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Medical Biochemistry Molecular Principles of Structural Organization of Cells

Medical Biochemistry Molecular Principles of Structural Organization of Cells

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Medical Biochemistry Molecular Principles of Structural Organization of Cells

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  1. Medical BiochemistryMolecular Principles of Structural Organization of Cells 4. PROTEINS

  2. PROTEINS • The major cell components of any living organism (25% of wet weight and 45-50% of dry weight) • Play important roles in all biological processes • Elementary composition: C 51-55%, O 21-23%, N 15-18%, H 6-7%, S 0.3-2.5% • Structure - they are • high-molecular (the mass of single-chain protein is 10-50 kilodaltons (350 dal-1000 kdal); multichain protein complexes >200 kdal. • N containing organic compounds (16% of dry weight), • with complex structural organization, • constructed from 20 different aminoacids, • linked in chains by peptide bonds. • Depending on the chain length peptides are classified in • Oligopeptides = 2-10 aa • Polypeptides = 10-40 aa • Proteins = >40 aa

  3. NATURE OF PROTEINS • Functions: • Enzymatic catalysis • Transport and storage of small molecules and ions • Structural (cytoskeleton), providing strength and structure to cells, forming components for intracellular and extracellular movements • Immune defense system (antibodies) • Hormonal regulation (hormones and receptors) • Control of genetic expression – activators, repressors • Show specificity of biological function, as a consequence of the uniqueness of three-dimensional structure

  4. AMINOACIDS – STRUCTURAL MONOMERS OF PROTEINS • Aminoacids contain at least 1 –NH2 group and 1 –COOH group. • L-aminoacids are classified in α-, β-, γ- depending on the position of C bearing –NH2 group with respect to –COOH. There are >200 aa in different species, 60 in human, only 20 in the structure of proteins. • Aa are classified in: • proteogenic - in the structure of proteins • nonproteogenic – not incorporated in proteins • Three classifications are adopted: • Structural • Electrochemical • Biological (physiological) • All protein aa are L-aminoacids and α-aminoacids

  5. AMINOACIDS – FUNDAMENTAL UNITS OF PROTEINSSTRUCTURAL CLASSIFICATION 1. ACYCLIC 1.1. Aliphatic unsubstituted Glycine (Gly) Alanine(Ala) Valine (Val) Leucine (Leu) Isoleucine (Ile) 1.2. Aliphatic substituted 1.2.1.Hydroxy aa Serine (Ser) Threonine (Thr) (hydroxyamine a) 1.2.2.Thio- aa Cysteine (Cys) Cystine (Cys2) Methionine (Met) (thiamin a)

  6. 1.2.3. Monoamino- Aspartic acid Glutamic acid Asparagine Glutamine Aminocitric acid dicarboxylic (Asp) (Glu) (Asn) (Gln) (carboxyamine a) 1.2.4. Diamino- Lysine (Lys) Hydroxylysine (Lys-OH) Monocarboxylic (diamine acids) 1.2.5. Guanidine amine Arginine (Arg) acids

  7. 2. CYCLIC AMINOACIDS 2.1. Aromatic aa PhenylalanineTyrosine (Phe)(Tyr) 2.2. Heterocyclic aa HistidineTryptophan Proline Hydroxyproline (His)(Trp) (Pro) (Pro-OH) Rare aminoacids: Aminocitric acid, Lys-OH, Pro-OH

  8. AMINOACIDS - STRUCTURAL CLASSIFICATION • ACYCLIC AMINOACIDS 1.1. Aliphatic unsubstituted: • Glycine/glycocol: excretion of benzoic acid as benzoylglycine, constituent of glutathione, intermediate in the synthesis of creatine, hem, purine bases • Alanine, valine, leucine, isoleucine: nonpolar, hydrophobic bonds 1.2. Aliphatic substituted: 1.2.1. Hydroxyamine acids: • Serine: slightly acidic; role: • constituent of active sites of some enzymes, • binding site of olygosaccharides in glycoproteins • Phosphoserine in phosphoproteins (phosvitin, vitellin, casein, myosine), phosphorylated enzymes, • in phosphatidylserine • Threonine: slightly acidic; role: • active site of enzymes • binding site of olygosaccharides in glycoproteins • phosphothreonine in phosphoproteins (casein, tropomyosin)

