Substrate selectivity in arginine-dependent acid resistance in enteric bacteria
To successfully colonize the human gut, enteric bacteria must activate acid resistance systems to survive the extreme acidity (pH 1.5–3.5) of the stomach. The antiporter AdiC is the master orchestrator of the arginine-dependent system. Upon acid shock, it imports extracellular arginine (Arg) into th...
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Published in | Proceedings of the National Academy of Sciences - PNAS Vol. 110; no. 15; pp. 5893 - 5897 |
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Main Authors | , |
Format | Journal Article |
Language | English |
Published |
United States
National Academy of Sciences
09.04.2013
National Acad Sciences |
Subjects | |
Online Access | Get full text |
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Summary: | To successfully colonize the human gut, enteric bacteria must activate acid resistance systems to survive the extreme acidity (pH 1.5–3.5) of the stomach. The antiporter AdiC is the master orchestrator of the arginine-dependent system. Upon acid shock, it imports extracellular arginine (Arg) into the cytoplasm, providing the substrate for arginine decarboxylases, which consume a cellular proton ending up in a C–H bond of the decarboxylated product agmatine (Agm ²⁺). Agm ²⁺ and the “virtual” proton it carries are exported via AdiC subsequently. It is widely accepted that AdiC counters intracellular acidification by continuously pumping out virtual protons. However, in the gastric environment, Arg is present in two carboxyl-protonation forms, Arg ⁺ and Arg ²⁺. Virtual proton pumping can only be achieved by Arg ⁺/Agm ²⁺ exchange, whereas Arg ²⁺/Agm ²⁺ exchange would produce no net proton movement. This study experimentally asks which exchange AdiC catalyzes, an issue previously unapproachable due to the absence of a reconstituted system mimicking the situation of bacteria in the stomach. Here, using an oriented liposome system able to hold a three-unit pH gradient, we demonstrate that Arg/Agm exchange by AdiC is strongly electrogenic with positive charge moved outward, and thus that AdiC mainly mediates Arg ⁺/Agm ²⁺ exchange to support effective virtual proton pumping. Further experiments reveal a mechanistic surprise—that AdiC selects Arg ⁺ against Arg ²⁺ on the basis of gross valence, rather than by local scrutiny of protonation states of the carboxyl group, as had been suggested by Arg-bound AdiC crystal structures. |
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Bibliography: | http://dx.doi.org/10.1073/pnas.1301442110 ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 Edited* by H. Ronald Kaback, University of California, Los Angeles, CA, and approved March 4, 2013 (received for review January 22, 2013) Author contributions: M.-F.T. and C.M. designed research; M.-F.T. performed research; M.-F.T. and C.M. analyzed data; and M.-F.T. and C.M. wrote the paper. |
ISSN: | 0027-8424 1091-6490 |
DOI: | 10.1073/pnas.1301442110 |