As a rule, it is proteins that are responsible for the functional activity of membranes. These include a variety of enzymes, transport proteins, receptors, channels, and pores. Prior to this, it was believed that membrane proteins have an exclusively β folded structure, the secondary structure of the protein, but these works have shown that the membranes contain a large number ofα spirals. Further studies have shown that membrane proteins can penetrate deeply into the lipid bilayer or even penetrate it and their stabilization is carried out due to hydrophobic ...

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Lecture 5

The structure and functions of membrane proteins

Cell membranes contain protein from 20 to 80% (by weight). As a rule, it is proteins that are responsible for the functional activity of membranes. These include a variety of enzymes, transport proteins, receptors, channels, pores, etc. etc., which ensure the uniqueness of the functions of each membrane. The first advances in the study of membrane proteins were achieved when biochemists learned to use detergents to isolate membrane proteins in a functionally active form. These were works on the study of enzyme complexes of the inner membrane of mitochondria. Prior to this, it was thought that membrane proteins have exclusively β pleated structure (the secondary structure of the protein), but these works have shown that membranes contain a large number of α helices. Much less common is the β helix, which, however, is of great biological importance. The fact is that in the areas surrounded by lipids, the β helix is ​​a hollow cylinder, in the outer wall of which non-polar (hydrophobic) amino acid residues are concentrated, and in the inner hydrophilic. Such a cylinder could form a channel in the membrane through which ions and water-soluble substances can freely pass. Further studies have shown that membrane proteins can penetrate deeply into the lipid bilayer or even permeate it and their stabilization is carried out due to hydrophobic interactions. There are at least four types of protein arrangement in membranes: The first type is transmembrane, when the protein permeates the entire membrane, and the hydrophobic region of the protein has an α configuration. A similar arrangement in the membrane has a bacteriorhodopsin molecule from Halobacterium halobium its α helices successively cross the bilayer; The second type is hydrophobic anchoring, when the protein has a short stretch of hydrophobic amino acid residues near the carboxyl end. This is the so-called hydrophobic anchor, which can be removed by proteolysis, and the released protein becomes water-soluble. This arrangement in the membrane is inherent in many cytochromes. The third type is binding to the surface of the bilayer, when the interaction of proteins is primarily of an electrostatic or hydrophobic nature. This type of interaction can be used as an addition to other interactions, such as transmembrane anchoring. The fourth type is binding to proteins embedded in the bilayer, this is when some proteins bind to proteins that are located inside the lipid bilayer. For example, F 1 - part H + - ATPase, which binds to F0 part immersed in the membrane, as well as some proteins of the cytoskeleton.

At the heart of modern ideas about the structure of membrane proteins is the idea that their polypeptide chain is folded so that a non-polar, hydrophobic surface is formed that is in contact with the non-polar region of the lipid bilayer. The polar domains of the protein molecule can interact with the polar heads of lipids on the surface of the bilayer. Many proteins are transmembrane and span the bilayer. Some proteins seem to be associated with the membrane only through their interaction with other proteins.

Many membrane proteins typically bind to the membrane through non-covalent interactions. However, there are proteins that are covalently linked to lipids. Many plasma membrane proteins belong to the class of glycoproteins. Carbohydrate residues of these proteins are always located on the outside of the plasma membrane.

Usually, membrane proteins are divided into external (peripheral) and internal (integral). The criterion is the degree of severity of processing required to extract these proteins from the membrane. Peripheral proteins are released when membranes are washed with buffer solutions of low ionic strength, low or, conversely, high pH, ​​and in the presence of chelating agents (eg, EDTA) that bind divalent cations. It often happens that it is very difficult to distinguish peripheral membrane proteins from proteins bound to the membrane during isolation.

Detergents or even organic solvents must be used to release integral membrane proteins.

Many eukaryotic and prokaryotic membrane proteins are covalently linked to lipids that are attached to the polypeptide after translation.

Membrane proteins covalently linked to lipids

prokaryotes Lipoproteins of the outer membrane of bacteria E. coli Penicillase Cytochrome reaction center subunit eukaryotes (BUT) Proteins to which myristic acid is attached cAMP catalytic unit protein kinase NADPH cytochrome c 5 reductase α Guanine nucleotide-binding protein subunit (B) Proteins to which palmitic acid is attached G glycoprotein vesicular stomatitis virus HA Influenza virus glycoprotein Transferrin receptor Rhodopsin Ankirin (AT) Proteins with a glycosylphosphatidylinositol anchor Glycoprotein Thy 1 Acetylcholinesterase Alkaline phosphatase 4. Adhesive molecule of nerve cells

In some cases, these lipids act as a hydrophobic anchor by which the protein is attached to the membrane. In other cases, lipids are likely to function as an assistant in protein migration to the appropriate region of the cell or (as in the case of viral envelope proteins) in membrane fusion.

