3′-Phosphoadenosine-5′-phosphosulfate (PAPS) is a derivative of

adenosine monophosphate Adenosine monophosphate (AMP), also known as 5'-adenylic acid, is a nucleotide. AMP consists of a phosphate group, the sugar ribose, and the nucleobase adenine. It is an ester of phosphoric acid and the nucleoside adenosine. As a substituent it t ...

(AMP) that is

phosphorylated In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writt ...

at the 3′ position and has a

sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...

group attached to the 5′

phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

. It is the most common

coenzyme A cofactor is a non-protein chemical compound or Metal ions in aqueous solution, metallic ion that is required for an enzyme's role as a catalysis, catalyst (a catalyst is a substance that increases the rate of a chemical reaction). Cofactors can ...

sulfotransferase In biochemistry, sulfotransferases (SULTs) are transferase enzymes that catalyze the transfer of a sulfo group () from a donor molecule to an acceptor alcohol () or amine (). The most common sulfo group donor is 3'-phosphoadenosine-5'-phosphos ...

reactions and hence part of

sulfation pathways Sulfation (sometimes spelled sulphation in British English) is the chemical reaction that entails the addition of SO3 group. In principle, many sulfations would involve reactions of sulfur trioxide (SO3). In practice, most sulfations are effected ...

. It is endogenously synthesized by organisms via the phosphorylation of adenosine 5′-phosphosulfate (APS), an intermediary metabolite. In humans such reaction is performed by bifunctional 3′-phosphoadenosine 5′-phosphosulfate synthases (

PAPSS1 Bifunctional 3'-phosphoadenosine 5'-phosphosulfate synthetase 1 is an enzyme that in humans is encoded by the ''PAPSS1'' gene. Three-prime-phosphoadenosine 5-prime-phosphosulfate ( PAPS) is the sulfate donor cosubstrate for all sulfotransferase ...

and

PAPSS2 Bifunctional 3'-phosphoadenosine 5'-phosphosulfate synthetase 2 is an enzyme that in humans is encoded by the ''PAPSS2'' gene. Sulfation is a common modification of endogenous (lipids, proteins, and carbohydrates) and exogenous (xenobiotics and ...

) using ATP as the phosphate donor.

Formation and reduction

APS and PAPS are intermediates in the reduction of sulfate to

sulfite Sulfites or sulphites are compounds that contain the sulfite ion (systematic name: sulfate(IV) ion), . The sulfite ion is the conjugate base of bisulfite. Although its acid (sulfurous acid) is elusive, its salts are widely used. Sulfites are ...

, an exothermic conversion that is carried out by

sulfate-reducing bacteria Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate () as termina ...

. In these organisms, sulfate serves as an electron acceptor, akin to the use of O₂ as an electron acceptor by aerobic organisms. Sulfate is not reduced directly but must be activated by the formation of APS or PAPS. These carriers of activated sulfate are produced by reaction with ATP. The first reaction is catalysed by

ATP sulfurylase In enzymology, a sulfate adenylyltransferase () is an enzyme that catalyzes the chemical reaction : ATP + sulfate ⇌ pyrophosphate + adenylyl sulfate Thus, the two substrates of this enzyme are ATP and sulfate, whereas its two products are p ...

: :SO₄²⁻ + ATP ⇌ APS + PP_i The conversion of APS to PAPS is catalysed by APS kinase: :APS + ATP ⇌ PAPS + ADP APSchemdraw

Reduction of APS leads to sulfite, which is further reduced to

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

, which is excreted. This process is called dissimilatory sulfate reduction. Reduction of PAPS, a more elaborated

sulfate ester In organosulfur chemistry, organosulfates are a class of organic compounds sharing a common functional group with the structure . The core is a sulfate group and the R group is any Organyl group, organic residue. All organosulfates are formally ...

, leads also to hydrogen sulfide. But in this case, the product is used in biosynthesis, e.g. for the production of

cysteine Cysteine (; symbol Cys or C) is a semiessential proteinogenic amino acid with the chemical formula, formula . The thiol side chain in cysteine enables the formation of Disulfide, disulfide bonds, and often participates in enzymatic reactions as ...

. The latter process is called assimilatory sulfate reduction because the sulfate sulfur is assimilated.M. T. Madigan, J. M. Martinko, J. Parker “Brock Biology of Microorganisms” Prentice Hall, 1997. .

References

{{DEFAULTSORT:Phosphoadenosine-5'-phosphosulfate, 3'- Nucleotides Coenzymes Sulfate esters Sulfur metabolism