STRUCTURE AND PROPERTIES OF BIOMOLECULES - A. E. Zemlyakov - 2017

05. CELLULAR AND MOLECULAR BASIS OF THE IMMUNE SYSTEM

❖ Immunity — the innate, heritable ability of an Organism to recognize and neutralize foreign material introduced from the outside or generated As a result of pathologies.

There are two main, interconnected immune systems within the body.

♦ Cellular immunity — a system of Cells possessing phagocytic activity. These include macrophages, neutrophilic granulocytes, etc.

♦ Humoral immunity — The production of specific substances (Antibodies) in the body against a foreign object (antigen).

The foundations of modern immunological theory were laid by the Russian biologistI. Mechnikov, who discovered the phagocytic action of immune cells, and the German researcher P. Ehrlich, who discovered lymphocyte differentiation and The Role of Bone Marrow in the immune system, and proposed the receptor theory of immune action. In 1908, both scientists received the Nobel Prize in Physiology or Medicine for these studies.

Class="center">

A wide variety of Organs are involved in the functioning of the human immune system, including the bone marrow, Lymph Nodes, Spleen, Thymus, and Tonsils.

A large number of cells participating in the immune system have been identified in the Tissues, circulatory, and lymphatic systems of the body. The main immune cells include macrophages, lymphocytes, and natural killer cells.

❖ Macrophages. Cell diameter is 10-20 µm; they contain a large number of Lysosomes where enzymatic degradation takes place. They originate from a common precursor for many cell types — stem cells, which sequentially transition into promonocytes, circulating monocytes, and tissue macrophages.

Macrophages are involved in Cell Recognition and also ensure the capture of foreign cells or other Antigens via numerous pseudopodia, followed by pinocytosis — engulfment and degradation using lysosomal Enzymes.

Along with phagocytic Functions, macrophages perform antigen recognition, activate lymphocytes, produce monokines (interleukin-1, tumor necrosis factor), and synthesize interferon (see below), Lysozyme, and protein components of The Complement System (see below).

❖ Lymphocytes. They are formed from bone marrow stem cells. Two main populations are distinguished: T cells, whose maturation and differentiation take place in the thymus, and B cells, which presumably differentiate in the bone marrow.

✵ T lymphocytes. Diameter ~4.5 µm, surface relatively smooth, containing A number of specific receptors. They lack an Endoplasmic reticulum. The main subpopulations are:

♦ T helper cells — participate in B cell activation;

♦ T suppressor cells — are responsible for terminating the Immune Response;

♦ T differentiating cells — direct the differentiation of stem cells;

♦ T killer cells — cytotoxic cells;

♦ T amplifier cells — promote The Development of the immune process;

♦ T memory cells — retain information about the antigen and facilitate the secondary immune response.

✵ B lymphocytes. Diameter ~5.8 µm, villous surface, containing numerous immunoglobulin-like receptors. Endoplasmic reticulum with polyribosomes. The main subpopulations are:

♦ B₁ precursor cells capable of synthesizing antibodies without T-helper activation;

♦ B₂ plasmacytic cells responsible for antibody synthesis;

♦ B₃ (B-killers) - cytotoxic cells;

♦ B-suppressors - involved in halting the immune response;

♦ Memory B cells - retain antigen information and facilitate the secondary immune response.

❖ Key principles of immune response development.

An immune response begins when an antigen (Ag) enters the body—any object (molecule, complex, virus, cell) that triggers an immune response. The antigen is captured by macrophage receptors (A), drawn into The Cell interior, and degraded (pinocytosis). Specific structures (antigenic determinants) are excised from the antigen surface and presented on the macrophage surface as a complex with class II Major Histocompatibility Complex antigens (MHC II), which are specialized Proteins responsible for transmitting information about the antigen.

Such an activated macrophage stimulates T-helpers by transferring antigen information to lymphocyte receptors. Concurrently, T-killers, capable of independently destroying antigens, are stimulated. Activated T-helpers interact with specific B-cell receptors. It is at this stage that antigen "recognition" occurs. It is estimated that B-cell populations are capable of recognizing ~10⁶–10⁹ antigens. B cells differentiate into plasmacytic cells, which then secrete specific antibodies (Ig) that bind to antigens to form complexes (agglutination or clumping). If Bacteria act as the antigens, their lipid membranes may be disrupted. When antibody concentrations are insufficient, antigen cells labeled with antibodies are destroyed by complement. Once the antigen is eliminated, T-suppressors return the immune system to a steady state.

At all stages requiring intercellular interactions, mediators (messengers) of immune reactions play a vital role: monokines—proteins secreted by macrophages, and lymphokines—proteins produced by lymphocytes. For example, activated macrophages secrete interleukin-1 (IL-1, see below), which stimulates T-helpers. These, in turn, secrete interleukin-2 (IL-2), which is essential for B-cell activation.

