Dr. Samer Zahran

2. Dr. Samer Zahran CHEMISTRY OF AMINO ACIDS AND PROTEINS

3. Protein • Definition: Proteins are formed of amino acid residues (more than 100 amino acid) linked together by peptide bonds. • Chemically, polymerisation of aminoacids into protein is a dehydration reaction. They are of high molecular weight (more than 5000), colloidal in nature, non dialysable and heat labile.

4. The conformation of proteins(orders of protein structure) • In its native form, protein molecule has a characteristic three dimensional shape (primary, secondary, tertiary structure), which is required for its specific biological function or activity. Proteins formed of two or more polypeptide chains have quaternary structure. 1. Primary structure of proteins: • This refers to the number and sequence of amino acids in the polypeptide chain or chains linked by peptide bonds.

5. 2. Secondary structure of proteins: • Coiling, folding or bending of the polypeptide chain leading to specific structure kept by interactions of amino acids close to each other in the sequence of polypeptide chain. There are two main regular forms of secondary structure; α-helix and β-pleated sheets .

6. Regular secondary structure: α-helix: • It is a rod like structure formed from a tightly packed, coiled polypeptide chain arranged in spiral structure. • The coil of the polypeptide chain in α-helical structure brings 3.6 amino acids per turn.

8. β-pleated sheet: • It’s called so because it was the second structure discovered after α-helix which is the first. Unlike α-helix, β-pleated sheet may be composed of two or more peptide chains that are almost fully extended.

10. 3. Tertiary structure of proteins: • Occurs when certain attraction occurs between α-helix and β-pleated sheets to gives the overall shape of the protein molecules. • It is the three dimensional structure of each polypeptide chain. There are two main forms of tertiary structure: fibrous and globular types.

11. 4. Quaternary structure of proteins: • Certain polypeptides will aggregate to form one functional protein. • Proteins possess quaternary structure if they consist of 2 or more polypeptide chains, structurally identical , or totally unrelated united by non-covalent bonds (hydrogen , electrostatic bonds and hydrophobic interaction).

13. Denaturation of proteins Definition: • It is loss of native structure (natural conformation) of protein by many physical or chemical agents leading to changes in the secondary, tertiary and quaternary structure of proteins • due to rupture of the non-covalent bonds (hydrogen bonds, hydrophobic bonds and electrostatic bonds and may be disulphide, but not peptide bonds), with loss of biological activity.

14. N.B. The primary structure of proteins is not changed in denaturation since there is no hydrolysis of peptide bonds. • Denaturation disrupt all orders of protein structure except primary structure.

16. Causes of denaturation: • Physical agents: • e.g. heating above 70 ºC, vigorous shaking and stirring, repeated freezing and thawing, ultraviolet rays, X-rays and exposure to high pressure.

17. 2. Chemical agents: e.g. • – Urea. • Salts of heavy metals as Mg2+ and Pb2+ disrupt ionic bonds • Strong acids and bases (extreme pH).. • -Sulfhydral reagents e.g. mercaptoethanol (destroys S-S bonds by reduction). • Alkaloidal reagents e.g. picric acid and phosphotungestic acid • Alcohol.

18. Effects of denaturation: • Physical changes: • - Decreased solubility (due to exposure of internal non-polar groups) and decreased rate of diffusion through membranes. • - Increased viscosity of proteins (due to unfolding of chains and increase of their molecular size).

19. 2. Chemical changes: • - Rupture of non- covalent (and may be disulfide) bonds. • - Exposure of some groups which are present in the interior of the protein molecule e.g. – SH.

20. 3. Biological changes: • - Loss of biological activity of enzymes and protein hormones. • - Changes of antigenic property of proteins. • - Denatured proteins are easily digested due to unfolding of the peptide chains.

21. Classification of proteins I- According to shape: • On the basis of axial ratios of proteins (the ratios of length to width) two classes of proteins are found: 1- Globular proteins: • They have an axial ratio of less than 10 times its width (usually about 3 or 4) e.g. plasma albumins and globulins and many enzymes. They have spheroidal shape. 2- Fibrous proteins: • They have an axial ratio of more than 10 times its width e.g. keratin, myosin, fibrin and collagen.

