J. Malecki et al., Kinetic studies of cAMP-induced allosteric changes in cyclic AMP receptor protein from Escherichia coli, J BIOL CHEM, 275(12), 2000, pp. 8480-8486
Cyclic AMP receptor protein (CRP) regulates the expression of several genes
in Escherichia coli. The ability of CRP to bind specific DNA sequences and
stimulate transcription is achieved as result of binding of an allosteric
Ligand: cAMP. Stopped-flow fluorimetry was employed to study the kinetics o
f the conformational changes in CRP induced by cAMP binding to high and low
affinity receptor sites. Results of experiments using CRP labeled at Cys-1
78 with 1,5-I-AENS indicate change in conformation of the helix-turn-helix,
occurring after the formation of CRP-cAMP(2) complex, i.e, after saturatio
n of the high affinity sites. The observed conformational change occurs acc
ording to sequential model of allostery and is described by rate constants:
k(c) = 9.7 +/- 0.1 s(-1) and k(-c) = 0.31 +/- 0.05 s(-1), for the forward
and backward reaction, respectively. Results of experiments monitored using
CRP intrinsic fluorescence suggest that conformational change precedes the
formation of CRP-cAMP(4) complex and results from displacement of equilibr
ium between two forms of CRP-cAMP(2), caused by binding of cAMP to low affi
nity sites of one of these forms only. The observed conformational change o
ccurs according to concerted model of allostery and is described by rate co
nstants: k(on) = 28 +/- 1.5 s(-1) and k(off) = 75.5 +/- 3 s(-1). Results of
experiments using single-tryptophan-containing CRP mutants indicate that T
rp-85 is mainly responsible for the observed total change in intrinsic fluo
rescence of wild-type CRP.