Inhibition of melibiose transporter by amiloride in Escherichia coli

A Possible Role for Iron-Sulfur Cluster N2 in Proton Translocation by the NADH:Ubiquinone Oxidoreductase (Complex I)

The proton-pumping NADH:ubiquinone oxidoreductase, the respiratory complex I, couples the transfer of electrons from NADH to ubiquinone with the translocation of protons across the membrane. The enzyme mechanism is still unknown due to the lack of a high-resolution structure and its complicated composition. The complex from Escherichia coli is made up of 13 subunits called NuoA through NuoN and contains one FMN and nine iron-sulfur (Fe/S) clusters as redox groups. The pH dependence of the midpoint redox potential of the Fe/S cluster named N2 and its spin-spin interaction with ubiquinone radicals made it an ideal candidate for a key component in redox-driven proton translocation. During the past years we have assigned the subunit localization of cluster N2 to subunit NuoB by site-directed mutagenesis and predicted its ligation by molecular simulation. Redox-induced FT-IR spectroscopy has shown that its redox reaction is accompanied by the protonation and deprotonation of individual amino acid residues. These residues have been identified by site-directed mutagenesis. The enzyme catalytic activity depends on the presence of cluster N2 and is coupled with major conformational changes. From these data a model for redox-induced conformation-driven proton translocation has been derived.

Ion translocation by the Escherichia coli NADH:ubiquinone oxidoreductase (complex I)

The energy-converting NADH:ubiquinone oxidoreductase, also known as respiratory complex I, couples the transfer of electrons from NADH to ubiquinone with the translocation of ions across the membrane. It was assumed that the complex exclusively works as a proton pump. Recently, it has been proposed that complex I from Klebsiella pneumoniae and Escherichia coli work as Na+ pumps. We have used an E. coli complex I preparation to determine the type of ion(s) translocated by means of enzyme activity, generation of a membrane potential and redox-induced Fourier-transform infrared spectroscopy. We did not find any indications for Na+ translocation by the E. coli complex I.

The Escherichia coli NADH:Ubiquinone Oxidoreductase (Complex I) Is a Primary Proton Pump but May Be Capable of Secondary Sodium Antiport

The NADH:ubiquinone oxidoreductase (complex I) couples the transfer of electrons from NADH to ubiquinone with the translocation of protons across the membrane. Recently, it was demonstrated that complex I from Klebsiella pneumoniae translocates sodium ions instead of protons. Experimental evidence suggested that complex I from the close relative Escherichia coli works as a primary sodium pump as well. However, data obtained with whole cells showed the presence of an NADH-induced electrochemical proton gradient. In addition, Fourier transform IR spectroscopy demonstrated that the redox reaction of the E. coli complex I is coupled to a protonation of amino acids. To resolve this contradiction we measured the properties of isolated E. coli complex I reconstituted in phospholipids. We found that the NADH:ubiquinone oxidoreductase activity did not depend on the sodium concentration. The redox reaction of the complex in proteoliposomes caused a membrane potential due to an electrochemical proton gradient as measured with fluorescent probes. The signals were sensitive to the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP), the inhibitors piericidin A, dicyclohexylcarbodi-imide (DCCD), and amiloride derivatives, but were insensitive to the sodium ionophore ETH-157. Furthermore, monensin acting as a Na(+)/H(+) exchanger prevented the generation of a proton gradient. Thus, our data demonstrated that the E. coli complex I is a primary electrogenic proton pump. However, the magnitude of the pH gradient depended on the sodium concentration. The capability of complex I for secondary Na(+)/H(+) antiport is discussed.

Mutational analysis of amiloride sensitivity of the NhaA Na+/H+ antiporter from Vibrio parahaemolyticus

The activity of the NhaA Na+/H+ antiporter of Vibrio parahaemolyticus is inhibited by amiloride. We found an amino acid sequence in the NhaA that was identical to a putative amiloride binding domain of the Na+/H+ exchanger in mammalian cells. We constructed mutant NhaAs that had amino acid substitutions in the putative amiloride binding domain by site-directed mutagenesis. These include V62L (Val62 replaced by Leu), F63Y, F64Y, and L65F. Most mutant NhaAs showed decreased sensitivity for amiloride. Among these, the F64Y mutant NhaA showed the least amiloride sensitivity, with a Ki value 7 to 10 times greater than that in the wild type. Thus, the sequence between residues V62 and L65 in NhaA, especially F64, is very important for the inhibitory effect of amiloride on the antiporter.