3 -Phosphoadenosine-5 -phosphosulfate, synthesis

Sulfation is expensive in energy terms for the cell, since two molecules of ATP are necessary for the synthesis of one molecule of 3 -phosphoadenosine 5 -phosphosulfate (PAPS). Both enzymes involved in the synthesis of PAPS, ATP sulfurylase, and APS kinase, reside within a single bifunctional cytosolic protein of approximately 56 kDa, where substrate channeling of APS from ATP sulfurylase to APS kinase occurs. Several group VI anions other than sulfate can also serve as substrates, although the resultant anhydrides are unstable. Because this instability would lead to the overall consumption of ATP, these other anions can exert a toxic effect by depleting the cell of ATP. 

The sulfate molecule used in conjugation reactions is derived from 3-phosphoadenosine-5 -phosphosulfate (PAPS). Efficient synthesis of PAPS requires an adequate amount of inorganic sulfate with L-cysteine, D-cysteine, or L-methionine serving as precursors for the inorganic sulfate. PAPS is likely synthesized in every vertebrate cell, with high concentrations produced in the liver. 

Sulfate can be converted to the sulfate donor compound 3 -phosphoadenosine-5 -phosphosulfate (PAPS) in a two-step reaction (Figure 17-17). PAPS participates in the sulfate esterification of alcoholic and phenolic functional groups (e.g., in synthesis of sulfolipids and glycosamino-glycans). 

Synthesis of glycosphingolipids and sulfoglycosphin-golipids involves the addition of sugar and sulfate residues to ceramide from UDP-sugar derivatives or the activated sulfate donor 3 -phosphoadenosine-5 -phosphosulfate (Chapter 17), and appropriate transferases. These pathways are discussed in Chapter 16. Catabolism of sphingolipids is by specific lysosomal hydrolases. Several inherited disorders associated with the deficiencies of these enzymes are discussed below. 

Sulfated glycosphingolipids. Sulfatide (3 -sulfo-GalCer) synthesis is catalyzed by GalCer sulfatotransferase, which utilizes the activated sulfate donor 3 -phosphoadenosine-5 -phosphosulfate. The cDNA encoding the sulfotransferase has been cloned (K. Honke,... 

Fig. 33.34. The synthesis of 3 -phosphoadenosine 5 -phosphosulfate (PAPS), an active sulfate donor. PAPS donates sulfate groups to cerebrosides to form sulfatides and is also involved in glycosaminoglycan biosynthesis (see Chapter 49). Ad = adenosine.

Vitamin A deficiency characteristically results in hypoplasia of mucus-secreting tissues and keratinizing metaplasia of epithelial surfaces (171). The inhibition of mucus secretion appears to be related to a defect in mucopolysaccharide synthesis which can be attributed to a diminished incorporation of sulfate into 3 -phosphoadenosine-5 -phosphosulfate (active sulfate) (172). The impaired synthesis of active sulfate can be demonstrated in cell-free colon preparations from vitamin A deficient rats and can be restored to normals by a metabolite of vitamin A. 

Conjugation of phenols, aliphatic and steroid alcohols with sulfate occurs in mammals, birds, reptiles, amphibia, but not in fish. In addition, active sulfate in the presence of transferase will conjugate aromatic amines and form sulfamates [70] in mammals, birds, and spiders. The synthesis of sulfate derivatives occurs in the soluble fraction of liver homogenates through the formation of adenosine-5 -phosphosulfate (APS) and 3 -phosphoadenosine-5 -phosphosulfate (PAPS) [21]. The reactions may be written as follows ... 

The sulfatide isolated was soluble in chloroform-methanol (2—1) and failed to partition into an aqueous phase. Further identification of the sulfolipid was not made. Goldberg (1961) has postulated several pathways which may be involved in cerebroside sulfate synthesis. The initial steps are the activations of sulfate to 3 -phosphoadenosine-5 -phosphosulfate, i.e., PAPS, as described by Bandurski et al. (1956, 1958), Wilson and Bandurski (1956), and Robbins and Lipmann (1956, 1958a, b). 

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