Amino Acids & Proteins 2024 PDF

Proteins & Amino acids 2024 Proteins and amino acids - Proteins are large polymer of L--amino  acids - Proteins are the most abundant and functionally molecules in living systems. Main functions of proteins: 1- Structural functions: Collagen...

Proteins & Amino acids 2024 Proteins and amino acids - Proteins are large polymer of L--amino  acids - Proteins are the most abundant and functionally molecules in living systems. Main functions of proteins: 1- Structural functions: Collagen enters in the structure of bones and elastin in lungs. 2- Enzymes: Pepsin, lipase, urease are proteins that accelerate the chemical reactions. 3- Hormones: Growth hormone, insulin, glucagon and oxytocin are protein in nature. 4- Transporters: Lipoproteins transport lipids in plasma, hemoglobin transports oxygen. 5- Blood clotting factors: Fibrinogen (factor I), prothrombin (factor II). 6-Receptors: Insulin receptor, estrogen receptor. 7- Defence molecules: Immunoglobulins (IgM, IgA, IgG, IgE and IgD). 8-Storage as myoglobin stores oxygen and ferritin which is storage form of iron. 9- Contractile proteins e.g. actin and tubulin. - Nitrogen balance: Nitrogen intake = nitrogen loss -Each 100 grams of protein contain about 16 grams of nitrogen. - Negative nitrogen balance: Nitrogen intake < Nitrogen loss e.g. fasting, starvation, DM, diarrhea, fever, burns - Positive nitrogen balance: Nitrogen intake > nitrogen loss e.g. growing children, pregnancy, body building - Although more than 300 different amino acids have been discovered in nature, only 20 (standard amino acids) are commonly found in mammalian proteins. Note: These 20 amino acids are the only amino acids that are coded for by DNA. 1  Proteins & Amino acids 2024 -All of these 20 amino acids are (L-α-amino acids), the amino group is attached to the second carbon(next to the carboxyl group), and the amino group is on the left side Structure of the amino acids: Each amino acid has α-carbon atom attached to: 1- a carboxyl group (COOH) 2- a primary amino group (NH₂) 3- a distinctive side chain (R) 4- a hydrogen atom Note: proline has a secondary amino group (NH) also called imino group Classification of amino acids: Amino acids are classified according to chemical structure, nutritional importance and metabolic fate. I. Chemical classification: 1- Amino acids with non-polar side chains: These amino acids have non-polar side chains (9 amino acids). Thus these amino acids are called hydrophobic 2-Amino acids with uncharged polar side chains: These amino acids have zero net charge at neutral pH. These are 6 amino acids: 2  Proteins & Amino acids 2024 1- Serine and threonine have alcoholic OH, while tyrosine has phenolic OH. 2- Aspargine and glutamine have amide groups, which can form hydrogen bond in protein. 3-Cysteine contains a thiol (sulfhydryl) group (–SH), which is an important component the active site of many enzymes. - In proteins, the –SH groups of two cysteines can be oxidized to form (cystine), which contains a covalent cross-link called a disulfide bond (–S– S–). 3-Amino acids with charged polar side chains: These amino acids are subclassified to: a) Acidic amino acids: 2 amino acids have an additional carboxyl group. - At physiologic pH, the side chains of these amino acids are ionized, forming negative charged carboxylate group (COO¯). They are, therefore, called aspartate or glutamate. b) Basic amino acids: These amino acids contain an additional amino group. - At physiologic pH the side chains of lysine and arginine are ionized forming positive charge. In contrast, histidine is weakly basic. 3  Proteins & Amino acids 2024 Summary of the chemical classification of amino acids: Non-polar a.as Glycine, Alanine, Valine, Leucine, Isoleucine, Phenylalanine, Tryptophan, Methionine, and Proline. Polar a.as Serine, Threonine, Tyrosine, Cysteine, Aspargine, Glutamine, Aspartic acid, Glutamic acid, Lysine, Arginine and Histidine Serine, Threonine, Tyrosine, Cysteine, Polar neutral a.as Aspargine and Glutamine Aspartic acid, Glutamic acid, Lysine, Arginine and Histidine Polar charged a.as Aspartic acid, Glutamic acid Lysine, Arginine and Histidine Acidic a.as Phenylalanine, Tryptophan, Tyrosine Serine, Threonine Basic a.as Tyrosine Aromatic a.as