  9. 1.2.2. Thiamin acids: • Cysteine: slightly acidic, converted by oxidoreduction in cystine, forms disulfide bonds between peptide chains; role: in the structure of glutathione, metallothioneines, excretion of aromatic substances • Cystine: reduced to cysteine; in the structure of keratin, hair, insulin • Methionine: nonpolar, furnishes the 8 atoms of C in cysteine synthesis; crystalline in the lens contains N-acetylmethionine 1.2.3. Carboxy acids: • Aspartic acid: enzymes active sites, urea cycle, synthesis of nitrogenous bases • Glutamic acid: glutathione, folic acid, collagen; transamination, glutaminogenesis 1.2.4. Diamine acids: • Lysine: cationic at pH 7; binds cofactors at the active site of enzymes • Lysine-OH: collagen, bonding site for olygosaccharides 1.2.5. Guanidinamine acids: • Arginine: basic, binds phosphate group; takes part in urea cycle, biosynthesis of creatine

  10. 2. CYCLIC AMINOACIDS 2.1. Aromatic: • Phenylalanine: nonpolar, in the synthesis of tyrosine • Thyrosine: slightly acidic; enzyme bonding with substrate, synthesis of tyroxine, catecholamines, melanins 2.2. Heterocyclic: • Histidine: active site of enzymes, binds metal ions, in the structure of anserine and carnosine (dipeptides); generated histamine • Tryptophan: precursor of serotonin, crystalline in lens • Proline: role in folding the polypeptide chain • Proline-OH: collagen, elastin, acethylcolinesterase

  11. AMINOACIDS – ELECTROCHEMICAL CLASSIFICATION • Acidic – additional -COOH groups in the sidechain, polar: • aspartic acid, • glutamic acid, • aminocitric acid • Basic - cary additional basic groups: amino, guanidine, imidazole), polar: • lysine, • arginine, • histidine • Neutral - nonpolar, hydrophobic acids

  12. AMINOACIDS – BIOLOGICAL CLASSIFICATION • Essential (8) - cannot be synthesized in the organism • Val, Leu, Ile, Thr, Lys, Met, Phe, Trp • Half-essential (3) - can be synthesized not in sufficient amounts • Arg, Tyr, His • Nonessential - can be synthesized by the organism

  13. NONPROTEOGENIC AMINOACIDS • ornithine and citrulline – intermediates in urea cycle, synthesis of arginine • γ-aminobutyric acid (GABA)– free in the brain, lungs, heart; neurotransmitter • β-alanine– in the strucutre of vitamin B3, CoA-SH, carnosine and anserine; product of pyrimidines catabolism • dihydroxyphenylalanine (DOPA)– intermediate in the synthesis of hormones of adrenal medulla ornithine citrulline γ-aminobutyric acid β-alanine DOPA GABA

  14. AMINOACIDS – PHYSICAL AND CHEMICAL PROPERTIES • Acid-base properties • Aa have amphoteric properties (have both acidic and basic groups) • Monoamino-monocarboxylic aa exist in aqueous solutions as zwitterions (dipolar molecules): carboxyl is dissociated and negatively charged, amino is protonated and positively charged; they are electrically neutral. • At low pH COO- accepts H+ and becomes uncharged; the molecule is positive • At high pH the NH3+ loses H+ and becomes uncharged; the molecule is negative • The aminoacids having side chains that contain dissociating groups: • Aspartic acid, glutamic acid are acidic • Lysine, arginine, histidine are basic • Cysteine, tyrosine have a negative charge on the sidechain when dissociated • The state in which the net charge on the aa is 0 = isoelectric point (pHi) a very accurate indicator of acid-base properties: • for nonpolar aa – close to neutral (5.5 for Phe, 6.3 for Pro) • for acidic aa – low values (3.2 for Glu) • weak acidic – for Cys, Cys-S-S-Cys (5) • for the rest, especially Lys, Arg, His – higher than 7