In prokaryotes, Brown's lipoprotein, the main lipoprotein of the outer membrane, is most fully characterized. E. coli . The mature form of this protein contains acylglycerol, which is linked by a thioether bond to N terminal cysteine. Besides, N The terminal amino acid is linked to the fatty acid by an amide bond. The membrane-bound form of penicillase attaches to the cytoplasmic membrane via N terminal acylglycerol is similar to membrane lipoproteins.

Eukaryotic membrane proteins are covalently linked to lipids, as shown in the table, and can be divided into three classes. Proteins of the first two classes, apparently, are localized mainly on the cytoplasmic surface of the plasma membrane, and proteins of the third class on the outer surface.

There are membrane proteins that are covalently linked to carbohydrates. These include surface proteins of cells mainly performing the functions of transport and reception. It is still unclear what is the matter here. Perhaps this is due to the fact that proteins need to be sorted when directed to the plasma membrane. Sugar residues can protect the protein from proteolysis or participate in recognition or adhesion. Therefore, sugar residues in membrane glycoproteins are localized exclusively on the outer side of the membrane.

Two main classes of oligosaccharide structures of membrane glycoproteins can be distinguished: 1) N glycosidic oligosaccharides associated with proteins through the amide group of aspargin; 2) O-glycosidic oligosaccharides linked through the hydroxyl groups of serine and threonine. This class of oligosaccharides consists of three subclasses.

1. A simple or mannose-enriched complex in which the oligosaccharide contains mannose and N Acetylglucosamine.
2. A normal complex in which the mannose-enriched core has additional side branches containing other saccharide residues, such as sialic acid.
3. Large complex that is associated with the anionic transporter of the erythrocyte membrane

Most membrane glycoprotein oligosaccharides belong to subclass 1 or 2.

Membrane proteins of bacteria

As noted above, proteins in the cytoplasmic membrane make up about 50% of its surface. Approximately 10% of the membrane is formed by tightly bound protein and lipid complexes. A molecule of any protein built into the membrane is surrounded by 45 130 or more lipid molecules. About half of the free lipids are associated with peripheral membrane proteins.

The protein composition of the cytoplasmic membrane of bacteria is more diverse than the lipid composition. So, in the cytoplasmic membrane E. coli K 12 about 120 different proteins have been found. Depending on the orientation in the membrane and the nature of the connection with the lipid bilayer, as noted above, proteins are divided into integral and peripheral. The peripheral proteins of bacteria include a number of enzymes such as NADH dehydrogenase, malate dehydrogenase, etc., as well as some proteins that are part of the ATPase complex. This complex is a group of protein subunits located in a certain way, contacting the cytoplasm, periplasmic space and forming a channel in the membrane through which the proton passes. The section of the complex marked F1 , immersed in the cytoplasm, a and c components of the site F0 The hydrophobic sides of the molecules are immersed in the membrane. Subunit b partially immersed in the membrane with its hydrophobic part and carries out the connection of the membrane and cytoplasmic parts of the enzyme complex, as well as the connection of ATP synthesis in the area F1 with the proton potential in the membrane. subunits a, b and c provide a proton channel. Other components of the complex ensure its structural and functional integrity.

to integral proteins E. coli, which lipids are necessary for the manifestation of enzymatic activity, can be attributed to succinate dehydrogenase, cytochrome b . Highly interesting properties possesses the antibiotics gramicidin A, alamethicin, amphotericin and nystacin. When interacting with the bacterial membrane, they become integral proteins (antibiotics are polypeptides and macrocycles).

Gramicidin A is a hydrophobic peptide consisting of 15 L-D -amino acids. When embedded in the membrane, it forms channels that allow monovalent cations to pass through. This channel, which forms gramicidin A , has been characterized most fully. The channel is formed by two molecules of gramicidin A. As a result of the alternation L- and D - amino acids, a helix is ​​formed in which the side chains are located outside, and the carboxyl groups of the backbone are inside the channel. This type of helix is ​​not found in any other proteins and is formed due to the unusual alternation of stereoisomers of amino acids in gramicidin A. The gramicidin channel, as noted above, is cation-selective. Small inorganic and organic cations pass through it, while the permeability Cl is equal to zero.