❖ The complement system. Blood Plasma contains 11 proteins with molecular weights ranging from 75 to 400 kDa, which form 9 complexes capable of binding in a cascade to foreign cells labeled with antibodies. Complement "perforates" the membrane, leading to cell lysis.

The process begins when protein C1, consisting of 3 subunits, binds to the constant region of the immunoglobulin "antigen-antibody" complex. Upon binding, the C1 complex acquires the enzymatic ability to cleave protein C4 into two parts, C4a and C4b. The former degrades, while the latter forms a complex with C1.

Next, proteins C2a and C3b bind sequentially, each being cleaved from larger proteins C2 and C3. The resulting aggregate interacts with the antigen membrane on the one hand, promoting bacterial phagocytosis, and exhibits protease activity on the other.

The Cleavage of protein C5 and the attachment of the C5b fragment facilitate the insertion of components C6 and C7 into the lipid membrane. Subsequent association with protein C8 triggers the lysis process, and The addition of component C9 significantly accelerates it. Pores form in the membrane, disrupting the ionic COMPOSITION OF THE Cytoplasm, causing the cell to swell and rupture.

❖ Antibodies—a class of proteins produced by B cells that bind antigens. Also known as IMMUNOGLOBULINS (Ig), antibodies account for over 20% of total Plasma Proteins. The animal immune system is capable of synthesizing up to 10⁸ antibody molecules of varying Specificity, ensuring interaction with a wide array of antigens.

The Structure of immunoglobulins is based on a complex of two pairs of polypeptide chains: light (L) and heavy (H). There are Two Types of light chains (λ and χ) and five types of heavy chains (γ, α, μ, δ, and ε). The Nature of the heavy chains determines the immunoglobulin class. Vertebrates possess 5 classes of antibodies:

♦ IgG (composition χ₂γ₂ or λ₂γ₂; molecular weight ~140–170 kDa)—the principal class of antibodies, accounting for up to 75% of total antibodies, with a serum concentration of 6–16 g/L;

♦ IgM (molecular weight ~800–950 kDa)—produced at the onset of the immune response, accounting for 5–10% of total immunoglobulins, with a serum concentration of 0.5–1.8 g/L; structured as a pentamer (χ₂μ₂)₅ or (λ₂μ₂)₅;

♦ IgA (molecular weight from 160 to 500 kDa)—forms The basis of antibodies in secretions (milk, mucus, tears), accounting for 7–15% of the immunoglobulin fraction, with a serum concentration of 1–5 g/L; can exist as a monomer χ₂α₂ or λ₂α₂, a dimer (two subunits joined by an additional J chain and a glycoprotein secretory component), or a trimer;

♦ IgD (composition χ₂δ₂ or λ₂δ₂, molecular weight ~180 kDa) and IgE (composition χ₂ε₂ or λ₂ε₂, molecular weight ~190 kDa)—minor serum components, accounting for 0.3% and 0.03% of total antibodies, respectively; they function as B-cell receptors (IgD) and mediate allergic reactions (IgE).

Immunoglobulin protein chains contain carbohydrate chains attached via N-glycosidic bonds. Each class of these Glycoproteins is characterized by a specific number of monosaccharide residues; for instance, IgG contains 22 Monosaccharides across 2 carbohydrate chains, whereas IgM contains 82 across 10 oligosaccharide chains.

❖ Structure of IgG. The molecule consists of four polypeptide structures: two light chains, each comprising approximately 220 amino acid residues, and two heavy chains containing about 450 amino acid residues. The heavy chains each contain a single oligosaccharide fragment. The protein chains are linked by disulfide bridges and non-covalent interactions to form a Y-shaped structure.

Protein chains have a domain architecture. Compact structured regions (domains) of ~110 amino acid residues are connected by an unstructured peptide chain, which provides the molecule with increased flexibility and facilitates adaptation to the antigen (especially in the so-called hinge region).

The chains comprise a variable region—which is unique to each antibody and ensures high-specificity binding to a specific antigen—as well as a constant region, which is common to the entire class of immunoglobulins and is responsible for General Properties, such as The ability to bind complement or anchor to membranes.

Due to the presence of two antigen-binding sites, antibodies are capable of "cross-linking" antigens into aggregates. The clumping of antigens caused by antibodies is referred to as agglutination.

❖ Structure of IgM. The macromolecule of this immunoglobulin has a pentameric structure in serum and a monomeric structure as a receptor On the surface of mature B cells. Each subunit of the pentamer is linked by Disulfide Bonds in the constant chain regions to each other and to an additional J peptide chain. Compared to IgG, an additional domain is present in the constant region of the peptide chains. Simplified, its Spatial Structure can be visualized as a ring featuring 10 antigen-binding sites.

