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Catalysis by Acetylcholinesterase. The Rate-Limiting Steps Involved in the Acylation of Acetylcholinesterase by Acetic Acid Esters and Phosphorylating Agents

Terrone L. Rosenberry ; Departments of Biochemistry and Neurology, College of Physicians and Surgeons, Columbia University, New York, N. Y., USA

Sažetak

Inferences about the catalytic mechanism of acetylcholinesterase
are frequently made on the basis of a presumed analogy with
chymotrypsin. Although both enzymes are serine hydrolases, ;several
differences in the steady-state kinetic properties of the two have
been observed. In this report particular attention is focused on the
second-order reaction constant, k0a1/Kapp· While the reported pH
dependence and deuterium oxide isotope effect associated with this
parameter for chymotrypsin are generally consistent with simple
models involving rate-limiting general acid-base catalysi·s, this
study finds a more complicated situation with acetylcholinesterase.
The apparent pKa of k0.JKapp for acetylcholinesterase varies
between 5.5 and 6.3 for neutral substrates and involves non-linear
inhibition by H+. Deuterium oxide isotope effects of k0.JKapp range
from 1.1 for acetylchoHne to 1.9 for p-nitrophenyl acetate. The
bimolecular reaction rate appears rate-limiting for acetylcholine
at low concentrations, while a rate-limiting induced-fit ·step is
proposed to account for apparent pKa values and low deuterium
oxide isotope effects associated with low concentrations of phenyl
acetate and isoamyl acetate. Other neutral acetic acid esters give
apparent pK3 values and deuterium oxide isotope effects consistent
with rate-limiting general-base catalysis at all substrate concentrations.
The pH dependence of second-order acylation by two
phosphorylating agents was also examined; one of these agents,
diisopropylfluorophosphate, gave the very unusual observation of
an increase in acylation rate as the pH decreased from 6 to 5.

Ključne riječi

Hrčak ID:

196588

URI

https://hrcak.srce.hr/196588

Datum izdavanja:

3.12.1975.

Posjeta: 586 *