  15. 2. All proteogenic aa except glycine have an asymmetric C*, exibiting optical activity. They exist as stereoisomers or enantiomers (L- or D-) R R l l H2N – C* – H H – C* – NH2 l l COOH COOH L-aminoacid D-aminoacid All the native aa are levorotatory as they rotate to the left the plane-polarized light; they belong to L- series D-aminoacids exist in bacterial products (cell walls), peptide antibiotics, but not incorporated in proteins via ribosomal synthesis

  16. STRUCTURE AND LEVELS OF STRUCTURAL ORGANIZATION OF PROTEINSPRIMARY STRUCTURE • The simplest level of structural organization = a linear polypeptide chain that is composed of aminoacids radicals linked through covalent peptide bonds formed between the α-amino group of one aa and the α-carboxyl group of the next aa. R1 R2 R1 R2 l l l l H2N-CH-COOH + H2N-CH-COOH → H2N-CH-CO-HN-CH-COOH -H2O peptide bond aminoacid1 aminoacid2 dipeptid • Specific characters of the peptide bond: • coplanarity (all the atoms – CO-NH- in a single plan) O R2 • two resonance forms (keto- and enol) ll l • trans position of the substituent to C-N bond - CH - C – N – CH - • ability to form H bonds ll R1 H

  17. Nomenclature for peptides: • 2 aa (aa residues or radicals) = dipeptide; • 3 aa = tripeptide; and so on examples: • carnosine = β-alanyl-histidine • anserine = β-alanyl-N-methyl-histidine • glutathione = γ-glutamyl-cysteinyl-glycine • synthesized in the erythrocytes, liver, intestinal mucosa, brain; • a systemic protectant against oxidative stress, detoxification from peroxides, cofactor for antioxidative GPx enzyme, transmembrane transport, receptor action, antitoxic • takes part in redox processes, coenzyme that donates H, activator of SH-dependent enzymes, • Polypeptides > 10 aa residues • Proteins > 40 aa residues

  18. All peptides or proteins contain: R1 R2 R3 R4R5R6 l l l l l l H2N-CH-CO-HN-CH-CO-HN-CH-CO-HN-CH-CO-HN-CH-CO-HN-CH-COOH N-terminal aa = free -NH2 C-terminal aa = free –COOH (written to the left) (written to the right) Aa are named consecutively beginning with the N-terminal aa, bearing the suffix –yl, except the C-terminal aa that has its name ended in –ine (e.g. valinyl-leucinyl-alanine)

  19. STRUCTURE AND LEVELS OF STRUCTURAL ORGANIZATION OF PROTEINSSECONDARY STRUCTURE • Refers to the way the peptide is folded into an ordered structure owing to hydrogen bonds between the peptide groups of the same or juxtaposed polypeptide chain • Classified in: • -helix • -structure

  20. SECONDARY STRUCTURE • 1. helical structure (α-helix): • helical configuration, right-handed (clockwise turns) • H bonds are formed between peptide groups within the same polypeptide chain, between the 1st and 4th aminoacid radical; there are 3.6 aminoacid residues per turn • regularity of turns along the helix length • equivalence of all aa residues (irrespective the R structure) • nonparticipation of R groups in H bonding Barker R: Organic Chemistry of Biological Compounds, Englewood Cliffs, NJ, Prentice Hall, 1971

  21. SECONDARY STRUCTURE 2. pleated sheets (β-structure): • the chains lie side by side, with the H bonds formed between the • -CO- group of one peptide bond • –NH- group of another peptide bond in the neighboring chain • the chains may run • in the same direction (parallel β-sheet) or • in opposite direction (antiparallelβ-sheet) Barker R: Organic Chemistry of Biological Compounds, Englewood Cliffs, NJ, Prentice Hall, 1971

