Proteins. Dr. Basima Sadiq Jaff. /3 rd class of pharmacy. PhD. Clinical Biochemistry

Transcription

1 Proteins /3 rd class of pharmacy Dr. Basima Sadiq Jaff PhD. Clinical Biochemistry

2 a Greek word that means of first importance. It is a very important class of food molecules that provide organisms not only with C and H but also with N and Sulfer which are not available from fats and carbohydrates. Dietary proteins are important for synthesis of nitrogen-containing molecules: nucleic acids, enzymes & other proteins, nerve transmitters & many hormones. Foods with high over (10%) protein content are including: Animal sources for proteins: meat, fish, poultry (chicken), milk, cheese, eggs Plant sources for proteins: cereals, nuts, legumes (peas, beans, and soya beans) Complete protein: contains all essential amino acids Incomplete protein: deficient in one or more essential amino acids o Maize (corn) contains considerable methionine but deficient lysine & tryptophan o Beans abundant in lysine & tryptophan but very little methionine Shwan M. A._College of Medicine 2

3 Proteins classification according to their solubility 1. Albumins: Soluble in water and salt solutions. Examples: Albumin of plasma, egg albumin and lactalbumin of milk. 2. Globulins: Sparingly soluble in water but soluble in salt solutions. Examples: Globulins of plasma, ovoglobulins of egg, lactoglobulin of milk. 3. Glutelins: Soluble in dilute acids and alkalies. Examples: Glutenin of wheat, oryzenin of rice, zein of maize. 4. Protamins: Soluble in ammonia and water. Examples: Salmine from salmon fish, sturine of sturgeon. 5. Histones: Soluble in water and dilute acids. Example: Histones present in chromatin. 6. Prolamines: Soluble in dilute alcohol and insoluble in water and alcohol. Examples: Gliadin of wheat, zein of corn. 7. Sclero proteins: Insoluble in water and dilute acids and alkalies. Examples: Collagen, elastin and keratin. 3

4 Acute Phase Proteins or Reactants (APR) ; 1. The concentration of these proteins increases markedly during acute inflammation. 2. They are α1-antitrypsin, haptoglobin, ceruloplasmin, complement-3, fibrinogen and c-reactive protein. Their concentration increases in conditions like surgery, myocardial infraction, infections and tumours. 3. Acute phase reaction is general to any infection. They all play part in complex defensive process of inflammation. 4. The synthesis of these proteins by liver is triggered by interleukin at the site of injury. 5. The plasma levels of these APR raises at different rates. The levels of c-reactive protein raises first followed by α1-antitrypsin. The level of complement-3 raises at the end 4

5 Classification of Protein according to Structure Simple proteins: On hydrolysis ONLY amino acids will be yield -Egg(albumin) -Serum(globulins) -Wheat(Glutelin) Conjugated Proteins: On hydrolysis amino acids and a prosthetic group (nonamino acid part) will be yield. 5

6 Classification of Protein according to Shape & Solubility Globular proteins: They are spherical proteins; hydrophilic (polar) amino acids distributed outside (on folding) and the hydrophobic (nonpolar) amino acids inside. They are soluble in aqueous solutions. E.g: albumin, globulin, casein, myoglobin, haemoglobin, all of the enzymes, and protein hormones. Fibrous proteins: These proteins have a rod like structure. They are insoluble in water. Example: collagen occurs in cartilage and connective tissues, keratin occurs inhair & nails 6

7 Exampls of Proteins classification according to their solubility 1. Albumins: Soluble in water and salt solutions. Examples: Albumin of plasma, egg albumin and lactalbumin of milk. 2. Globulins: Sparingly soluble in water but soluble in salt solutions. Examples: Globulins of plasma, ovoglobulins of egg, lactoglobulin of milk. 3. Glutelins: Soluble in dilute acids and alkalies. Examples: Glutenin of wheat, oryzenin of rice, zein of maize. 4. Protamins: Soluble in ammonia and water. Examples: Salmine from salmon fish, sturine of sturgeon. 5. Histones: Soluble in water and dilute acids. Example: Histones present in chromatin. 6. Prolamines: Soluble in dilute alcohol and insoluble in water and alcohol. Examples: Gliadin of wheat, zein of corn. 7. Sclero proteins: Insoluble in water and dilute acids and alkalies. Examples: Collagen, elastin and keratin. 7

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9 Level structures of protein Primary structure Amino acid sequence of the protein Secondary structure H bonds in the peptide chain backbone -helix and -sheets Tertiary structure Non-covalent interactions between the R groups within the protein Quaternary structure Interaction between 2 polypeptide chains Shwan M. A._College of Medicine 9

10 Level structures of protein Primary structure of protein It is the linear sequence of amino acids covalently linked by peptide bonds. This sequence defines how the protein will fold. Also, it defines how the protein will function. A single change in the amino acid sequence of hemoglobin can cause the proteins to clump together, resulting in the disease sickle cell anemia. 10

11 Level structures of protein Secondary structure of protein Hydrogen bonds between amide hydrogens and carbonyl oxygens of the peptide bond among amino acids form two particularly stable structural elements in protein: α-helix and β-pleated sheets by folding of primary sequences. For example: fibrous proteins like α-keratin and silk fibroin. 11

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13 Level structures of protein Tertiary structure of protein Globular proteins contain varying amounts of α-helix and β-pleated sheets folded into higher levels of structure called tertiary structure. It is maintained by attractive forces including: Van der Waals forces between the nonpolar R groups of amino acids Hydrogen bonds between the polar R groups of amino acids Ionic bonds (salt bridges) between oppositely charged amino acids Disulfide bonds between polar cysteine 13

14 Quaternary structure of protein Two or more folded polypeptide chains can come together by non-covalent bonds to form one functional molecule with several subunits. Such multi subunit proteins include enzymes and transport proteins. Shwan M. A._College of Medicine 14

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20 Quaternary structure of protein Two or more folded polypeptide chains can come together by non-covalent bonds to form one functional molecule with several subunits. Such multi subunit proteins include enzymes and transport proteins. Shwan M. A._College of Medicine 20

21 Denaturation of protein Proteins can be denatured, that is, unfolded and disorganized, making them nonfunctional. Denaturing agents include heat, organic solvents, mechanical mixing, strong acid or bases, detergents, and ions of heavy metals such as lead and mercury. 21

22 Denaturation of protein Protein function depends on its structure. When the three-dimensional conformation of a protein is denatured; its biological activity will be lost. Peptide bonds (primary structure of proteins) are not affected. But; the H-bonds, disulfide bonds, salt bridges, and hydrophobic interactions can all be disrupted, leading to the alteration (disruption) of quaternary, tertiary, & secondary structure of protein respectively. In some cases, once the denaturing agent is removed the enzyme may be re-fold and become active again. 22

23 Denaturation of protein Effect of heat: H-bonds and hydrophobic interactions are disrupted. Effect of organic solvents: Alcohols and ethers can disrupt H-bonds. 23

24 Denaturation of protein Effect of heavy metal salts: Heavy metals are reacting with sulfhydryl (-SH) groups of protein forming stable metal bonds, that prevents the formation of needed disulfide bonds. Also, they can combine with carboxylate ions, preventing their participations in salt bridges. Effect of acids and bases: Change in ph can disrupt salt bridges. 24

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26 ESSAY QUESTIONS 1. Classify proteins based on composition. Give examples for each class. 2. Explain terms primary, secondary, tertiary and quaternary structure of proteins. Write various forces that stabilize protein structure 3. Write an essay on functions of proteins with examples. Shwan M. A._College of Medicine 26

27 THANKs Dr.Basima.