Open AccessVariable stoichiometry of proton: Lactose symport in Escherichia coli cells
Author(s) -
Ten Brink Bart,
Lolkema Juke S.,
Hellingwerf Klaas J.,
Konings Wil N.
Publication year - 1981
Publication title -
fems microbiology letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.899
H-Index - 151
eISSN - 1574-6968
pISSN - 0378-1097
DOI - 10.1111/j.1574-6968.1981.tb07648.x
Subject(s) - symporter , escherichia coli , stoichiometry , lactose , proton , chemistry , enterobacteriaceae , microbiology and biotechnology , biochemistry , biology , physics , organic chemistry , gene , transporter , quantum mechanics
in which A 0 and A i are the external and internal solute concentration; m the charge of the solute; n the number of protons symported and Z a factor which converts pH-units into millivolts (Z equals 60 at room temperature). A steady-state level of solute accumulation will be reached when this driving force equals zero. The validity of Eqn. (1) has been tested for several solutes, both in isolated membrane vesicles [2-6] and in whole cells [7,8] of Escherichia coli. Ramos and Kaback [3,4] determined the steadystate levels of A/irl+, ApH, Aq~ and solute accumulation (A~soj~e) in E. coli membrane vesicles at external pH values ranging from 5 to 8. For the neutral solute lactose the A~1a~to~ was at all pH values proportional with A/Ill+. However, above pH 6.3 the A/in+ was thermodynamically insufficient to account for the concentration gradient of lactose, if the stoichiometry between protons and lactose is 1 during symport. Therefore, the ratio A~lactose/A/ir~÷ (which equals n) has a value Of 1 at pH 5.5 and 2 at pH 7.5. These results indicate that in E. coil membrane vesicles lactose is symported with a variable number of protons, as outlined by Rottenberg [9]. According to this model the number of protons symported with a solute may depend on the external pH and varies from n at pH 5.5 to (n + 1) at pH 7.5. Also for other solutes, such as lysine [3], proline [4], lactate [3,5] and glucose-6-phosphate [3,6], the experimental data indicated a pHdependent variable stoichiometry of proton : solute symport. The physiological relevance of the phenomenon of variable stoichiometry for intact cells has been questioned [7,8]. In E. coli cells m/.tlact . . . . A~ and ApH were measured, and at all pH values tested between 5 and 8, the A/Ill+ exceeded the A/qacto =. Thus, a proton:lactose stoichiometry of 1 could account for the observed values of A~ac,ose in the entire pH range studied. This apparent discrepancy between the results obtained with isolated membrane vesicles and intact cells of E. coli might however be due to an overestimation of the A/in+ values in intact cells. In this paper we demonstrate that especially the