-bisphosphate
\b[Adenosine 3′,5′-bisphosphate], often abbreviated PAP, is a <nucleotide> with separate <phosphate> groups attached at the 3′ and 5′ positions of the <ribose> in <adenosine>. The positions are part of its identity: two <phosphate> groups on different sugar positions do not make the same structure as the linked phosphate chain of <adenosine diphosphate>.<Adenosine 3',5'-bismonophosphate in ChEBI>[CHEBI:17985, Definition, structure and Synonyms.]{ref}
An important reaction example is <sulfation> using <PAPS>. In the reaction catalyzed by <heparan sulfate glucosamine N-sulfotransferase>, <PAPS> transfers a sulfur-containing group to a <glucosamine> unit in <heparan sulfate>, leaving <adenosine 3',5'-bisphosphate>. The product retains the two phosphate groups but lacks the transferred sulfo group. Counting phosphorus atoms alone would therefore fail to distinguish donor from product.<Heparan sulfate glucosamine N-sulfotransferase in IUBMB Enzyme Nomenclature>[EC 2.8.2.8, Reaction and Glossary.]{ref}
This is a useful example of why a <molecular formula> or a count of functional groups is less informative than a <chemical structure>: attachment positions and connectivity determine which molecule is present.
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