Membrane Proteins and Their Natural
Environment
1
Summary
This chapter offers a compact introduction to membrane proteins and their natural
environment. An overview is presented of the cellular location and functions of
membrane proteins, of lipid bilayers and the physical-chemical constraints they impose
on membrane-spanning molecules, of the impact of these constraints on the structure of
protein transmembrane regions, of lipid/protein interactions, and of membrane protein
synthesis. Background information that is indispensable as a frame for the rest of the
book is recalled, but the accent is put on notions that are essential to understanding
how surfactants work and to optimizing their use. The nature and extent of conformational changes undergone by protein transmembrane regions during functional cycles
are illustrated, taking as examples three membrane proteins, bacteriorhodopsin, the
nicotinic acetylcholine receptor, and the sarcoplasmic reticulum calcium pump, whose
stability and functionality in the presence of various surfactants have been studied in
some detail and will be discussed in subsequent chapters.
1.1
Introduction
Living organisms are made up of aqueous compartments that maintain and exploit concentration
differences between them and with the outside world. Biological molecules and ions are kept from
diffusing away, nutrients are imported and stored, by-products are expelled, and toxic molecules or
ions are kept out. Differences in physical-chemical potentials between compartments are created and
maintained. They are used to provide chemical and physical driving forces for an organism to sustain
itself, act on its environment, and store and exchange energy and information. In its simplest form –
from the point of view of compartmentalization – a living cell, such as a bacterium, is a little bag whose
molecular content reproduces itself until enough material has accumulated to permit it to divide into
two daughter cells, etc. In more complex organisms, the content of the bag can itself be
compartmentalized, and cells can associate with one another and cooperate, exchange long-distance
messages, etc.
Outside the cell is a liquid, water-based world. This may not seem so intuitive to us, humans,
who breathe air and walk on soil, but the environment all cells live in is aqueous, be it the sea,
# Springer International Publishing AG, part of Springer Nature 2018
J. -L. Popot, Membrane Proteins in Aqueous Solutions, Biological and Medical Physics,
Biomedical Engineering, https://doi.org/10.1007/978-3-319-73148-3_1
1
Environment
1
Summary
This chapter offers a compact introduction to membrane proteins and their natural
environment. An overview is presented of the cellular location and functions of
membrane proteins, of lipid bilayers and the physical-chemical constraints they impose
on membrane-spanning molecules, of the impact of these constraints on the structure of
protein transmembrane regions, of lipid/protein interactions, and of membrane protein
synthesis. Background information that is indispensable as a frame for the rest of the
book is recalled, but the accent is put on notions that are essential to understanding
how surfactants work and to optimizing their use. The nature and extent of conformational changes undergone by protein transmembrane regions during functional cycles
are illustrated, taking as examples three membrane proteins, bacteriorhodopsin, the
nicotinic acetylcholine receptor, and the sarcoplasmic reticulum calcium pump, whose
stability and functionality in the presence of various surfactants have been studied in
some detail and will be discussed in subsequent chapters.
1.1
Introduction
Living organisms are made up of aqueous compartments that maintain and exploit concentration
differences between them and with the outside world. Biological molecules and ions are kept from
diffusing away, nutrients are imported and stored, by-products are expelled, and toxic molecules or
ions are kept out. Differences in physical-chemical potentials between compartments are created and
maintained. They are used to provide chemical and physical driving forces for an organism to sustain
itself, act on its environment, and store and exchange energy and information. In its simplest form –
from the point of view of compartmentalization – a living cell, such as a bacterium, is a little bag whose
molecular content reproduces itself until enough material has accumulated to permit it to divide into
two daughter cells, etc. In more complex organisms, the content of the bag can itself be
compartmentalized, and cells can associate with one another and cooperate, exchange long-distance
messages, etc.
Outside the cell is a liquid, water-based world. This may not seem so intuitive to us, humans,
who breathe air and walk on soil, but the environment all cells live in is aqueous, be it the sea,
# Springer International Publishing AG, part of Springer Nature 2018
J. -L. Popot, Membrane Proteins in Aqueous Solutions, Biological and Medical Physics,
Biomedical Engineering, https://doi.org/10.1007/978-3-319-73148-3_1
1