Cysteine, Methionine Alcoholic OH a.as Phenolic OH a.as Aspargine, Glutamine Sulfur containing a.as Proline (contains pyrrolidine ring) Amide containing Arginine a.as Tryptophan Imino acid Guanidine Histidine containing a.a Indole containing Cysteine a.a Imidazole Valine, Leucine and Isoleucine containing a.a Proline, Tryptophan and Histidine Thiol containing a.a Branched a.as Heterocyclic a.as 4  Proteins & Amino acids 2024 II. Nutritional classification: Amino acids can be classified into 3 main groups: 1-Essential amino acids (8) cannot be synthesized in the human body. 2-Semi-essential amino acids (2) can be synthesized in amount enough for adults, but not enough for the growing children. 3-Non-essential amino acids (10) can be synthesized in the human body in amount enough for adults and growing children. They include the rest of amino acids. III- Metabolic classification: Amino acids may be classified according to their metabolic fate into: 1- Ketogenic amino acid (only one): Leucine is only one pure ketogenic amino acid because it only gives ketone bodies. 2-Ketogenic and glucogenic (glycogenic) amino acids (5 amino acids): - These amino acids give both ketone bodies and glucose (glycogen). - These amino acids include: - phenylalanine - tyrosine - tryptophan - lysine - isoleucine 3-Glucogenic (glycogenic) amino acids: These give glucose in the human body. They include the rest of amino acids. Amino acid Abbreviati SymbAmino acidAbbreviatioSymbol 5  Proteins & Amino acids 2024 on ol n Glycine Gly G Threonine Thr T Alanine Ala A Tryptophan Trp W Valine Val V Tyrosine Tyr Y Leucine Leu L Aspargine Asn N Isoleucine Ile I Glutamine Gln Q Serine Ser S Aspartic Asp D Methionin Met M acid Glu E e Cys C Glutamic Lys K Cysteine His H acid Arg R Histidine Pro P Lysine Phe F Proline Arginine Phenylalani ne Non-standard amino acids: These are amino acids which are found in the human body. 1)Some D-amino acids are present in human body, but not enter in the structure of proteins e.g D-serine is present in forebrain and D- aspartate is present in brain and peripheral tissues. 2) Some L-amino acid derivatives e.g. hydroxy lysine, hydroxy proline, cystine and GABA 3) L-ornithin and L-citrullin and L-arginosuccinic acid (in urea cycle). Note: 1-(Homo+name) means one carbon more. 2-(Nor+name) means one carbon less. Functions of amino acids: 1-Building blocks: amino acids enter in the structure of: a) Body proteins e.g plasma proteins (albumin & globulin), and tissue proteins (collagen, keratin & elastin). b) Enzymes e.g pepsin and amylase are proteins. c) Hormones e.g growth hormone and prolactin are proteins. 6  Proteins & Amino acids 2024 2-Amines: Some amino acids are converted to important amines by decarboxylation e.g. histamine is vasodilator 3-Neurotransmitters: e.g glycine, glutamate, serotonin which is derived from tryptophan. 4-Synthesis of heme: glycine → porphyrins → Heme 5- Synthesis of nucleotides for DNA and RNA: Glycine, glutamine and aspartate. 6-Production of NO gas (hormone): Arginine 7-Health and growth: Essential amino acids are needed for normal growth in infants and maintain health in adults. Optical properties of amino acids: - All amino acids (except glycine) are optically active compounds, because they contain at least one asymmetric (chiral) carbon atom. -Two amino acids contain two asymmetric carbon atoms e.g. isoleucine and threonine - Amino acids can exist in two forms (D and L) that are not superimposable mirror images of each other (called enantiomers). - All amino acids found in proteins are of the L-configuration. - Absorption of spectrum: - All proteins absorb high-wavelength (280 nm) ultraviolet light due to the presence of tryptophan. Amphoteric properties of amino acids: 1-Amino acid acts as an acid (proton donor) in alkaline medium (COOH → COO⁻ + H⁺). 