  22. SECONDARY STRUCTURE The α-helix can be reversibly converted to β-structure due to the reorganization of the H bonds (e.g. keratin, the protein in hair) The same protein has both types of structure: • paramyosin has 95-100% α-helix, • myoglobin, hemoglobin, have high percentage of α-helix; • keratin, collagen (skin, tendon) have β-structure

  23. 3. Collagen triple helix • Constituent of skin, bones, teeth, blood vessels, tendons, cartilage, connective tissue, the most abundant protein in the human (30% of total body mass) • Contains 33% Gly, 21% Lys-OH or Pro-OH, almost absent Cys • The tropocollagen structure (the repetitive unit) is formed of 3 protein strands that wrap around each other forming a left-handed superhelix, held together by hydrogen bonds formed by the –OH in the Lys-OH or Pro-OH • 10 different types: I in tendons and bones, II in hyaline cartilage, III in connective tissue, IV in basement membranes, VI in placenta SECONDARY STRUCTURE

  24. STRUCTURE AND LEVELS OF STRUCTURAL ORGANIZATION OF PROTEINSTERTIARY STRUCTURE • Is referred to as a specific mode of spatial arrangement of the polypeptide chain: globular (ellipsoidal shape) and fibrous species (elongated) • Due to the associations between segments of α-helix and β-structure, representing a state of lowest energy and greatest stability • Bonds formed between the sidechain radicals of aminoacids stabilize the structure: • Strong bonds: • Covalent • Disulfide (-S-S-) • Isopeptide (peptide-like, -CO-NH-) • Ester (-CO-O-) • Weak bonds: • Polar bonds: • Hydrogen bonds • Ionic or electrostatic • Nonpolar bonds (van der Waals) Barker R: Organic Chemistry of Biological Compounds, Englewood Cliffs, NJ, Prentice Hall, 1971

  25. TERTIARY STRUCTURE • Specific features: • The conformation is determined by the properties of the sidechain radicals and medium • The molecule tends to adopt an energetically favorable configuration corresponding to the minimum of free energy • The nonpolar R form an interior region with hydrophobic radicals • The polar, hydrophylic R extend outside, oriented to the water molecules • There are regions formed as α-helix or β-structure and random coils • The tertiary structure determines the protein activity

  26. STRUCTURE AND LEVELS OF STRUCTURAL ORGANIZATION OF PROTEINS QUATERNARY STRUCTURE • Represents the aggregation of 2 or more polypeptide chains (protomers or subunits) with tertiary structure, organized into a single functional protein molecule, named oligomer. • Configuration of their tertiary structure, globular or fibrous. • Contacts between the subunits are possible through the polar groups in R, as the nonpolar aminoacids radicals are oriented to the interior • Bonds: • Weak: • ionic bonds • hydrogen bonds • Covalent: • disulfide

  27. Examples: • hemoglobin (Hb) the blood pigment is a tetramer, constituted of 4 protomers: 2 identical α-chains, 2 identical β-chains. The four protomers form 2 subunits (αβ). The association can be represented: 2 α + 2 β →α2β2 → 2 αβ • allosteric enzymes –phosphorylase a is a dimer, formed of 2 identical subunits (that separately are inactive)

  28. PHYSICAL AND CHEMICAL PROPERTIES OF PROTEINS 1. Amphoteric – as they combine acidic and basic properties • due to the acid-base groups of the side-chain radicals of the protein constituting aminoacids. • the majority of the polar groups are located on the surface of globular proteins, providing the acid-base properties and the charge of the protein molecules • Acidic aminoacids (glutamic, aspartic, aminocitric) → acidic properties • Basic aminoacids (lysine, arginine, histidine) → basic properties Buffering properties – the proteins containing a large amount of histidine radicals, because its side-chain exibit buffering properties within a pH range close to the physiological pH, for example hemoglobin (8% histidine)