Alameticin is a peptide antibiotic of 20 amino acid residues, capable of forming electrically excitable channels in the membrane. The amino acid sequence of alamethicin includes the unusual residues α aminobutyric acid and L phenylalanine. When bound to the membrane, unlike gramicidin A, it forms a pore. It is much smaller than the channel that gramicidin A forms. This is primarily due to the fact that the space around the α helix is ​​too small for an ion to pass through.

Marcolide antibiotics such as nystatin and amphotericin bind to cholesterol and form channels. The channels form 8 10 molecules of these polyene antibiotics, through which, however, ions penetrate at low speeds.

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Most membrane proteins are integral components of membranes. (they interact with phospholipids); almost all sufficiently fully studied proteins have a length , exceeding 5-10 nm, is the value, equal to the thickness of the bilayer . These integral proteins are usually globular amphiphilic structures . Both their ends are hydrophilic, and the section crossing the core of the bilayer is hydrophobic. After establishing the structure of integral membrane proteins, it became clear that some of them (for example, molecules of carrier proteins) can cross the bilayer multiple times , as shown in Fig. 12.

integral proteins distributed in bilayer asymmetrically (Fig. 13). If a membrane containing asymmetrically distributed integral proteins is dissolved in a detergent (small amphipathic molecules that form micelles in water; with their help, transmembrane proteins can be solubilized. When a detergent is mixed with a membrane, the hydrophobic ends of its molecules bind to hydrophobic regions on the surface of membrane proteins, displacing lipid molecules from there.Since the opposite end of the detergent molecule is polar, such binding leads to the fact that membrane proteins go into solution in the form of complexes with detergent), and then the detergent is slowly removed, then self-organization of phospholipids and integral proteins will occur and a membrane structure will form, but proteins it will no longer be oriented in a specific way. In this way, the asymmetric orientation in the membrane of at least some proteins can be set when they are included in the lipid bilayer. The outer hydrophilic portion of the amphiphilic protein, which is, of course, synthesized inside the cell, must then cross the hydrophobic layer of the membrane and end up outside.

Peripheral proteins do not interact directly with phospholipids in the bilayer; instead they form weak bonds with hydrophilic sites specific integral proteins . For example, ankyrin, a peripheral protein, is associated with an integral protein of band III of the erythrocyte membrane. Spectrin, which forms the backbone of the erythrocyte membrane, is in turn bound to ankyrin and thus plays an important role in maintaining the biconcave shape of the erythrocyte (see below). Immunoglobulin molecules are integral proteins of the plasma membrane and are released only together with a small fragment of the membrane. Many receptors for various hormones are integral proteins, and specific polypeptide hormones that bind to these receptors can thus be considered peripheral proteins. . Such peripheral proteins as peptide hormones can even determine the distribution of integral proteins, their receptors, in the bilayer plane.

1. TRANSPORT OF HYDROPHILIC MOLECULES, and, in particular, charged particles. For example, the transport of sodium and potassium ions is carried out by the K,Na pump.

2. ENZYMATIVE ROLE.

Enzymes enclosed in a membrane have a number of features of catalytic properties. These enzymes are particularly sensitive to environmental factors.

RECEPTOR ROLE. Interaction with hormones, mediators is carried out by membrane proteins-glycoproteins. The carbohydrate component itself does not participate in the construction of the membrane, but lipids and proteins contain carbohydrates.

The role of carbohydrate components of membranes

a) Participate in the reception.

b) Provide interaction of cells with each other.

c) Some carbohydrate components provide antigenic specificity of cells. For example, erythrocytes different groups blood differ from each other in the composition of carbohydrate components.

The membranes are asymmetrical. 2 monolayers differ from each other in their composition. For example, plasma membrane glycolipids are always found in the outer monolayer. Asymmetry is also characteristic of protein components.

Adenylate cyclase. Its active site is located on the inside of the membrane. Receptor proteins contain their carbohydrate component on the outside of the membrane.

The most important component of plasma membranes is cholesterol.

Cholesterol interacts with the hydrophobic tails of polar molecules and limits the rate of lipid diffusion. Therefore, cholesterol is called a stabilizer of biological membranes. Membrane components not only move in space, but are constantly updated. New molecules take their place.

The curriculum includes only the exchange of HFL and cholesterol. Lipoids are synthesized on the membranes of the endoplasmic reticulum. There is a constant movement of lipoids from the EPS membranes to other membranes.