2-Amino acid acts as a base (proton acceptor) in acidic medium (NH₂ + H⁺ → NH₃⁺). - Zwitter ion are molecules that contain equal numbers of opposite charges (+ & ─ ). - Amino acid molecule carries 2 opposite charges (+ & –). Isoelectric point (pI) is the pH at which an amino acid is electrically neutral (net charge = 0). For an amino acid such as alanine that has only two dissociating groups. The first pKa₁ (R-COOH) is 2.35 and the 7  Proteins & Amino acids 2024 second pKa₂ (R-NH₃⁺) is 9.69. The isoelectric pH (pI) of alanine thus is: -Important features about isoelectric point of amino acids: 1- Amino acids will carry no net charge 2- No mobility in electrical field 3- Solubility will be minimum 4- Precipitation will be maximum 5- Buffering capacity and viscosity will be minimum. Reactions of amino acids: 1-Transamination reactions: All the protein amino acids except lysine, threonine, proline, and hydroxy proline undergo transamination reactions. Transamination is reversible reaction. 2-Deamination reactions: a) Oxidative deamination by removal of hydrogen and removal of amine as (NH₃). i- By L-glutamate dehydrogenase: ii- By L-amino acid oxidase which acts only on L-amino acids: 8  Proteins & Amino acids 2024 iii- By D-amino acid oxidase which acts only on D-amino acids: b) Non-oxidative deamination: removal of amine group from glutamine and aspargine. 3-Decaroxylation: Removal of (CO₂) from amino acid produces biologically active amine e.g 4-Reaction with ninhydrin involves decarboxylation and deamination: a) All amino acids (except proline) reacts with ninhydrin giving a blue color. b) Proline reacts with ninhydrin giving yellow color. 5- Buiret's reaction: (for all proteins and peptides) Protein + NaOH + CuSO₄ → violet color (complex) 9  Proteins & Amino acids 2024 6-Reactions due to (R) side chains: a) Millon´s reaction: (for phenolic amino acid) Tyrosine + Hg²⁺ (salt) → red color b) Xanthoproteic reaction: (For aromatic amino acids) Phenyl alanine and tyrosine + HNO₃ (conc.) → orange color c) Rosenheim test: (for indole) Tryptophan + formaldehyde concentrated sulfuric acid → A purple ring appears between the two layers. Peptides: Peptides are compounds formed of (2 to 49) amino acids linked together by peptide bonds. Peptides can be classified to: 1-Dipeptides: consist of 2 amino acids and one peptide bond 2-Oligopeptides: consist of (3 to 10) amino acids e.g. glutathion is a tripeptide. 3-Polypeptides: consist of (11 to 49) amino acids e.g. glucagon contains 29 a.as Peptide bonds are covalent bonds (amide linkages) between the α-carboxyl group of one amino acid and the α-amino group of another. Functions of peptides: A-Hormones: e.g. 1)Insulin and glucagon are secreted form pancreas. 2) Vasopressin and oxytocin (each contains 9 a.as) from posterior pituitary gland. 3) β- Lipotropin (93 a.as.) from anterior pituitary gland which is a precursor of β-endorphin. - β-Endorphin (31 a.as) has analgesic effect powerful 18-30 times than morphine. 4) Atrial natriuretic factor (28 a.as.): (ANF) is produced from the right atrium of heart. - ANF increases the excretion of Na⁺ in urine leading to vasodilation and hypotension. 10  Proteins & Amino acids 2024 5) Bradykinin causes relaxation of vascular smooth muscles leading to vasodilation and hypotension. Example of some peptides: A- Penicillin is a dipeptide consists of cysteine and D-valine residues. - It is a cyclic peptide. - It is an antibiotic used clinically for killing bacteria and stopping their growth. B- Glutathione (G-SH): 1) G-SH is a tripeptide present in the cell membranes 2) It is consists of 3 amino acids: glutamate, cysteine and glycine 3) The functional group of glutathione is (–SH) thiol of cysteine. 4) Glutathion has two forms (reduced form G-SH and oxidized form G-S-S-G). Functions of glutathione: 1) removes the toxicity of hydrogen peroxide H₂O₂ which causes cell damage especially RBCs leading to hemolytic anemia. 2) Transports of amino acids across intestinal cell membranes. 3) Inactivation of insulin hormone after the insulin performs its action. 