  29. 2. Colloidal and osmotic properties: aqueous solution of proteins are stable and equilibrated (do not precipitate), homogeneous. • Properties of colloidal solutions: • Characteristic optical properties • Low diffusion rate • Inability to pass across semipermeable membranes • High viscosity • Property of gelation 3. Hydration of proteins and factors affecting solubility • Proteins are hydrophilic • Factors affecting solubility • The charge on protein molecules (the higher the number of polar aminoacids – the greater the amount of water bound) • Neutral salts in small concentrations enhance the solubility • The medium pH values • Temperature influences differently, depending on the specific protein

  30. Salting-out is the selective precipitation of a protein by a neutral salt solution, used for separation and purification of proteins; after removing the salting-out agent, the protein retains its native properties and functions unchanged. • Denaturation and renaturation: agents destroy the higher levels of structural organization of protein molecules (secondary, tertiary, quaternary) by the breakdown of bonds that stabilize them and retention of the primary structure; as the result, the protein loses its native physical-chemical and biological properties = denaturation; the protein separates from solution as a precipitate.

  31. Factors producing denaturation: • Physical: temperature, pressure, mechanical action, ultrasonic, ionizing irradiation • Chemical: acids, alkali, organic solvents, detergents, certain amides (urea, guanidine)alkaloids, heavy metal salts (Hg, Cu, Ba, Zn, Cd) Properties of denaturated proteins: • An increased number of reactive and functional groups – the unfolding of polypeptide chain • Reduced solubility, increased ability to precipitate • Alteration of configuration • Loss of biological activity • A facilitated cleavage by proteolytic enzymes Denaturation is used to deproteinize a mixture of protein-containing materials. Removing the proteins one can obtain a protein-free solution Denaturation was thought to be irreversible; in certain conditions the protein restores its biological activity = renaturation

  32. CLASSIFICATION AND NOMENCLATURE OF PROTEINS • Physical-chemical classification: • Electrochemical properties: • Acidic (polyanionic proteins) • Basic (polycationic proteins) • Neutral • Polar properties • Polar / hydrophylic • Nonpolar / hydrophobic • Amphipathic / amphyphylic • Functional classification – biological functions • Structural classification • Simple/unconjugated/apoproteins = polypeptide chain • Conjugated/proteids = polypeptide chain + nonprotein moiety (glycoproteins, lipoproteins, phosphoproteins, nucleoproteins, metalproteins, cofactor-proteins)

  33. SIMPLE PROTEINS • Histones • form reversible complexes with DNA = chromatin; histone-like proteins exist in ribosomes • stabilize the spatial structure of DNA and chromosomes • interrupt the genetic information transfer from DNA to RNA • Protamines • the most low-molecular proteins, basic, bound to DNA in the chromatin of spermatozoa • Prolamines • plant proteins in grain gluten of cereals: gliadin of wheat, avenin from oats, zein from corn • nonpolar aminoacids and proline - insoluble in water, salt solution, acid and bases • Glutelins • plant proteins, high content of arginine, low content of proline - insoluble in water, salt solution, ethanol; soluble in diluted alkali, • Scleroproteins • bones, cartillage, ligaments, tendons, nails, hair • fibrous protein soluble in special solvents

  34. 6. Albumins and globulins are heterogeneous groups of proteins contained in the blood plasma, in cells and biological fluids, with highly diversified functions. Albumins: • Relatively small molecular mass (15,000-17,000 Daltons) • Possess a negative charge • Acidic properties (isoelectric point 4.7); high content of glutamic acid • Strongly hydrated – are precipitated only at high concentrations of water-absorbing agents • High absorbtive capacity for both polar and nonpolar molecules (transport agents) Globulins: • Higher molecular mass (>100,000) • Insoluble in pure water, soluble in dilute salt solutions • Weakly acidic or neutral (isoelectric point 6-7.3) • Weakly hydrated – are precipitated in low-concentrated solutions of ammonium sulfate • Some of them specifically bind various materials (specific transport agents) others nonspecifically bind lipid-soluble materials

  35. Can be separated by electrophoresis because they have different mobility under an applied electric field. • Albumins are polyanionic proteins and move to the anode faster than globulins • Globulins are divided into 3 major fractions α (α1, α2), β (β1, β2), γ

