Interaction of sulfur‐containing ATP analogs with rabbit muscle phosphofructokinase

.beta.- and .gamma.-Thio analogs of adenosine triphosphate as probes of the Escherichia coli valyl transfer ribonucleic acid synthetase reaction pathway. A novel stereospecific interchange of adenosine 5'-O-(2-thiotriphosphate) to adenosine 5'-O-(3-thiotriphosphate)

“…Thus, if the two-step sequence is applicable to the thio analogues, step 1 must become partially, or wholly, rate limiting. The position of the sulfur affects the rate of aminoacylation catalyzed by the valyl enzyme to a greater degree than for the rates reported for the phenylalanyl enzyme; with the valyl enzyme, the rates fall in the order ATP > ATPyS > ATP/3S.2 The pattern agrees with the generalization (Nogc et al, 1979) that substitution of an oxygen of ATP by sulfur reduces the rate of reaction progressively as the position of the sulfur approaches the site of bond cleavage.2 The results of experiments in which PP¡ exchanges for SPP¡ to form ATP from ATP/3S follow the same pattern. The rates relative to ATP are as follows: ATPyS, 0.29; ATPSSA, 0.0066; ATPSSB, 0.00074.…”

Stereoselectivity of ectonucleotidases on vascular endothelial cells

“…This is in agreement with the view that these compounds are in general stable to degradation by phosphatases (see Eckstein, 1975); the only exception is the finding by Edwards & Maguire (1970) that 5'-nucleotidase purified from rat heart degraded AMPS almost as well as AMP itself. It should, perhaps, be noted that the terminally substituted phosphorothioates can participate in other enzymic reactions; for example, ATP[yS] is a good substrate for myosin and for several, but not all, kinases (Eckstein, 1975;Ngoc et al, 1979;Goody & Hofmann, 1980). Experiments with phosphorothioate analogues where sulphur is substituted on the penultimate phosphate (ADP[aS], ATP[fJSI) revealed that the endothelial nucleoside diphosphatase and the triphosphatase were both apparently stereospecific: in the presence of Mg2+ only the Sp isomer of ADP [aS] and only the Rp isomer of ATP[(1S] were catabolized.…”

“…First, although all the enzymes previously studied show stereoselectivity (usually marked) for one isomer of ATP[11S1 or ADP[aSI, they also usually show stereoselectivity for ATPlaSI (Frey et al, 1982). Of more than fifteen enzymes examined so far only phosphofructokinase (Ngoc et al, 1979) and carbamate kinase (Pillai et al, 1980) have shown no obvious stereoselectivity for ATP[aSl. Secondly, the favoured diastereoisomer of ATP[,fS] or ATP[aS] in a range of kinase reactions often interacts less well with the enzyme concerned than does ATP itself; Km values increase somewhat and Vmax values decline substantially (see e.g.…”