❖ Mediators of the immune response.

✵ Interferons are broad-spectrum antiviral agents produced by various cells, notably in response to viral infection. They are small proteins or glycoproteins with a Molecular Weight of 15–25 kDa. Depending on the producing cells, they are classified into leukocyte (α-interferons), fibroblast (β-interferons), and immune (γ-interferon) types. α-Interferon is a protein, whereas the other two types are glycoproteins.

Interferons (IFs) are hormone-like compounds. Their interaction with cellular receptors on virus-infected cells triggers the activation of oligoadenylate synthetase (OAS). The oligo(2’ → 5’) adenylate (2-5A) synthesized by OAS in turn activates protein kinase R (PKR). Phosphorylation of the initiation factor eIF2 by PKR renders it inactive, thereby blocking The Biosynthesis of both host and viral proteins. On the other hand, oligoadenylate activates Ribonuclease L (RNase L), which degrades both messenger and viral RNAs. Consequently, along with the destruction of viral components, Protein Synthesis is inhibited. Furthermore, the resulting RNA fragments stimulate the PRODUCTION OF β-interferon, which is released from the cell to exert further antiviral effects. As a result, the infected cell undergoes apoptosis along with the virus trapped inside it.

Additionally, interferons activate Cells of the immune system. For instance, activated cytotoxic T Lymphocytes and natural killer cells are capable of destroying virus-infected cells. Interferon-activated macrophages stimulate the immune response against viral determinants by inducing T helper cells.

Currently, genetically engineered interferon (recombinant interferon) is one of the most effective drugs in modern medicine.

✵ Interleukins are polypeptide factors synthesized by cells of the immune system that act on immunocompetent cells. To date, nearly four dozen interleukin proteins have been identified. The most thoroughly studied include interleukins-1, 2, 4, and 6.

♦ Interleukin-1 (IL-1) exists in two forms: IL-1α and IL-1β. These are proteinaceous compounds consisting of 159 and 153 Amino Acids, with molecular weights of 18 and 17.4 kDa, respectively.

Macrophages are the primary producers of IL-1. The most potent Inducers (substances that enhance production) of IL-1 are lipopolysaccharide (LPS) and MDP (see below). Overall, IL-1 enhances the functions of T AND B lymphocytes, including the activation of IL-2, IL-6, IL-12, and γ-interferon production. It also acts on the Brain—specifically, increasing body Temperature (the pyrogenic effect) and promoting the slow-wave Sleep phase.

♦ Interleukin-2 (IL-2) consists of 133 amino acids, contains an intramolecular disulfide bond, and is glycosylated at Thr-3. Its molecular weight is 15.4 kDa. The primary producers of IL-2 are T helper cells, and to a lesser extent, cytotoxic T cells. Inducers of IL-2 production include IL-1, interferon, and MDP. IL-2 is a key compound in the Development of the immune response, playing a vital role in stimulating both T and B cells.

✵ Tumor necrosis factor (TNF). This term encompasses a family of cytokines comprising 19 distinct proteins. A membrane-bound form with a molecular weight of 26 kDa is distinguished, which is subsequently cleaved into a secreted form with a molecular weight of 17 kDa. The name of this cytokine derives from the ability of TNF-activated cytotoxic T cells to destroy certain tumor cells.

It has now been established that the TNF trimer, by acting on specific receptors TNF-R1 and TNF-R2, participates in immune system activation, inhibits tumor cell growth, and suppresses viral Replication.

❖ Cyclosporine A. A cyclopeptide consisting of 11 amino acids, a significant portion of which are N-methylated, including Sarcosine (Sar), MeVal, and MeLeu. It is produced by a number of mycelial Fungi.

Cyclosporine A is used in medical practice as an immunosuppressant to prevent the rejection of transplanted tissues and organs, as well as to treat various autoimmune diseases.

The biological MECHANISM OF ACTION is based on the suppression of T helper cells by inhibiting the production and synthesis of certain lymphokines, such as IL-2, which acts as a T-cell growth factor.

❖ Glycopeptide immunomodulators. Over 50 years ago, it was demonstrated that Freund's adjuvant—a Water-in-oil emulsion containing killed cells of Mycobacterium tuberculosis in the organic phase and an antigen in the aqueous phase—substantially enhances the immune response in both humoral and cellular immunity assays.

An adjuvant is a compound that enhances the immune response when co-administered with an antigen.

The minimal active structure within Freund's adjuvant is the glycopeptide muramyl dipeptide (MDP), a peptidoglycan cleavage product generated by lysozyme action. Structurally, the MDP molecule consists of an N-acetylmuramic acid carbohydrate residue and the L-alanyl-D-isoglutamine dipeptide.