  36. CONJUGATED PROTEINS Heteromacromolecules = macromolecular complexes composed of 2 components of different chemical classes. Conjugated proteins (protein-nonprotein complexes) • Nucleoproteins = proteins + nucleic acids • DNA-protein complexes (DNA + histones/nonhistones) = deoxyribonucleoproteins (DNP) in the chromosomes • RNA-protein = ribonucleoproteins (RNP), in the cell

  37. 2. Glycoproteins = proteins (80-90%) + heteropolyglucide • Have higher thermal stability • Difficult to be digested by proteolytic enzymes (pepsin, trypsin) • Exist in the blood, cell membrane (with the carbohydrate residue always located on the external surface), inside the cell • Biological functions: • Transport of hydrophobic materials and ions (ceruloplasmin, transferrin, haptoglobin,transcortin) • Blood coagulation (prothrombin, fibrinogen) • Immunity (immunoglobulins) • Enzymes (cholinesterase) • Hormones (corticotropin, gonadotropins) • Specificity of intercellular contacts - on the membrane surface – act as recognition and binding sites (receptors) for the substances to be taken up by the cell • Blood-group specificity - on the surface of the erythrocytes, are antigens that determine whether an individual has type A (N-acetyl galactosamine), B (D-galactose), AB (both) or O (absence of both) blood • Mucus secreted by the epithelial cells lubricates and protects the tissues lined by these cells

  38. Lipoproteins= lipids (triglycerides, cholesterol, cholesterides, phospholipids) + proteins (apolipoproteins) • The high content in lipid determines the higher molecular mass and lower density • The apolipoprotein differ in structure and composition: A1, A2, A3, B, C1, C2, C3, D, E • Micelles-like structure: • hydrophobic core of nonpolar lipids (triacylglycerides, cholesterol esters) • hydrophylic envelope of polar lipids (cholesterol, phospholipids) and proteins • By ultracentrifugation (or electrophoresis) the lipoproteins are separated in: Proteins Major lipid Function Chylomicrons, 2% TG transport exogeneous TG Very Low Density Lipoproteins(VLDL)/ pre-β-lipoproteins 5-10% TG transport TG livertissues Intermediate Density Lipoproteins (IDL) 15-20% TG, C, PL Low Density Lipoproteins (LDL) / β-lipoproteins 20-25% C transport C livertissues High Density Lipoproteins (HDL)/ α-lipoproteins 45% PL, C transport C tissues  liver • Functions: • Structural – biological membranes providing the physiological function of cells, nerves and transport of materials • Plasmatic - transport of lipids supplied by the intestinal absorption and their distribution among lipid-synthesizing and lipid-consuming tissues and transport of fat-soluble vitamins, acyclic alcohols, β-carotene

  39. Phosphoproteins–contain a phosphate residue esterifying the –OH of serine; example: casein in milk • Cofactor-proteins= protein + a nonprotein moiety. The colored cofactor-proteins are chromoproteins: • Hemoproteins (contain heme) • Chorophyllo proteins (chlorophyll) • Cobamine proteins (vitamin B12) • Retinal proteins (vitamin A – aldehyde) • Flavoproteins (flavins) Hemeproteinsare classified in: • Nonenzymic: hemoglobin, myoglobin • Enzymic: cytochromes, catalases, peroxidases The prosthetic group (non-protein component) = heme a metalloporphyrin complex

  40. Hemoglobin Globular protein in the erythrocytes with molecular mass of 66,000-68,000 Structure: primary structure: • protein moiety (globin) + prosthetic group (heme) 1.The globin is an oligomer formed of 4 polypeptide chains in 2 subunits: • 2 α chains containing 141 aminoacid radicals and • 2 β chains containing 146 aminoacid radicals • 2 α + 2 β →α2β2 → 2 αβ

  41. The secondary structure – 8 α-helical segments (lettered A-H); • the polar (hydrophylic) residues tend to be on the outside of the molecule, • almost all the nonpolar (hydrophobic) residues are on the inside of the molecule The tertiary structure: inside each subunit there is a hydrophobic pocket in which one heme is held due to van der Waals bonds and ionic bonds