CHOLESTEROL SYNTHESIS

It occurs mainly in the liver on the membranes of the endoplasmic reticulum of hepatocytes. This cholesterol is endogenous. There is a constant transport of cholesterol from the liver to the tissues. Dietary (exogenous) cholesterol is also used to build membranes. The key enzyme in cholesterol biosynthesis is HMG reductase (beta-hydroxy, beta-methyl, glutaryl-CoA reductase). This enzyme is inhibited by the principle of negative feedback by the end product - cholesterol.

CHOLESTEROL TRANSPORT.

Dietary cholesterol is transported by chylomicrons and enters the liver. Therefore, the liver is a source for tissues of both dietary cholesterol (which got there as part of chylomicrons) and endogenous cholesterol.

In the liver, VLDL are synthesized and then enter the bloodstream - very low density lipoproteins (consist of 75% of cholesterol), as well as LDL - low density lipoproteins (they contain apoprotein apoB 100.

Almost all cells have receptors for apoB 100 . Therefore, LDL are fixed on the cell surface. In this case, the transition of cholesterol into cell membranes is observed. Therefore, LDL are able to supply tissue cells with cholesterol.

In addition, cholesterol is released from tissues and transported to the liver. High-density lipoproteins (HDL) transport cholesterol from tissues to the liver. They contain very few lipids and a lot of protein. Synthesis of HDL takes place in the liver. HDL particles are disc-shaped and contain apoproteins apoA, apoC and apoE. In the bloodstream, an enzyme protein attaches to LDL lecithincholesterol acyltransferase(LHAT) (see figure).

ApoC and apoE can switch from HDL to chylomicrons or VLDL. Therefore, HDL are donors of apoE and apoC. ApoA is an LCAT activator.

LCAT catalyzes the following reaction:

This is the transfer of a fatty acid from the R2 position to cholesterol.

The reaction is very important, because the resulting cholesterol ester is a very hydrophobic substance and immediately passes into the HDL core - this is how excess cholesterol is removed from them upon contact with HDL cell membranes. Then HDL goes to the liver, where it is destroyed, and excess cholesterol is removed from the body.

Violation of the ratio between the amount of LDL, VLDL and HDL can cause cholesterol retention in tissues. This leads to atherosclerosis. Therefore, LDL is called atherogenic lipoproteins, and HDL is called anti-atherogenic lipoprotein. With hereditary deficiency of HDL, early forms of atherosclerosis are observed.

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To membrane proteins include proteins that are embedded in or associated with the cell membrane or the membrane of a cell organelle. About 25% of all proteins are membrane proteins.

Biochemical classification

According to the biochemical classification, membrane proteins are divided into integral and peripheral.

• Integral membrane proteins are firmly embedded in the membrane and can only be removed from the lipid environment with the help of detergents or non-polar solvents. In relation to the lipid bilayer, integral proteins can be transmembrane polytopic or integral monotopic.
• Peripheral membrane proteins are monotopic proteins. They are either bound by weak bonds to the lipid membrane or are associated with integral proteins by hydrophobic, electrostatic, or other non-covalent forces. Thus, unlike integral proteins, they dissociate from the membrane when treated with an appropriate aqueous solution (eg, low or high pH, ​​high salt concentration, or chaotropic agent). This dissociation does not require the destruction of the membrane.

Membrane proteins can be built into the membrane due to fatty acid or prenyl residues or glycosylphosphatidylinositol attached to the protein during their post-translational modification.

Another important point is the methods of attaching proteins to the membrane:

1. Binding to proteins immersed in the bilayer. Examples include the F1 part of H + -ATRase, which binds to the Fo part embedded in the membrane; some cytoskeletal proteins can also be mentioned.

2. Binding to the bilayer surface. This interaction is primarily electrostatic in nature (eg myelin basic protein) or hydrophobic (eg surfactant peptides and possibly phospholipases). On the surface of some membrane proteins there are hydrophobic domains that are formed due to the features of the secondary or tertiary structure. These surface interactions can be used in addition to other interactions such as transmembrane anchoring.

3. Binding with a hydrophobic "anchor"; this structure usually appears as a sequence of non-polar amino acid residues (for example, in cytochrome 65). Some membrane proteins use covalently linked fatty acids or phospholipids as anchors.

4. Transmembrane proteins. Some of them cross the membrane only once (for example, glycophorin), others - several times (for example, lactose permease; bacteriorhodopsin).