4) Removes the xenobiotics (foreign compounds) 5) Maintains the SH groups of enzymes in the reduced form. 6) Serves as a coenzyme for certain enzymes e.g. prostaglandin PGE2 synthetase Proteins: -Proteins are macromolecules formed of 50 amino acids or more linked together by peptide bonds. -Proteins are natural substances with high molecular weights ranging from 5,000 to many millions. - Besides carbon, hydrogen and oxygen, they also contain 11  Proteins & Amino acids 2024 nitrogen. - Many proteins contain sulfur and Phosphorous. Structure of Proteins: -Each protein in its native state has 3 dimensional structure called the conformation of protein. -This conformation is essential for the function. - Any change in protein conformation leading to specific disease. - The protein molecule is termed by four organizational levels named primary, secondary, tertiary, and quaternary structure. A) Primary Structure of Proteins is the sequence (arrangement) of amino acids in a protein chain. - The bond responsible for the primary structure is the peptide bond. -Each polypeptide chain starts on the left side by free amino group (called N-terminal) and ends on the right side by free carboxyl group (called C-terminal). - The remaining amino acids in the chain are termed amino acid residues. - Primary structure of proteins can be determined by: 1-Sanger’s method by using Sanger’s reagent FDNB (1-Fluoro-2,4- DiNitroBenzene). 2- Edmans method by using Edman reagent (Phenyl isocyanate). - Pronase is a mixture of non-specific proteolytic enzymes that causes complete hydrolysis of proteins during the determination of the primary structure. B) Secondary structure: It is a spatial relationship of adjacent amino acids residues, and the bond responsible is hydrogen bond. -Secondary structure can be determined by: 1- Circular dichroism (CD) spectroscopy 2- Fourier transform infrared spectroscopy (FTIR) spectroscopy I- Regular forms of secondary structure: 12  Proteins & Amino acids 2024 A. The α-helix: The alpha-helix is generally right handed. It is a rod like structure with the peptide bonds coiled tightly inside and the side chains (R) extending outward from the chain. Each turn of α- helix contains 3.6 amino acids. Proline and hydroxy proline will not allow the formation of alpha-helix. B. The β-sheet structure is formed between 2 or more separated polypeptide chains. - It may be formed between segments of the same polypeptide chain. Two types of β-sheet are: 1-Parallel β-sheets in which the polypeptide chains run in the same direction. 2- Antiparallel β-sheets in which the polypeptide chains run in opposite direction. 13  Proteins & Amino acids 2024 II-Other forms of secondary structure: a) Loop regions: - They are rich in charged amino acids. - These regions like hairpin loop connecting the antiparallel β- sheets. b) β-Bends (reverse turns, β-turns): β-Bends help in the formation of a compact, globular shape on the surface of protein molecules, and often include charged residues. c) Disordered regions: with low flexibility III-Supersecondary structures (motifs): Globular proteins are constructed by combining secondary structural elements (α-helices, β-sheets) e.g β-α-β, Greek key and β-meander C) Tertiary structure: This is the final arrangement of domains in the polypeptide chain Domains are the fundamental functional and three-dimensional structural units of polypeptides. -Tertiary structure can be determined by: NMR spectroscopy and X- ray crystallography. - There are 2 forms of tertiary structure: 1-Globular proteins are compact forms and result from the folding of polypeptide chain e.g myoglobin, albumin and globulin 14  Proteins & Amino acids 2024 2-Fibrous proteins are extended forms of proteins e.g keratin, collagen and myosin Bonds responsible for tertiary structure are: 1- Hydrogen bonds within chain or between chains 2- Hydrophobic interactions between non-polar side chains 3- Disulfide bonds between cysteine residues 4- Ionic interactions between the carboxyl group (-COO¯) of aspartate or glutamate and positively charged groups, such as the amino group (-NH₃⁺) in the side chain of lysine. D) Quaternary structure: This structure is only found in proteins that consist of two or more polypeptide chains (may be structurally identical or different). Each polypeptide chain is called subunit. The protein will lose its function when the subunits are dissociated. - The forces that keep the quaternary structure are hydrogen bonds, electrostatic bonds, hydrophobic bonds and van der Waals forces. - For example, 2 alpha-chains and 2 beta-chains form the Hemoglobin (tetramer). Similarly, 2 heavy chains and 2 light chains form one molecule of immunoglobulin G. Creatine kinase (CK) is a dimer. Lactate dehydrogenase (LDH) is a tetramer. Role of chaperones in protein folding: -Chaperone is a specialized group of proteins, which are required for the proper folding of many other of proteins. - The chaperones also known as “heat shock” proteins. - Chaperones prevent the faulty folding of specific other proteins. 