Bacterial Membrane Transport: Organization of Membrane Activities

Abstract

Bacteria have transport systems enabling them to accumulate needed nutrients, extrude unwanted by‐products and modify their cytoplasmic content of protons and salts so as to maintain a composition conducive to growth and development. Most bacterial transport systems resemble their counterparts in eukaryotic cells, and similar principles operate in both cell types. Because bacterial transporters are often easier to deal with experimentally, they have become important models for their eukaryotic brethren. The recent determination of the crystal structure of several bacterial transporters including ones that share homology with human transporters that are important in health and disease has been a major breakthrough. It opened the way both to the understanding of the mechanism of active transport as well as to rational drug design.

Key concepts

  • Cytoplasmic membrane separates the cytoplasm from the cell‐environment and at the same time communicates between these compartments by transducing signals and transporting molecules and ions across the membrane.

  • Channels facilitate the passive transport of molecules or ions across the membrane.

  • Transporters couple energy to actively transport molecules or ions across the membrane.

  • Primary transport system couples transport to chemical or light energy.

  • Secondary transport system couples transport to chemiosmotic energy.

  • Chemiosmotic circuit couples several secondary‐ and primary transporters in a biological membrane.

  • A virtual proton pump emerges from combining transport and metabolism.

  • The proton motive force or the electrochemical gradient of protons across the membrane provides a reservoir of potential energy to be used in all forms of energy‐transduction across the membrane.

  • Crystal structure of membrane proteins are critical for understanding the mechanism of transporters and for drug design for transporters involved in human's health and disease.

Keywords: chemiosmotic circuits; primary proton pump; virtual proton pump; primary transporters; secondary transporters

Figure 1.

Chemiosmotic organization at the bacterial plasma membrane. A chemiosmotic H+ circuit initiated by electron transport‐linked proton pumps is found in aerobes and in facultative organisms (e.g. E. coli) that are growing aerobically. Secondary transporters that complete this circuit (nos 1–5) are described in the text. The H+/ATPase is using the proton electrochemical gradient to synthesize ATP. In anaerobic bacteria similar circuit can be maintained by anaerobic electron transport or by the H+/ATPase that can function in reverse, hydrolizing ATP to maintain a chemiosmotic circuit (see Harold and Maloney, ). ETC proton pumps: electron transport‐linked proton pumps.

Figure 2.

A virtual proton pump. In Oxalobacter formigenes, the thermodynamic equivalent of a proton pump emerges from the functional association of OxlT, an oxalate2−:formate1− antiporter, and an intracellular decarboxylation system. Modified from Anantharam et al..

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Further Reading

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Padan, Etana(Sep 2009) Bacterial Membrane Transport: Organization of Membrane Activities. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001418.pub2]