Kinetic alpha-deuterium isotope effects for Escherichia coli purine nucleoside phosphorylase-catalyzed phosphorolysis of adenosine and inosine
Kinetic alpha-deuterium isotope effects have been measured for the purine nucleoside phosphorylase-catalyzed phosphorolysis of adenosine and inosine by a competitive double label technique at saturating concentrations of the second substrate, phosphate. Under these conditions the observed isotope effect, kH/kD, is on the second order rate constant, Vmax/Km, for reaction of nucleoside with the Michaelis complex of enzyme and phosphate. For adenosine, at neutral pH, the isotope effect is unity. For inosine, kH/kD was determined as a function of pH. These values suggest a mechanism for purine nucleoside phosphorylase involving a change in rate-limiting step from one at pH values near neutrality for which cleavage of the nucleoside C-N bond is not rate limiting to a step at extremes of pH with a transition state having considerable oxocarbonium ion-like character.
- OSTI ID:
- 5952855
- Journal Information:
- J. Biol. Chem.; (United States), Journal Name: J. Biol. Chem.; (United States) Vol. 256:2; ISSN JBCHA
- Country of Publication:
- United States
- Language:
- English
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59 BASIC BIOLOGICAL SCIENCES
ACID ANHYDRASES
ADENOSINE
BACTERIA
CHEMICAL REACTION KINETICS
DEUTERIUM COMPOUNDS
DOUBLE LABELLING
ENZYMES
ESCHERICHIA COLI
HETEROCYCLIC COMPOUNDS
HYDROGEN COMPOUNDS
HYDROLASES
INOSINE
ISOTOPE EFFECTS
KINETICS
LABELLING
LYSIS
MICROORGANISMS
NUCLEOSIDES
NUCLEOTIDES
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
OXYGEN COMPOUNDS
PH VALUE
PHOSPHATES
PHOSPHOHYDROLASES
PHOSPHORUS COMPOUNDS
PURINES
REACTION KINETICS
RIBOSIDES