Protein structure

1. Protein structure

2. • Proteins catalyze metabolic reactions, power cellular motion, and forms structural integrity to hair, bones, tendons and teeth • Human proteins therefore reflects the sophistication and diversity of their biologic roles

3. • The 20 amino acids commonly found in proteins are joined together by peptide bonds. PEPTIDES: Two AA covalently joined through a substituted amide linkage – peptide bond by removal of H2O OH- Carboxyl group of one AA and H+ from amino group of another AA • ?

4. • Two AA reacts to form dipeptides. Three AA can be joined by two peptide bonds to form a tripeptide and so on. Oligopeptide: When a few AA are joined by various peptide linkage • When many amino acids are joined, the product is called a polypeptide. Proteins may have thousands of amino acid residues • • • •

5. • The linear sequence of the linked amino acids contains the information necessary to generate a protein molecule with a unique three- dimensional shape. Therefore maturation of a newly synthesized polypeptide into a biologically functional protein – Requires folding into a specific three- dimensional arrangement, or conformation •

6. • The complexity of protein structure is best analyzed by considering the molecule in terms of four organizational levels: Primary: linking amino acid residues in a polypeptide chain Secondary: stable arrangements of amino acid residues giving rise to recurring structural patterns into geometrically ordered units; twisting resulting in α-helix or pleated tertiary: the three-dimensional assembly of secondary structural units to form larger functional units Quaternary: : It’s the arrangement in space of protein having two or more polypeptide subunits 1. 2. 3. 4.

8. • An examination of these hierarchies of increasing complexity has revealed that certain structural elements are repeated in a wide variety of proteins, suggesting that there are general “rules” regarding the ways in which proteins achieve their native, functional form. • These repeated structural elements range from simple combinations of α-helices and β- sheets forming small motifs, to the complex folding of polypeptide domains of multifunctional proteins.

9. CLASSIFICATION OF PROTEINS CLASSIFICATION OF PROTEINS • Proteins are classified: On the basis of shape and size On the basis of shape and size I. II. On the basis of functional properties II. On the basis of functional properties III. On the basis of solubility and physical III. On the basis of solubility and physical properties. properties.

10. I. On the basis of shape and size I. On the basis of shape and size • Fibrous proteins: Fibrous proteins: When the axial ratio of length: width of a protein molecule is more than 10, it is called a fibrous protein . fiber like in shape, insoluble in water and resistant to digestion Examples Examples : : α-keratin from hair, collagen. • Globular protein: Globular protein: When the axial ratio of length: width of a protein molecule is less than 10, it is called as globular protein. Examples: Examples: Myoglobin, haemoglobin, ribonuclease, etc. • •

11. II. On the basis of functional properties: II. On the basis of functional properties: The second way of classifying proteins makes use of their functional properties, functional properties, such as: Enzymes: Enzymes: Proteins catalyzing the biological reactions. Hormones: Hormones: Proteins regulating physiological responses. Defence proteins: Defence proteins: Immunoglobulins involved in defence mechanisms. Contractile proteins: Contractile proteins: Proteins of skeletal muscle involved in muscle contraction and relaxation. Respiratory proteins: Respiratory proteins: Involved in the function of respiration, like haemoglobin, myoglobin, cytochromes. Structural proteins: Structural proteins: Proteins of skin, cartilage, nail. • • • • • • • • •

12. III. On the basis of solubility and physical properties: III. On the basis of solubility and physical properties: However, both the above classification schemes have many overlapping features. Therefore a third most acceptable scheme of classification of proteins is adopted. According to this scheme proteins are classified on the proteins are classified on the basis of their solubility and physical properties and are divided basis of their solubility and physical properties and are divided in three different classes. in three different classes. A. Simple proteins: A. Simple proteins: These are proteins which on complete hydrolysis yield only amino acids. Albumin, Histone B. Conjugated proteins: B. Conjugated proteins: These are proteins which in addition to amino acids contain a non-protein group called prosthetic group contain a non-protein group called prosthetic group in their structure. Lipoproteins, Glycoproteins, Nucleoprotein C. Derived proteins: C. Derived proteins: These are the proteins formed from native protein by the action of heat, physical forces or chemical factors. Fibrin •

13. Digestion in stomach • Food particles that enter the stomach are thoroughly mixed with gastric secretions to form the solution defined earlier as chyme. • Chyme contains molecular fragments of proteins and polysaccharides; droplets of fat; and ions, water, and various other molecules ingested in the food. Virtually none of these ions or molecules, except water, can cross the epithelium of the gastric wall, and thus little absorption of nutrients occurs in the stomach. In addition to playing a role in the breakdown of proteins, the stomach functions as a storage vessel that periodically empties some chyme into the small intestine at a rate that favors the complete digestion and absorption of a meal. • •

14. The three major exocrine secretions of the stomach—mucus, acid, and pepsinogen—is secreted by a different cell type The cells at the opening of the gastric glands secrete a protective coating of mucus and HCO 3−. Lining the walls of the glands are parietal cells, cells, which secrete acid and intrinsic factor. Intrinsic factor Intrinsic factor is a protein that binds and allows the absorption of vitamin B12. chief cells, chief cells, which secrete pepsinogen • • • parietal • • The gastric glands also containenteroendocrine cells called G cells, which secrete gastrin. In addition, enterochromaffin-like (ECL) cells, enterochromaffin-like (ECL) cells, which release the paracrine substance histamine histamine •

15. HCl Production and Secretion HCl Production and Secretion • about 2 L of hydrochloric acid per day. the parietal cell contains the enzyme carbonic anhydrase that catalyzes the reaction between CO2 with water to produce carbonic acid, which dissociates to H+ and HCO 3− The stomach secretes •

16. • H+/K+- ATPase pumps the parietal cells pump hydrogen ions into the lumen of the stomacH. This primary active transporter also pumps K+ into the cell, which then leaks back into the lumen through K+ channels. As H+ is secreted into the lumen, HCO 3− is moved across the membrane and into the capillaries in exchange for Cl−, which maintains electroneutrality. In this way, production and secretion of H+ are coupled. • • •