Medical Biochemistry Molecular Principles of Structural Organization of Cells

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Medical BiochemistryMolecular Principles of
Structural Organization of Cells

• 4. PROTEINS

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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 gt200 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 gt40 aa

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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

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AMINOACIDS STRUCTURAL MONOMERS OF PROTEINS

• Aminoacids contain at least 1 NH2 group and 1
  COOH group.
• L-aminoacids are classified in a-, ß-, ?-
  depending on the position of C bearing NH2 group
  with respect to COOH. There are gt200 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 a-aminoacids

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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)

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• 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

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• 2. CYCLIC AMINOACIDS
• 2.1. Aromatic aa
• Phenylalanine Tyrosine
• (Phe) (Tyr)
• 2.2. Heterocyclic aa
• Histidine Tryptophan Proline Hydroxyproline
• (His) (Trp) (Pro) (Pro-OH)
• Rare aminoacids Aminocitric acid, Lys-OH,
  Pro-OH

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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)

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• 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

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• 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, acethylcolinesteras
  e

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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

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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

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NONPROTEOGENIC AMINOACIDS

• ornithine and citrulline intermediates in urea
  cycle, synthesis of arginine
• ?-aminobutyric acid (GABA) free in the brain,
  lungs, heart neurotransmitter
• ß-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 ?-aminobutyric
  acid ß-alanine DOPA
• GABA

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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

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• 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

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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 a-amino group of
  one aa and the a-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
  l l
• R1 H

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• Nomenclature for peptides
• 2 aa (aa residues or radicals) dipeptide
• 3 aa tripeptide and so on
• examples
• carnosine ß-alanyl-histidine
• anserine ß-alanyl-N-methyl-histidine
• glutathione ?-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 gt 10 aa residues
• Proteins gt 40 aa residues

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• All peptides or proteins contain
• R1 R2 R3
  R4 R5
  R6
• 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)

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STRUCTURE AND LEVELS OF STRUCTURAL ORGANIZATION
OF PROTEINS SECONDARY 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

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SECONDARY STRUCTURE

• 1. helical structure (a-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
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SECONDARY STRUCTURE

• 2. pleated sheets (ß-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 ß-sheet) or
• in opposite direction (antiparallel ß-sheet)

Barker R Organic Chemistry of Biological
Compounds, Englewood Cliffs, NJ, Prentice Hall,
1971
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SECONDARY STRUCTURE

• The a-helix can be reversibly converted to
  ß-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 a-helix,
• myoglobin, hemoglobin, have high percentage of
  a-helix
• keratin, collagen (skin, tendon) have ß-structure

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• SECONDARY STRUCTURE

• 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

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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
  a-helix and ß-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
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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 a-helix or
  ß-structure and random coils
• The tertiary structure determines the protein
  activity

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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

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• Examples
• hemoglobin (Hb) the blood pigment is a tetramer,
  constituted of 4 protomers 2 identical a-chains,
  2 identical ß-chains. The four protomers form 2
  subunits (aß). The association can be
  represented
• 2 a 2 ß ? a2ß2 ? 2 aß
• allosteric enzymes phosphorylase a is a dimer,
  formed of 2 identical subunits (that separately
  are inactive)

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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)

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• 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

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• 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.

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• 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

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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)

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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

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• 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 (gt100,000)
• 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

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• 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 a
  (a1, a2), ß (ß1, ß2), ?

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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

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• 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,transcort
  in)
• 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

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• 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-ß-lipoproteins 5-10 TG transport TG
  liver?tissues
• Intermediate Density Lipoproteins (IDL)
  15-20 TG, C, PL
• Low Density Lipoproteins (LDL) /
  ß-lipoproteins 20-25 C transport C liver?tissues
• High Density Lipoproteins (HDL)/
  a-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, ß-carotene

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• 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)
• Hemeproteins are classified in
• Nonenzymic hemoglobin, myoglobin
• Enzymic cytochromes, catalases, peroxidases
• The prosthetic group (non-protein component)
  heme a metalloporphyrin complex

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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 a chains containing
• 141 aminoacid radicals and
• 2 ß chains containing
• 146 aminoacid radicals
• 2 a 2 ß ? a2ß2 ? 2 aß

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• The secondary structure 8 a-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
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• 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 (-CHCH2) at positions 2, 4
• propionyl groups (-CH2-CH2-COOH) at positions 6,
  7
• Fe2

2
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• Fe2 is bound 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

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Hemoglobin - Types

• Normal
• Primitive (embryonal) Hb P (Gower 1, Gower 2)
• 
  (disappears in 3 month)
• Fetal Hb F a2?2 70 of the Hb at birth
  moment
• Adult Hb A a2ß2
• Hb A2 a2d2
• Hb A3 structurally changed ß-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 ß4,
• Barts Hb ?4,
• Hb S (Sickle-cell Hb) glutamic acid in position 6
  of ß-chain is changed with valine

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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 a-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
  a-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