Membrane protein

Introduction

The membrane proteins are proteins found within the cellular membrane. The quantity of Gène S coding a membrane protein is estimated at approximately 25% of the Génome. This very significant number reflects the importance of this protein class, as well as the most fundamental diversity of the provided functions, being the bond between the extracellular medium and the intracellular medium.

Classification of membrane proteins

Membrane topologies

The membrane proteins can be classified according to the structures enabling them to interact with the membrane and the way in which those are arranged. The types of structures put in plays and their organizations are gathered under the membrane term of topology .

Polytopic proteins

The polytopic proteins are proteins in contact with the two compartments defined by the membrane. The structures in contact with the membranes can be:

A. one or more transmembrane propellers α (of 1 with more than 20 per protein). It is by far about obervée transmembrane conformation.
B. a transmembrane barrel β composed of 8 to 22 bits β.

Monotopic proteins

The monotopic proteins are proteins in contact with only one of the compartments defined by the membrane. The structures in contact with the membranes can be:

C. one or more propellers α parallel with the plan of the membrane (from 1 to 3 per protein). These propellers have the characteristic to be Amphipathique;
D. of the loops hydrophopes;
E. of the covalent bonds to a Lipid, or lipidation: it is often of N-acylation N-final (connection with a Fatty-acid) or about C-final glipiation (connection with a glycosylphosphatidylinositol);
F. of the electrostatic connections to lipids. These links can be made directly with phospholipides or via an ion;

Classification biochemical of membrane proteins

The membrane proteins can be classified according to biochemical criteria, in particular of their difficulty of being extracted the membranes.

Integral or intrinsic proteins

They are mono or polytopic proteins strongly interacting with the membrane. A strong detergent or a hydrophobic solvent breaking the membrane structure is necessary to extract them. These proteins are visualized by cryofracture. This consists in freezing a cellular membrane and typing in the block to separate the full-course ones.

Peripheral or extrinsic proteins

They acts of montopic proteins interacting slightly with the membrane, that is to say by electrostatic connections , that is to say via other membrane proteins. It is not necessary of destructurer the membrane to extract them. A high ionic force or the use of an agent chaotropic can be enough.

Roles of membrane proteins

The functions of membrane proteins can be as varied as that of soluble proteins: enzymatic activities, structural functions, molecular engines… Because of their localization specific to the interface of two different compartments, the integral proteins of membrane can however have three types of completely specific additional functions:

the active transport or liability of molecules of through the membrane. Transport is known as liability when the compound diffuses simply from one with dimensions to another of the membrane. Transport is on the contrary active when it is coupled with a use of energy by membrane protein (hydrolysis of ATP, use of the force proton-motor coach). Active transport makes it possible to transport a molecule against its gradient of concentration.
reception/transmission of signals through the membrane.
modelling and adhesion enters the membranes
the intercellular recognition (Glycoprotéines)
fixatation of the extracellular matrix
to play an enzymatic part

References

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