22. II- According to the biological value: 1- Proteins of high biological value: i.e. contain all the essential amino acids e.g. animal proteins. 2- Proteins of low biological value: i.e. deficient in one or more essential amino acid e.g. plant proteins as Zein.

23. III- According to structure: 1- Simple proteins:Formed only of amino acids • a- Protamines and histones: are water soluble basic proteins rich in histidine, arginine and lysine. • They are present in nucleoproteins (protamines are present in fish and histones are present in plants and animals.) • * Globin; the protein moiety of Hb and myoglobin is considered as histone.

24. Protamines • Are essential in the process of sperm formation. • When mixed with insulin, protamines slow down the duration of insulin action. Histones • Are high molecular weight basic proteins • They play a role in gene regulation

25. b- Albumins (water soluble) and globulins,(soluble in dilute salt solutions). • These are heat coagulable, globular proteins of high biological value. • Globulins have larger molecular weight compared to albumins. • They are present mainly in blood plasma, egg white and milk.

26. c- Scleroproteins (connective tissue proteins or albuminoids) • These are fibrous structural proteins. • They are insoluble in most protein solvent. They include:

27. (1) α-keratin:They are the proteins of the outer surface of the skin, hair and nails. It is a helical polypeptide chain. It is insoluble due to its high content of hydrophobic amino acids. (2) β-keratin:Present in silk and formed of anti-parallel -sheets.

28. (3). Collagen: • It is present mainly in skin, cartilage, tendons and ligaments (hard tissues), • it has a special structure, the unit structure of its fibres is the tropocollagen. • The tropocollagen is formed of three polypeptide chains.

29. (4). Elastin:Found in yellow elastic tissue. (5). Ossein:The main protein of bone and teeth. (6).Reticulin:Found in reticular tissue.

30. 2- Conjugated proteins (compound proteins): • Those contain in addition to the protein moiety some other groups called the prosthetic group attached by covalent bonds. According to the prosthetic group; they are classified into:

31. a.Phosphoproteins: • They contain phosphoric acid as prosthetic group; conjugated to hydroxyl group of serine or threonine. • They are of animal origin e.g. caseinogen of milk and vitellin of egg yolk. • Caseinogen is converted by rennin to soluble casein which is precipitated by Calcium as Calcium caseinate (cheese).

32. b.Glycoproteins: • They are proteins that have carbohydrates (glycan) covalently attached to their polypeptide backbones.

33. c.Lipoproteins: • These are combinations of proteins with lipids. They are present in cell membranes, plasma lipoproteins and thromboplastin (a cephalin protein compound).

34. d.Nucleoprotiens: • They contain nucleic acids (DNA or RNA) as prosthetic groups attached to protamines or histones. They are found in cell nuclei and also in cytoplasm.

35. e.Chromoproteins: • These are proteins that contain colored prosthetic groups e.g. : • * Haemoglobin is composed of globin and haem (red), also cytochromes, catalases and peroxidases contain haem pigment conjugated to specific proteins (myoglobin is similar to Hb, but present in skeletal muscle).

36. Flavoproteinsare enzymes concerned with hydrogen transport contain riboflavin (B2) (yellow). • * Rhodopsin is composed of opsin (protein) and 11-cis retinal. It gives purple color (visual purple). It is present in retina and responsible for vision in dim light.

37. f.Metalloproteins : • They contain metals as prosthetic groups e.g.: • - Hb and ferritin contain iron. • - Ceruloplasmin contains copper. • - Carboxypeptidase and carbonic anhydrase contain zinc.

38. 3.Derived proteins: • They are hydrolytic products of proteins as a result of acids, alkalis or enzymes. • According to molecular weight they are classified into: • 1. Metaproteins. • 2. Proteoses. • 3. Peptones.

39. Where metaproteins have the higher molecular weight, while peptones have the smallest M.W. ,Gelatin which is a hydrolytic product of collagen is considered as a proteose. It is poor in essential amino acids, but it is used as a supplementary protein as it is easily digested.

40. Thank you