Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 98, Issue 11, Pages 6068-6073Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.111139698
Keywords
-
Categories
Funding
- NIDDK NIH HHS [R01 DK051131, R56 DK051131, DK51131] Funding Source: Medline
Ask authors/readers for more resources
A mechanistic model for lactose/H+ symport via the lactose permease of Escherichia coli proposed recently indicates that the permease must be protonated to bind ligand with high affinity. Moreover, in the ground state, the symported H+ is shared between His-322 (helix X) and Glu-269 (helix VIII), whereas Glu-325 (helix X) is charge-paired with Arg-302 (helix IX). Substrate binding at the outer surface induces a conformational change that leads to transfer of the H+ to Glu-325 and reorientation of the binding site to the inner surface. After release of the substrate, Glu-325 is deprotonated on the inside because of rejuxtapositioning with Arg-302. To test the role of Arg-302 in the mechanism, the catalytic properties of mutants Arg-302 --> Ala and Arg-302 --> Ser were studied. Both mutants are severely defective in active lactose transport, as well as in efflux or influx down a concentration gradient, translocation modes that involve net H+ movement. In marked contrast, the mutants catalyze equilibrium exchange of lactose and bind ligand with high affinity. These characteristics are remarkably analogous to those of permease mutants with neutral replacements for Glu-325, a residue that plays a direct role in H+ translocation. Th se observations lend strong support for the argument that Arg-302 interacts with Glu-325 to facilitate deprotonation of the carboxylic acid during turnover.
Authors
I am an author on this paper
Click your name to claim this paper and add it to your profile.
Reviews
Recommended
No Data Available