15  Proteins & Amino acids 2024 Properties of proteins: 1-Solubility: - Some proteins are water soluble because the surface of these proteins contain amino acids with polar side chains e.g albumin. - Some proteins are water insoluble because the surfaces of these proteins contain amino acids with non-polar side chains e.g keratin and collagen. - Some proteins are soluble in alcohol e.g protamines. Salting in and salting out: - Salting in means increase the solubility of proteins by adding small amount of salt as (NH₄)₂SO₄. Some proteins are soluble in water and salt solutions e.g. albumin, while other are weakly soluble in water but are soluble in salt solutions e.g. globulin. - Salting out means decrease the solubility (increase precipitation by increasing ionic strength) of proteins by adding large amount of salt as (NH₄)₂SO₄. -Salting out is used to isolate and purify large number of proteins (e.g. plasma). Example: Globulin is precipitated in half saturated (NH₄)₂SO₄ solution, while albumin is precipitated in full saturated (NH₄)₂SO₄ solution. 2-Amphoteric properties: - All proteins contain a free amino group (N-terminus) and a free carboxyl group (C- terminus). -Protein molecules also contain additional free carboxylic groups and free amino groups from the side chains of acidic and basic amino acids. - Protein molecules are positively charged in acidic medium and negatively charged in basic medium. Isoelectric point of proteins: -It is the pH value at which the protein molecule carries equal numbers of positive and negative charges. -At this pH the protein is precipitated and doesn´t migrate in the electrophorsis. - Flocculation: lt is a reversible process of protein precipitation at isoelectric pH. Casein (milk protein) can be easily precipitated when adjusted to isoelectric pH (4.6). 16  Proteins & Amino acids 2024 3-Hydrolysis of proteins: Proteins are hydrolyzed by heating with strong acids e.g. HCl, H₂SO₄. The products are amino acids. Denaturation of proteins: -Protein denaturation results in the unfolding and loss of the secondary and tertiary structures of protein. It is not accompanied by hydrolysis of peptide bonds (The primary structure is not affected). Denaturing agents include: 1- Heat 2- Organic solvents e.g. ethanol and Dimethyl Sulfoxide (DMSO) 3- Strong acids or bases (extreme low or high pH) 4-High concentration of salts 5- Urea and guanidine hydrochloride - Denaturation of protein can be reversed e.g. salt induced denaturation, and can be irreversible e.g. heat induced denaturation. - Effects of protein denaturation: Chemical changes: Breakdown of: a) H- bond b) S-S bond c) ionic interactions - Important features about denaturation of proteins: 1-All protein levels are lost except primary structure 2-The biological activity (function) is lost 3-Denaturated proteins are insoluble and easily precipitated 4-Denaturation may be reversible Classification of proteins: Proteins are divided into 3 classes: 1-Simple proteins: These proteins on hydrolysis give only amino acids, these include: a) Globular proteins: These are spherical or oval in shape, soluble in water or other solvents and digestible. These include: i) Albumins: Soluble in water and dilute salt solutions and coagulated by heat. ii) Globulins: Soluble in neutral and dilute salt solutions and coagulated by heat e.g. serum globulins. iii) Glutelins: Soluble in dilute acids and alkalies and mostly found in plants e.g. glutelin (wheat). 17  Proteins & Amino acids 2024 iv) Prolamines: They are rich in proline but lack in lysine. They are soluble in alcohol e.g. gliadin (wheat), zein (maize). v) Basic proteins e.g histones and protamins are strongly basic proteins. Protamins resemble histones but smaller in size and soluble in NH4OH. They are not coagulated by heating. They contain large number of arginine and lysine residues. b) Scleroproteins: e.g collagen, keratin and elastin 2- Conjugated Proteins: These proteins on hydrolysis give amino acids and other organic or inorganic components. Examples: a) Nucleoproteins (proteins contain Nucleic acids) b) Lipoproteins (proteins