TY - JOUR
T1 - The fast release of sticky protons
T2 - Kinetics of substrate binding and proton release in a multidrug transporter
AU - Adam, Yoav
AU - Tayer, Naama
AU - Rotem, Dvir
AU - Schreiber, Gideon
AU - Schuldiner, Shimon
PY - 2007/11/13
Y1 - 2007/11/13
N2 - EmrE is an Escherichia coli H+-coupled multidrug transporter that provides a unique experimental paradigm because of its small size and stability, and because its activity can be studied in detergent solution. In this work, we report a study of the transient kinetics of substrate binding and substrate-induced proton release in EmrE. For this purpose, we measured transient changes in the tryptophan fluorescence upon substrate binding and the rates of substrate-induced proton release. The fluorescence of the essential and fully conserved Trp residue at position 63 is sensitive to the occupancy of the binding site with either protons or substrate. The maximal rate of binding to detergent-solubilized EmrE of TPP+, a high-affinity substrate, is 2 × 107 M-1·s-1, a rate typical of diffusion-limited reactions. Rate measurements with medium- and low-affinity substrates imply that the affinity is determined mainly by the koff of the substrate. The rates of substrate binding and substrate-induced release of protons are faster at basic pHs and slower at lower pHs. These findings imply that the substrate-binding rates are determined by the generation of the species capable of binding; this is controlled by the high affinity to protons of the glutamate at position 14, because an Asp replacement with a lower pK is faster at the same pHs.
AB - EmrE is an Escherichia coli H+-coupled multidrug transporter that provides a unique experimental paradigm because of its small size and stability, and because its activity can be studied in detergent solution. In this work, we report a study of the transient kinetics of substrate binding and substrate-induced proton release in EmrE. For this purpose, we measured transient changes in the tryptophan fluorescence upon substrate binding and the rates of substrate-induced proton release. The fluorescence of the essential and fully conserved Trp residue at position 63 is sensitive to the occupancy of the binding site with either protons or substrate. The maximal rate of binding to detergent-solubilized EmrE of TPP+, a high-affinity substrate, is 2 × 107 M-1·s-1, a rate typical of diffusion-limited reactions. Rate measurements with medium- and low-affinity substrates imply that the affinity is determined mainly by the koff of the substrate. The rates of substrate binding and substrate-induced release of protons are faster at basic pHs and slower at lower pHs. These findings imply that the substrate-binding rates are determined by the generation of the species capable of binding; this is controlled by the high affinity to protons of the glutamate at position 14, because an Asp replacement with a lower pK is faster at the same pHs.
KW - Drug
KW - Fluorescence
KW - Ion-coupled transporter
KW - Membrane protein
KW - Resistance
KW - Transient kinetic
UR - http://www.scopus.com/inward/record.url?scp=36749095723&partnerID=8YFLogxK
U2 - 10.1073/pnas.0704425104
DO - 10.1073/pnas.0704425104
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C2 - 17984053
AN - SCOPUS:36749095723
SN - 0027-8424
VL - 104
SP - 17989
EP - 17994
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
IS - 46
ER -