Transition state of the sulfuryl transfer reaction of estrogen sulfotransferase.

Kinetic isotope effects have been measured for the estrogen sulfotransferase-catalyzed sulfuryl (SO3) transfer from p-nitrophenyl sulfate to the 5'-phosphoryl group of 3'-phosphoadenosine 5'-phosphate. 18(V/K)nonbridge = 1.0016 +/- 0.0005, 18(V/K)bridge = 1.0280 +/- 0.0006, and 15(V/K) = 1.0014 +/- 0.0004. (15(V/K) refers to the nitro group in p-nitrophenyl sulfate). The kinetic isotope effects indicate substantial S O bond fission in the transition state, with partial charge neutralization of the leaving group. The small kinetic isotope effect in the nonbridging sulfuryl oxygen atoms suggests no significant change in bond orders of these atoms occurs, consistent with modest nucleophilic involvement. A comparison of the data for enzymatic and uncatalyzed sulfuryl transfer reactions suggests that both proceed through very similar transition states.

Design and synthesis of pyrankacin: a pyranmycin class of broad-spectrum aminoglycoside antibiotic.

A novel broad-spectrum aminoglycoside antibiotic, pyrankacin, has been prepared. In addition to the synthetic innovation in dideoxygenation and regioselective Staudinger reduction, we have obtained prominent antibacterial activity against several clinically important pathogens in the course of this work.

Glycodiversification for the optimization of the kanamycin class aminoglycosides.

In an effort to optimize the antibacterial activity of kanamycin class aminoglycoside antibiotics, we have accomplished the synthesis and antibacterial assay of new kanamycin B analogues. A rationale-based glycodiversification strategy was employed. The activity of the lead is comparable to that of commercially available kanamycin. These new members, however, were found to be inactive against aminoglycoside resistant bacteria. Molecular modeling was used to provide the explanation. Thus, a new strategy for structural modifications of kanamycin class aminoglycosides is suggested.

A nonhydrolyzable analogue of phosphotyrosine, and related aryloxymethano- and aryloxyethano-phosphonic acids as motifs for inhibition of phosphatases.

Nonhydrolyzable analogues of both stereoisomers of phosphotyrosine, and a series of related aryloxy (or thio) methyl and aryloxy (or thio) ethyl phosphonic acids of the general formula RX-(CH(2))(n)-PO(3)H(2) (where X=O or S and n=1 or 2), have been tested as nonhydrolyzable mimetics of phosphatase substrates. These compounds were tested against a panel of phosphatases (two alkaline phosphatases, a protein-tyrosine phosphatase, and two serine/threonine phosphatases) with different active site motifs. The compounds exhibit competitive inhibition toward all enzymes tested, with the best inhibition expressed toward the Ser/Thr phosphatases. The stereoisomers of the phosphotyrosine analogues exhibited an unexpected difference in their inhibitory properties toward the protein-tyrosine phosphatase from Yersinia. The K(i) for the d isomer is 33-fold lower than that of the l isomer, and is more than an order of magnitude lower than the reported K(m) of the substrate l-phosphotyrosine.

Synthesis of an unusual branched-chain sugar, 5-C-methyl-L-idopyranose for SAR studies of pyranmycins: implication for the future design of aminoglycoside antibiotics.

The syntheses of a challenging branched-chain sugar and several L-sugars have been accomplished. Their application in studies of the antibacterial activity of pyranmycins is reported, which could provide new strategies for the future design of aminoglycoside antibiotics.

Transition state differences in hydrolysis reactions of alkyl versus aryl phosphate monoester monoanions.

Although aryl phosphates have been the subject of numerous experimental studies, far less data bearing on the mechanism and transition states for alkyl phosphate reactions have been presented. Except for esters with very good leaving groups such as 2,4-dinitrophenol, the monoanion of phosphate esters is more reactive than the dianion. Several mechanisms have been proposed for the hydrolysis of the monoanion species. (18)O kinetic isotope effects in the nonbridging oxygen atoms and in the P-O(R) ester bond, and solvent deuterium isotope effects, have been measured for the hydrolysis of m-nitrobenzyl phosphate. The results rule out a proposed mechanism in which the phosphoryl group deprotonates water and then undergoes attack by hydroxide. The results are most consistent with a preequilibrium proton transfer from the phosphoryl group to the ester oxygen atom, followed by rate-limiting P-O bond fission, as originally proposed by Kirby and co-workers in 1967. The transition state for m-nitrobenzyl phosphate (leaving group pK(a) 14.9) exhibits much less P-O bond fission than the reaction of the more labile p-nitrophenyl phosphate (leaving group pK(a) = 7.14). This seemingly anti-Hammond behavior results from weakening of the P-O(R) ester bond resulting from protonation, an effect which calculations have shown is much more pronounced for aryl phosphates than for alkyl ones.

Generality of solvation effects on the hydrolysis rates of phosphate monoesters and their possible relevance to enzymatic catalysis.

Previous work by Kirby and co-workers revealed a significant acceleration of the rate of hydrolysis of p-nitrophenyl phosphate by added dipolar solvents such as DMSO. Activation parameters and kinetic isotope effects have been measured to ascertain the origin of this effect. The generality of this phenomenon was examined with a series of esters with more basic leaving groups. Computational analyses of the effects of desolvation of dianionic phosphate monoesters were carried out, and the possible effect of the transfer from water to the active site of alkaline phosphatase was modeled. The results are consistent with a desolvation-induced weakening of the P-O ester bond in the ground state. Other aryl phosphate esters show similar rate accelerations at high fractions of DMSO, but phenyl and methyl phosphates do not, and their hydrolysis reactions are actually slowed by these conditions.