contain lipids) c) Phosphoproteins ( proteins contain phosphorous) d) Metalloproteins ( proteins contain metal ions e.g Fe²⁺) e) Glycoproteins (proteins contain carbohydrates) 3-Derived proteins: The derived proteins are of two types. The primary derived are the denatured or coagulated or first hydrolyzed products of proteins e.g. fibrin formed from fibrinogen. The secondary derived are the products of partial hydrolysis of simple and conjugated proteins e.g. proteoses and peptones. Example of simple proteins: (Scleroproteins) I-Keratins: 1- Two main types of keratins can be identified, α and β. 2- The  keratins are found in hair, nail, enamel and the outer layer of skin. 3- They are α-helical polypeptide chains. and consist of more than 300 amino acids in length. 4-They are rich in cysteine (Human hair keratin contains as much as 14% cysteine), which provides disulfide bonds between adjacent polypeptide chains. 5-The rigidity of keratin is determined by the number of S-S bonds. 6- Water insoluble due to their ↑ content of hydrophobic amino acids. II-Collagens: - Collagen is the most abundant proteins (25% of the human body protein). 18  Proteins & Amino acids 2024 - There are about 20 types of collagens. - Type I collagen is the most common in human body (90%) of body collagens. - A mature collagen fiber forms an elongated rod with an axial ratio of about 200. - Collagens are connective tissue proteins lacking tryptophan. Structure of collagen: 1-Tropocollagen is the basic unit of collagen, consists of three polypeptide chains of α-shape forming a triple helix. 2-Each chain contains about 1000 amino acids, and is twisted in the form of a tight left handed helix. 3-Each chain consists of repeated units of amino acids (Gly-X-Y)₃₃₃ where X is proline or alanine (may be leucine) and Y is hydroxy proline or hydroxy lysine. 4-Collagen contains about 33% glycine, 10% proline, 10% hydroxy proline and 1% hydroxylysine. Collagen molecules have very firm structure due to: 1- Each turn of α-helix contains only 3 amino acids. 2-The presence of 33% glycine (the smallest amino acids) make the polypeptide chains very close to each other. 3-The high content of OH-proline increases the number of hydrogen bonds. 4- The 3 left handed coiled polypeptide chains form a right handed super coiled. Functions of collagen: 1-Collagen is the protein of connective tissue present in skin, bones, cartilage, and blood vessels. 2- Bones and teeth are made by adding mineral crystals to the collagen. Properties of collagen: 1-Solubility: Collagen is insoluble in all solvents. 2-Collagen is protein of low biological value and not digestible. 3-Denaturation of collagen produces another protein which soaks the water like sponge forming gelatin. Gelatin is water soluble and is digestible. 5-Gelatin is given for patients during convalescence. - All the following diseases are seen in collagen diseases: 1-Osteogenesis imperfect is a genetic disorders of collagen biosynthesis characterized by fragile bones. 19  Proteins & Amino acids 2024 2-Homocystinuria: The accumulated homocysteine in this condition, reacts with lysyl aldehydes to block cross linking 3-Marfan’s syndrome: Mutations in the fibrillin gene causing connective tissue disorders. 4-Ascorbic acid deficiency leads to scurvy. 5-Copper deficiency → ↓↓ copper-containing protein required for the conversion of -amino group of lysine to aldehyde. III-Elastin: 1- It is a connective tissue protein formed of 4 polypeptide chains. 2- It is in large amount in lung, large arterial blood vessels, and some elastic ligaments. Smaller quantities of elastin are also found in skin. 3- It is rubber like and can be stretched to several times as their normal length. 4- It recoils to their original shape when the stretching force is relaxed. 5- It contains about 1/3 glycine, rich in proline, poor in OH-proline and free from OH-lysine. 