This is a reply to the following question from Ronald Koder Jr:
"If you are examining a ping-pong enzyme, and are trying out
different anaolgues of one of the substrates, would you
expect that the Km for the other substrate would
stay the same, or would it be the Vmax/Km for that substrate?"
This is a more subtle question than it may appear at first sight. Even
with the simplest assumptions there is no reason to expect the Km for
one substrate to remain the same when one changes the identity of the
other, but the case of Vmax/Km is more difficult than it seems.
On the simplest model the expression for Vmax/Km in terms of rate
constants contains no rate constants involving the other half of the
reaction, i.e. the half that involves the other substrate and its
corresponding product. On the face of it, therefore, one expects its
value to be independent of the identities of the reactants of the other
half.
That is NOT what is observed, however. John Westley and colleagues at
the University of Chicago studied several examples in which the
substituted-enzyme nature of the mechanism was known beyond any doubt
from other sorts of experiment, and found that in most cases the simple
expectation is NOT observed. The main exception was aspartate
transaminase, the archetype for this sort of enzyme, which behaved just
as one might have predicted, but several others did not. The paper of M.
Katz and J. Westley in Arch. Biochem. Biophys. (1980) 204, 464-470
describes one such investigation and gives references to others. They
interpret their observations in terms of enzyme memory. Several authors
(including some well known for their work in kinetics) have apparently
overlooked the work from Westley's group and have consequently drawn
mechanistic conclusions that are almost certainly erroneous.
A more recent discussion of this may be found on pp. 141-143 of my book
"Fundamentals of Enzyme Kinetics" (Portland Press, 1995), which is the
subject of a web page at
http://ir2lcb.cnrs-mrs.fr/lcbpage/athel/fek.html.
Regards,
Athel
