Structure of Proteins, Properties of Proteins, and Denaturation of Proteins

Tertiary and Quaternary structure of proteins
Physical and chemical properties of proteins
Precipitation of proteins
Denaturation of proteins

1. Chemistry of Proteins & Amino Acids Karthik G Kamath K Associate Professor Department of Biochemistry Mangala College of Allied Health Sciences

2. Tertiary Structure of Proteins 3-D arrangement of protein structure Compact structure Hydrophobic sidechains in the interior & Hydrophilic groups on the surface Ensures stability of the molecule

3. Bonds of tertiary structure Besides H-H bonds Disulfide bonds (-S-S) , ionic interactions (electrostatic bonds), & Van der Walls forces also contribute to the tertiary structure

4. Quaternary Structure of Proteins Some proteins consist of two/more polypeptide chains Which may be identical/non-related Such are called oligomers and possess a quaternary structure Monomers are held together by hydrogen bonds, hydrophobic interactions, and ionic bonds

5. Properties of proteins Solubility – form colloidal solutions in water Molecular weight – varies, dependent on the no. of amino acids . Vary from 4000 to 440000. Shape – globular, oval, fibrous or elongated Isoelectric pH ( pI ) – pH at which the protein exists as zwitter ion electrically neutral, maximum precipitability , and least buffering capacity pepsin 1.1, casein 4.6 etc Acidic & basic proteins – proteins with ratio ( ε Lys+ ε Arg)/( ε Glu+ ε Asp) >1 is basic. <1 is acidic

6. Precipitation of proteins Exists in colloidal solution due to hydration of polar groups Can be precipitated by dehydration or neutralization of polar groups Precipitation at pI Least soluble at pI Precipitation by salting out: Precipitation by addition of neutral salts – ammonium sulfate / sodium sulfate Increases protein-protein interaction Resulting in molecular aggregation and precipitation

7. Precipitation of proteins Amount depends on molecular weight Higher the molecular weight lower the salt required for precipitation Serum globulins are precipitated by half saturation and albumin by full saturation Precipitation by salts of heavy metals Pb 2+ , Hg 2+ , Fe 2+ etc cause precipitation Being positively charged, when added to protein in alkaline medium (negatively charged) results in precipitation

8. Precipitation of proteins Precipitation by anionic or alkaloid reagents By TCA, sulphosalicylic acid, phosphotungstic acid, picric acid. Proteins existing as cations are precipitated by anionic form of acids Precipitation by organic solvents Organic solvents are good protein precipitating agents They dehydrate the proteins by removing water envelope and causes precipitation

9. Denaturation of proteins Disorganization of the native structure Results in loss of secondary, tertiary, and quaternary structure Agents of denaturation: Physical agents – Heat, violent shaking, X-rays, UV rays Chemical agents – acids, alkalies , organic solvents, salts of heavy metals

10. Characteristics of denaturation Native helical structure is lost Primary structure with peptide linkages remains intact Loss of biological activity Becomes insoluble in solvents in which they were originally soluble Viscosity increases while surface tension decreases Increase in ionizable & sulfhydryl groups due to loss of hydrogen & disulfide bonds

11. Characteristics of denaturation More easily digested due to exposure of peptide bonds to the enzymes Usually irreversible Careful denaturation is sometimes reversible (renaturation) Cannot be crystallized