6-The major cross-links formed in elastin are the desmosines, each desmosine contains 4 lysine residues Collage Elastin Structure Triple helix Triple helix is absent Composition Rich in glycine and Rich in glycine and proline proline Cross link Aldol cross linkages Desmosine cross between lysine linkages between 3 aldehyde derivative lysine aldehyde derivatives and one lysine 20  Proteins & Amino acids 2024 Other HO-proline, HO-lysine Small amount of HO- components and glycosylated proline, but hydroxylysine are HO-lysine and present glycosylated Connective tissue hydroxylysine are absent Connective tissue Cojugated proteins on hydrolysis give protein part (apoprotein) and non-protein part (prosthetic group). I. Phosphoproteins: - These proteins are conjugated with phosphate group. - Phosphate group is attached to (OH) group of serine (phosphoserine) or threonine (phosphothreonine). Examples: a) Casein in a milk protein b) Vittelline in egg yolk c) Phosphoenzymes: Addition of phosphate group to an enzyme may activate or inactivate the enzyme according to its structure II. Lipoproteins: These are proteins loosely combined with lipid components. They occur in blood and on cell membranes. Serum lipoproteins are chylomicrons, LDL, VLDL, HDL. III. Glycoproteins and proteoglycans: These are proteins cojugated with carbohydrate in varying amounts. a) Glycoproteins are primarily formed of protein with small amount of carbohydrate e.g blood group enzymes and mucin. b) Proteoglycans are formed of over 95% carbohydrate e.g heparin. IV. Nucleoproteins: These are basic proteins (histones or protamines) conjugated with nucleic acids (DNA or RNA). Examples: a) Chromosomes consist of (proteins + DNA). b) Ribosomes consist of (proteins + rRNA) V. Chromoproteins: These are proteins cojugated with colored elements which may be metal element or pigment. a) Metallochromoproteins: - These contain metal ions e.g cytochromes, hemoglobin and 21  Proteins & Amino acids 2024 myoglobin (Fe²⁺) red color, ceruloplasmin (Cu²⁺) greenish-blue, tyrosinase (Copper) and carbonic anhydrase (Zinc). b) Non-metallochromoproteins: contain non-metal colored substances e.g -Flavoproteins (contain vitamin B₂) are yellow color. -Melanoproteins (contain melanin pigment) are brown to black color in hair and iris of eyes Hemoproteins are conjugated proteins formed of a protein part (globin) and a non-protein prosthetic group (heme) e.g hemoglobin, myoglobin and cytochromes. Myoglobin: 1- It is formed of one polypeptide chain (globin) and one heme unit. 2-It is found in the cytosol of red skeletal muscles and cardiac muscle. 3-It has a much higher affinity for oxygen than hemoglobin, thus it cannot release it except under very low oxygen tension. 4- Myoglobin concentration is elevated in plasma in patients of myocardial infarction (MI). Structure of hemoglobin - Hemoglobin is a conjugated protein having heme as the prosthetic group and the protein (globin). - It is a tetrameric protein with 4 subunits, each subunit having a prosthetic heme group and the globin polypeptide. - The 4 polypeptide chains are: - 2 identical  chains each contains 141 amino acids - 2 identical  chains each contains 146 amino acids - It contains 38 histidine residues - The 4 heme groups account for about 4% of the whole mass of Hb. Functions of hemoglobin: 1-Hemoglobin transports O₂ to tissues and removes CO₂ from them to the lungs. 2- It acts as a blood buffer in RBCs. Classification of proteins according to biological value: 1- High biological value proteins (complete proteins or Rich Proteins) which contain all the essential amino acids e.g casein of milk, egg protein and liver protein. 22  Proteins & Amino acids 2024 2- Low biological value proteins (poor proteins) which are lack in many essential amino acids e.g zein from corn lacks in tryptophan and lysine. 3- Incomplete proteins lack one essential amino acids. They cannot promote body growth in children e.g. cereals lack in lysine and pulses lack in methionine. Some important proteins and peptides: 1- Albumin: - It is the major protein in human plasma, soluble in water. - It is synthesized in liver. The half-life of albumin is about 20 days. - Human albumin consists of one polypeptide chain with a molecular weight of 69,000. - Albumin is the principal determinant of intravascular osmotic pressure. - Albumin is the carrier of various hydrophobic substances in the blood e.g Bilirubin, fatty acids, steroid hormones, thyroxine, aspirin, copper and calcium. Haptoglobin: - Haptoglobin (Hp) is a plasma glycoprotein that binds extracorpuscular hemoglobin (Hb) in a tight noncovalent complex (Hb-Hp). -It transports extracorpuscular hemoglobin in the plasma to reticuloendothelial system to be degraded. Haptoglobin is synthesized in the liver. Transferrin: - It is a glycoprotein, ₁- globulin, with molecular weight of 76,500. - It is synthesized in liver, with a half-life of 7-10 days. - One molecule of transferrin can transport 2 ferric atoms. Ferritin: - It is the storage form of iron. It contains about 23% iron. - It is seen in intestinal mucosal cells, liver, spleen and bone marrow. - The apoferritin has a molecular weight of about 440 kilo Daltons. It has 24 subunits. - It can take up to 4,000 iron atoms per molecule. 