Muramyl dipeptide activates immune cells and stimulates protective effects against

bacterial and viral infections, as well as antitumor immunity in the presence of synergists such as trehalose dimycolate (TDM), lipopolysaccharide (LPS), or lipid A.

Based on murein degradation products, the highly active disaccharide dipeptide GMDP was also developed and introduced into medical practice under the trade name "Licopid" (Russia) for the Treatment of Secondary Immunodeficiency States.

To investigate the structure-activity relationship, nearly a thousand MDP derivatives have been synthesized, several of which are used in clinical practice. For instance, MDP-L-Lys-COCi17H35 ("romurtide", Japan) is recommended for leukocyte count recovery in patients following tumor radio- and Chemotherapy; N-acetylmuramoyl-L-alanyl-D-glutamine butyl ester ("murabutide", France) is employed as a vaccine adjuvant; and the phosphoethanolamine derivative of MDP-L-Ala ("mifamurtide", Switzerland) is effective in the treatment of osteosarcoma.

Glycopeptides of the MDP group also exhibit neuropeptide properties. They raise body temperature (pyrogenic effect), affect blood pressure, and induce deep sleep. Structurally similar to GMDP is the "sleep factor" isolated from The Human Body, a substance that promotes slow-wave sleep. A unique feature of its structure is the presence of muramic acid in the form of a 1,6-anhydrosugar, along with the diamino dicarboxylic acid meso-diaminopimelic acid (mDAP) within its tetrapeptide fragment.

The hypothesis proposed by French scientist Edgar Lederer that MDP may function as a vitamin in the organism is quite compelling. The amount of muramyl dipeptide required to regulate the immune system can be generated endogenously through The breakdown of bacterial cell walls.

Control Tasks.

Test 1 (Sample)

1. The Primary Protein Structure is formed by:

A. peptide bonds; B. disulfide bonds; C. Hydrogen Bonds; D. ionic interactions; E. hydrophilic-hydrophobic interactions.

2. Draw the formula of L-isoleucine in Fischer projection.

This amino acid belongs to the following amino acid classes: A. neutral; B. acidic; C. basic; D. aromatic; E. aliphatic; F. heterocyclic; G. hydrophilic; H. hydrophobic; I. non-essential; J. essential.

3. Determine the configuration of the given amino acids using the D,L-nomenclature and name them.

4. Name the dipeptide.

Test 2 (Sample)

1. Select the terms corresponding to the following Definitions:

I. "proteins or Protein Complexes that act as specific and highly efficient catalysts for Chemical Reactions taking place in a living cell";

II. “biological Nucleus of origin”.

A. adjuvant; B. antigen; C. antibody; D. immunity; E. immunostimulant; F. coenzyme; G. lectin; H. selectin; I. receptor; J. toxin; K. phagocytosis; L. enzyme.

2. Activation of metabotropic receptors coupled to G_S-proteins leads to:

I. release of the DNA-binding domain; II. activation of adenylate cyclase; III. activation of phosphodiesterase; IV. activation of phospholipase C; V. activation of Tyrosine kinase.

A. increased cAMP concentration; B. decreased cAMP concentration; C. increased cGMP concentration; D. decreased cGMP concentration; E. increased Ca²⁺ concentration; F. decreased Ca²⁺ concentration; G. METABOLISM/31.html">Transcription initiation; H. transcription inhibition.

3. Match the terms to form logical pairs:

A. Hemoglobin; B. Chymotrypsin; C. Trypsin; D. Myoglobin; E. lysozyme

I. an enzyme that hydrolyzes PROTEINS AND Peptides at aromatic amino acid residues; II. an enzyme that hydrolyzes proteins and peptides at basic amino acid residues; III. an enzyme that hydrolyzes the polysaccharide components of bacterial cell walls; IV. a respiratory protein consisting of a single polypeptide chain; V. a respiratory protein consisting of four polypeptide chains.

4. Isomerases catalyze the following reactions:

A. oxidation-reduction; B. transfer of functional groups; C. Hydrolysis; D. addition across a double bond; E. isomerization; F. synthesis coupled with ATP energy.

5. Match the substances and processes with their corresponding positions in the steroid hormone receptor signaling pathway diagram.

1. hormone; 2. inhibitory subunit; 3. DNA; 4. hormone protein reservoir; 5. lipid membrane; 6. transcription; 7. activated receptor; 8. receptor.

6. Choose the correct name for the compound shown above.

1. muramyl dipeptide; 2. murabutide; 3. nitroglycerin; 4. nitrosorbid; 5. retinol; 6. all-trans-retinal; 7. 11-cis-retinal; 8. retinoic acid; 9. vitamin D₃.