  42. 2. The heme is a heterocyclic molecule composed of • protoporphyrin group = tetrapyrrole = four pyrrole groups linked by methene bridges (–CH= ); protoporphyrin IX possesses substituents: • methyl groups (-CH3) at positions 1, 3, 5, 8 • vinyl (-CH=CH2) at positions 2, 4 • propionyl groups (-CH2-CH2-COOH) at positions 6, 7 • Fe2+ 2+

  43. Fe2+ isbound in the center with 6 bonds: • 4 bonds with the N of the tetrapyrole ring, • 1 bond with the proximal hystidine in the F8 segment of the globin and • 1 coordination bond free for binding oxygen, on the other side of the heme plane; close to bond 6 there is a distal hystidine that influences the interaction of heme with other ligands

  44. Hemoglobin - Types Normal: • Primitive (embryonal): Hb P (Gower 1, Gower 2) (disappears in 3 month) • Fetal: Hb F α2γ2 = 70% of the Hb at birth moment • Adult: Hb A α2β2 Hb A2α2δ2 Hb A3 structurally changed β-chain, in old RBCs In the adult blood: 95-96% HbA, 2-3% HbA2, 0.1-2% HbF HbA2 and Hb F have a higher affinity for oxygen Abnormal Hb: • Hb H β4, • Bart’s Hb γ4, • Hb S (Sickle-cell Hb) glutamic acid in position 6 of β-chain is changed with valine

  45. Hemoglobin - Functions • Binds the oxygen and transfer it from the lungs to the tissues Hb + 4 O2 Hb(O2) 4 + 4 H2O deoxyhemoglobin oxyhemoglobin T-form (tense) R-form (relaxed) The process is dependent on pO2, pH, [CO2], [2,3-bisphosphoglycerate] • The first oxygen molecule becomes bound to the heme iron of α-chain which is pulled into the porphyrin ring plane which results in the displacement of proximal His. This detemines the rearrangement of the bonds with the other aminoacid radical in the same subunit and a rupture of some of the ionic bonds between the chains. • This facilitates the access of the second molecule of oxygen to the heme iron of the α-chain. The addition of the second molecule of oxygen ruptures other ionic bonds between the subunits. • The 3rd and 4th molecules of oxygen break the remaining ionic bonds. Thus the quaternary structure is changed from T-form (tense) to R-form (relaxed). The T-form Hb affinity for the oxygen is 300 time lower than of the R-form. • The gradual increase of the Hb affinity for the oxygen has a sigmoid shape to the oxygen binding curve and demonstrates the cooperative behavior of hemes

  46. Hemoglobin - Derivatives • Carbhemoglobin: in interaction with CO2, this is added to the globin -NH2 Hb-NH2 + CO2 Hb-NH-COO- + H+ carbhemoglobin • Carboxyhemoglobin (Hb-CO): Hb has an affinity 25,000 times greater for CO than for CO2; it cannot transfer O2 • Methemoglobin: is formed by the action of oxidants (nitrites, peroxides, ferricyanides, quinones); the Fe3+ can bind neither O2, nor CO2 inducing anoxia • Sulfhemoglobin: formed by irreversible reaction with hydrogen sulphide, sulfonamides, aromatic amines • Chlorhemine: Teichman crystals with species specific microscopic appearance (used in forensic laboratory)

  47. Myoglobin • 1 single-chain globin + 1 heme • The affinity for oxygen is 5 fold higher than the one of Hb • The curve of saturation with oxygen is a hyperbola

  48. Heme-enzymes • Cytochromes a, b, c, d • Cannot bind oxygen except Cyt a3 • Transfer of electrons as part in the respiratory chain of mitochondria -e- Cyt (Fe2+) Cyt (Fe3+) +e- • Catalases and peroxydases • Take part in the decomposition of hydrogen peroxide catalase H2O2 + H2O2 O2 + 2 H2O peroxydase S-H2 + H2O2 S + 2 H2O