23  Proteins & Amino acids 2024 Prothrombin - It is a single-chain glycoprotein synthesized in the liver. It is the zymogen form of thrombin. - The prothrombin is cleaved by Factor Xa producing active thrombin which is a dimer. - Thrombin has a protease enzyme activity with molecular weight of 34,000 D. - Thrombin activates fibrinogen to fibrin by cleaving of Arg-Gly peptide bonds of fibrinogen. - Thrombin converts factor XIII to factor XIIIa. This factor has transglutaminase enzyme activity. Fibrinogen: - Fibrinogen (factor I) is a soluble plasma glycoprotein that consists of 6 polypeptide chains. - All chains are synthesized in the liver. - Fibrinogen is activated to fibrin by thrombin. The fibrin monomers formed are insoluble. Insulin: - It is a polypeptide hormone secreted by β-cells of pancreas after carbohydrate meal. -It is composed of 51 amino acids arranged in 2 polypeptide chains (chain A 21 amino acids and chain B 30 amino acids). - The 2 polypeptide chains are linked together by 2 disulfide bridges, which are called interchain disulfide linkages. - Insulin also contain intrachain S-S linkage in a-chain between a.a (6) and a.a (11) -Insulin facilitates the transport of glucose across the cell membranes of skeletal muscles and adipose tissue. -Insulin controls glucose metabolism. -Insulin decreases the plasma glucose after carbohydrate meals. 24  Proteins & Amino acids 2024 Glucagon: - It is a polypeptide hormone secreted from the α-cells of pancreas during fasting. - It is composed of 29 amino acids arranged in one polypeptide chain. - It increases the plasma glucose level during fasting by increasing (glycogen→ glucose in the liver) Parathyroid hormone (PTH): - It is a polypeptide hormone secreted from parathyroid glands in hypocalcemia. -It is composed of 84 amino acids arranged in one polypeptide chain. -It increases the plasma Ca²⁺ in hypocalcemia. Calcitonin (or Thyrocalcitonin): -It is a polypeptide chain secreted from parafollicular cells of thyroid gland in hypercalcemia. - It is composed of 32 amino acids arranged in one polypeptide chain. -It decreases the plasma Ca²⁺ in cases of hypercalcemia. Fibrillin: - It is a fibrous protein secreted by fibroblasts into the extracellular matrix. - It is a large glycoprotein providing a scaffold for deposition elastin - It is found in the suspensory ligaments of lens, in periosteum and associated with elastin in aorta - Mutations in the fibrillin gene causing ectopia lentitis and dilation of ascending aorta (Marfan´s syndrome). Fibronectin: - It is a fibrous protein. 25  Proteins & Amino acids 2024 - It consists of 2 identical subunits joined together by 2 disulfide bonds (bridges). - It is a plasma glycoprotein which contributes in the adhesion of cells to the extracelllular matrix. Methods of separation of protein and amino acids: 1-Chromatography: The separated substances are divided between stationary phase (paper or gel) and mobile phase (liquid). 2-Electrophorsis: It is the movement of charged particles in the electrical field toward the oppositely charged electrodes. 3-Precpitation: (By salting out) -Albumin is precipitated by full saturated (NH₄)₂SO₄ solutions, while globulin is precipitated by half saturated (NH₄)₂SO₄ solution. 4-Ultracentrifugation: - A mixture of protein is separated into different fractions according to their densities by using a centrifuge of high velocity. 5-Dialysis: - If there is a mixture of proteins (colloids) and salts (crystalloids), they can be separated by using a semi-permeable membrane, where the crystalloids (small ions) can pass through this membrane. 26 Proteins & Amino acids 2024 Good luck 27