Membrane lipids

Membrane lipids are amphipathic molecules that spontaneously form bilayers. Lipids are insoluble in water, but readily soluble in organic solvents. In most animal cells, they make up about 50% of the mass of the plasma membrane. There are approximately 5 x 100 thousand lipid molecules in a 1 x 1 μm section of the lipid bilayer. Therefore, the plasma membrane of a small animal cell contains approximately 10 lipid molecules. There are three main types of lipids in the cell membrane:




1) phospholipids (the most common type); complex lipids containing glycerol, fatty acids, phosphoric acid and a nitrogenous compound.

A typical phospholipid molecule has a polar head and two hydrophobic hydrocarbon tails. The length of the tails varies from 14 to 24 carbon atoms in the chain. One of the tails typically contains one or more cis double bonds (unsaturated hydrocarbon) while the other (saturated hydrocarbon) has no double bonds. Each double bond causes a kink in the tail. These differences in tail length and saturation of hydrocarbon chains are important because they affect membrane fluidity.

Amphipathic molecules in an aqueous environment usually aggregate, with hydrophobic tails being hidden and hydrophilic heads remaining in contact with water molecules. Aggregation of this type is carried out in two ways: either by the formation of spherical micelles with tails turned inward, or by the formation of bimolecular films, or bilayers, in which hydrophobic tails are located between two layers of hydrophilic heads.

The two main phospholipids that are present in plasma are phosphatidylcholine (lecithin) and sphingomyelin. Synthesis of phospholipids occurs in almost all tissues, but the main source of plasma phospholipids is the liver. The small intestine also supplies plasma with phospholipids, namely lecithin, as part of the chylomicrons. Most of the phospholipids that enter the small intestine (including in the form of complexes with bile acids) are subjected to preliminary hydrolysis by pancreatic lipase. This explains why polyunsaturated lecithin added to food does not affect plasma phospholipid content of linoleate more than equivalent amounts of corn oil triglycerides.

Phospholipids are an integral component of all cell membranes. Phosphatidylcholine and sphingomyelin are constantly exchanged between plasma and erythrocytes. Both of these phospholipids are present in plasma as constituents of lipoproteins, where they play a key role in maintaining non-polar lipids such as triglycerides and cholesterol esters in a soluble state. This property reflects the amphipathic nature of phospholipid molecules - nonpolar fatty acid chains are able to interact with the lipid environment, and polar heads - with the aqueous environment (Jackson R.L. ea, 1974).

2) Cholesterol. Cholesterol is a sterol containing a four-ring steroid nucleus and a hydroxyl group.

This compound is found in the body both as a free sterol and as an ester with one of the long chain fatty acids. Free cholesterol is a component of all cell membranes and is the main form in which cholesterol is present in most tissues. The exceptions are the adrenal cortex, plasma and atheromatous plaques, where cholesterol esters predominate. In addition, a significant part of the cholesterol in the intestinal lymph and in the liver is also esterified.

Cholesterol is found in lipoproteins either in free form or in the form of long-chain esters. fatty acids. It is synthesized in many tissues from acetyl-CoA and excreted in bile as free cholesterol or bile salts. Cholesterol is a precursor to other steroids, namely corticosteroids, sex hormones, bile acids, and vitamin D. It is a compound typical of animal metabolism and is found in significant amounts in animal products such as egg yolk, meat, liver and brain.

Eukaryotic plasma membranes contain a fairly large amount of cholesterol - approximately one molecule for each phospholipid molecule. In addition to regulating flow, cholesterol increases the mechanical strength of the bilayer. Cholesterol molecules are oriented in the bilayer in such a way that their hydroxyl groups are adjacent to the polar heads of phospholipid molecules.

3) glycolipids

Glycolipids are lipid molecules belonging to the class of oligosaccharide-containing lipids that are found only in the outer half of the bilayer, and their sugar groups are oriented towards the cell surface.

Glycolipids are sphingolipids in which a fatty acid residue is attached to the NH group of sphingazine, and the following groups are attached to the oxygen of sphingazine: oligosaccharide chains, Gal, Glc, GalNAc (neuraminic acid) - gangliosides. Gal or Glc are cerebrosides. sulfosaccharides Glc-SO3H, Gal-SO3H are sulfolipids.

Glycolipids are found on the surface of all plasma membranes, but their function is unknown. Glycolipids make up 5% of the lipid molecules of the outer monolayer and vary greatly between different types and even in different tissues of the same species. In animal cells, they are synthesized from sphingosine, a long amino alcohol, and are called glycosphingolipids.

Their structure is generally similar to the structure of phospholipids formed from glycerol. All glycolipid molecules differ in the number of sugar residues in their polar heads. One of the simplest glycolipids is galactocerebroside.