Biological Membranes - A. N. Ogurtsov 2012

Structure and Functions of Biomembranes
Intermolecular Interactions in Biomembranes
Lipid-Protein Interactions

Lipid-Structure/156.html">Protein Interactions are driven by intermolecular dispersion and electrostatic forces, hydrogen bonding, or other binding effects.

Lipid-protein interactions and the phenomena they cause are generally classified as follows:

1) protein - lipid monolayer interactions;

2) protein - lipid bilayer interactions;

3) lipid-protein interactions in membranes, involving lipid-dependent Enzymes.

The interaction of Proteins with lipid monolayers is observed when radiolabeled proteins (albumin, cytochrome c) are incorporated into the monolayers. Electrostatic Interactions between proteins and the monolayer manifest as a sharp change in protein sorption on charged monolayers upon deviation from the proteins' isoelectric point.

Experiments with phospholipases have shown that electrostatic interactions determine the Initial Stages of the enzyme-lipid monolayer interaction. These initial stages significantly facilitate the subsequent correct stereochemical orientation of the Components of the enzyme-substrate complex.

Protein-lipid bilayer interaction is a highly specific and multi-step process characterized by intramembrane protein insertion alongside surface sorption. The experimental criterion for protein insertion into Cell/29.html">The Lipid Bilayer is typically A change in membrane ion permeability.

In model experiments, the insertion of Membrane Proteins into artificial bilayer systems plays a crucial role in their successful functional reconstitution.

Lipid-protein interaction in membranes manifests in The formation of a specific lipid environment around protein molecules within the membranes. Such Lipids are referred to as bound or annular lipids.

Using the EPR method, Changes in the mobility and packing pattern of hydrocarbon chains under METABOLISM/18.html">The Influence of proteins have been proven. Furthermore, EPR, NMR, fluorescence, and other Methods have demonstrated that the perturbing effect of various integral and peripheral proteins (cytochrome c oxidase, cytochrome c, polylysine, myelin, rhodopsin, thylakoid membrane proteins, etc.) extends up to the Fourth layer of lipids surrounding the protein molecule.

The Functional Significance of annular lipids is usually interpreted based on experimental observations showing that higher protein activity occurs in a less viscous lipid environment. This has been demonstrated, for example, for cytochrome c oxidase embedded in artificial lipid membranes of varying composition, or in the case of ATPases in membranes of auxotrophic microorganisms.

Currently, several dozen membrane enzymes whose activity depends on the presence of lipids have been described, some of which are listed in Table 3.

Class="center">Table 3 - Lipids required for the expression of specific enzymatic activity in Biomembranes

Some membrane enzymes, such as mitochondrial electron transport proteins, are weakly sensitive to Lipid Composition, but are effectively activated by the total lipid fraction containing a certain amount of unsaturated lipids.

Achieving maximum activity in Other Enzymes requires lipids of a strictly defined composition. These Enzymes exhibit Specificity toward the polar HEAD groups of lipids and show little dependence on the fatty acid composition. In contrast, the functional activity of rhodopsin, for example, depends on the length of the lipid hydrocarbon chains.

The lipid dependence of membrane enzyme activity can be clearly manifested under conditions of selective extraction of Membrane Lipids and subsequent addition of specific lipids to delipidated membranes. Thus, mild ether-butanol extraction of Liver Plasma Membranes leads to a decrease in basal adenylate cyclase activity and hormone-stimulated responses. Basal activity is fully restored by adding phosphatidylinositol to the membranes.

Almost complete restoration of hormone-stimulated activity is observed upon The addition of phosphatidylserine to the membranes. It is hypothesized that the interaction of adenylate cyclase with specific membrane lipids is necessary for the expression of catalytic center activity and the Formation of the active hormone-receptor complex.