Sulfuryl group transfer

Chen et al. (1999,2003) used cytosol prepared from various sections of the human intestine to study the occurrence and distribution of sulfotransferases in the gastrointestinal tract. They fortified the cytosol with PAPS. They utilized the sulfuryl group transfer from p-nitrophenol sulphate to PAP to generate PAPS for measurement of the phenol sulfotransferase activity by measurement of the colored product p-nitro-phenol. Cytosolic incubation were stopped by addition of Tris buffer, pH 8.7. 

Scheme 15 Transition structure for sulfuryl group transfer...

The sulfotransferases catalyze formation of sulfate esters of compounds having hydroxy or amino groups. The donor in this transfer of a sulfuryl group is 3 -phosphoadenylsulfate, also named previously as 3 -phosphoadenosine-5 -phos-phosulfate (PAPS). An example of this reaction is seen in Eq. (11) for phenol as the sulfuryl (S03) acceptor, with the products of the reaction being phenyl sulfate and adenosine 3, 5 -bisphosphate (PAP). 

Sulfotransferase enzymes (SULTs) catalyze the sulfation of substrates through the transfer of the sulfuryl group of adenosine 3 -phospate 5 -phosphosulfate... 

Stereochemistry is another powerful tool for determining the net reaction pathway of phosphatases and sulfatases. These enzymes catalyze the net transfer of a phosphoryl or sulfuryl group to water from a monoester, producing inorganic phosphate or sulfate. Inversion results when the reaction occurs in a single step (Scheme 2, pathway a). Phosphatases that transfer the phosphoryl group directly to water with inversion typically possess a binuclear metal center and the nucleophile is a metal-coordinated hydroxide. Examples of phosphatases that follow this mechanism are the purple acid phosphatases (PAPs) and the serine/threonine phosphatases (described in Sections 8.09.4.3 and 8.09.4.4.1). Net retention of stereochemistry occurs when a phosphorylated or sulfiirylated enzyme intermediate is on the catalytic pathway, which is hydrolyzed by the nucleophilic addition of water in a subsequent step (Scheme 2, pathway b). 

The reactive species under acidic conditions is the neutral ester. This reaction is believed to proceed by transfer of the proton from the sulfuryl group to the leaving group, as in reactions of phosphate monoester monoanions. A reduced value for d a solvent deuterium isotope effect of 2.43 " are consistent with proton transfer to the leaving group in the transition state. The intermediacy of free SO3 in the acid hydrolysis is sometimes assumed, but has not been proven. 

Sulfotransferase, sulfokinase an enzyme that catalyses the transfer of sulfuryl groups from phospho-adenosinephosphosulfate (PAPS) to oxygen and nitrogen functions of suitable acceptors. Transfer to an oxygen function (alcoholic and phenolic hydroxyl... 

Besides radical additions to unsaturated C—C bonds (Section III.B.l) and sulfene reactions (see above), sulfonyl halides are able to furnish sulfones by nucleophilic substitution of halide by appropriate C-nucleophiles. Undesired radical reactions are suppressed by avoiding heat, irradiation, radical initiators, transition-element ion catalysis, and unsuitable halogens. However, a second type of undesired reaction can occur by transfer of halogen instead of sulfonyl groups (which becomes the main reaction, e.g. with sulfuryl chloride). Normally, both types of undesired side-reaction can be avoided by utilizing sulfonyl fluorides. 

Figure 2 A loose transition state for phosphoryl or sulfuryl transfer is one in which bond fission is ahead of bond formation to the nucleophile, and resides in the lower right region of the More-O Ferrall Jencks diagram. A tight transition state is the reverse situation, residing in the upper left region. If the sum of bond order to nucleophile plus leaving group is unity, the transition state will lie on the synchronicity diagonal.

Scheme 2 Two potential reaction mechanisms for phosphatases or sulfatases are shown here using a phosphate ester. In (a), the phosphoryl group is transferred directly to a water molecule, which is typically bound to one or two metal ions if the substrate is made chiral at phosphorus, the stereochemical outcome is inversion. In (b), the phosphoryl group is first transferred to an enzymatic nucleophile E-POs " is a covalent phosphoenzyme intermediate. In a subsequent step, this intermediate is hydrolyzed. Since each step occurs with inversion of configuration at phosphorus, the net outcome is retention. The same principles apply to sulfuryl transfer. P, = inorganic phosphate.

Other examples of crossover activity are known. ASA has been shown to possess cyclic phosphodiesterase activity. Adenylate kinase transfers the 7-phosphoryl group of ATP, but can also accept as a substrate the sulfuryl analogue 7-sulfiiryl-ADP. Sulfoenolpyruvate, the sulfate analogue of phosphoenolpyruvate, is a substrate for pyruvate kinase, producing pyruvate and adenosine-5 -sulfatopyrophosphate. Many other examples probably remain undiscovered. 

Sulfuryl transfer, also referred to as sulfation or sulfonation is the transfer reaction of the sulfate group from the ubiquitous donor 3 -phosphoadenosine 5 -phosphosulfate (PAPS) to an oxygen atom of the acceptor substrate leading to a sulfuric acid ester (R — O — SO3, RR — N — O — SO3) (Fig. 31.41). 

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