Properties of protein k.padhmasri

properties of protein

1. PROPERTIES OF PROTEIN PRESENTATION BY, K.PADHMASRI

2. PROPERTIES Solubility Molecular weight Shape Isoelctric PH Acid and basic proteins Preciptation of proteins 1.Pi, 2.Salting in and salting out 3.By salt of heavy metals 4.By anionic/alkaloid detergent 5. By organic solvents Colouring reaction of protein

3. 1.SOLUBILITY Solubility : Proteins form Collaidol solutions instead of true solutions in water. This is due to huge size of protein molecules.

4. 2. MOLECULAR WEIGHT: The proteins vary in their molecular weights, which, in turn, is dependent on the number of amino acid residues . Each amino acid on an average contributes to a molecular weight of about 1 10K Da.

5. Majority of proteins/ p olypeptides may be composed of 40 to 4,000 amino acids with a molecular weight ranging from 4,000 to 440,00O. A few proteins with their molecular weights are listed below : Insulin -5,700 Myoglobin -1700 Hemoglobin -64,450 Serum albumin-69,000 .

6. 3.SHAPE There is a wide variation in the Protein shape. it may be globular( insulin) oval (albumin) fibrous or elongated ( fibrinogen )

7. 4.ISOELECTRIC POINT The nature of the amino acids (particularly their Ionizable groups )determines the pl of a protein . The acidic amino acids (Asp, Clu ) and basic amino acids (His, Lys, Arg ) strongly influence the pl

8. At isoelectric pH, the proteins exist a zwitterions or dipolar ions . They are electrically neutral (do not migrate in the electric field) with minimum solubility, maximum precipitability least buffering capacity.

9. The isoelectric pH(pl ) for some proteins are given here Pepsin-1.1 Casein-4.6 Human albumin-4.7; Urease - 5.0 Hemoglobin-6.7 Lysozyme-1. 0.

10. 5.ACID AND BASE PROPERTIES Proteins in which the ratio is great 1 er than are referred to as basic proteins. For acidic proteins, the ratio is less than 1

11. 6.PRECIPITATION OF PROTEIN Proteins exist in colloidal solution due to hydration of polar groups ( -COO- , - NH , -OH) . Proteins can be Precipitated by dehydration neutralization of polar groups polar groups

12. 7.PRECIPITATION OF Pi The proteins in general are least soluble at isoelectric pH . Certain proteins ( e.g.casein ) get easily precipitated when pH is adjusted to pl ( 4.6 for casein). Formation of curd from milk is a marvellous example of slow precipitation of milk protein, casein at pl. This occurs due to the lactic acid produced by fermentation of bacteria which lowers the pH to the pl of casein .

13. 8.PRECIPITATION BY SALTING OUT The process of protein precipitation by the additional of neutral salts such as ammonium sulfate or sodium sulfate is known as salting out . This phenomenon is explained on the basis of dehydration of protein molecules by salts This causes increased protein protein interaction , resulting in molecular Aggregation and precipitatio n

14. the amount of salt precipitation depends on the size ( molecular weight)of the protein molecule The higher is the protein molecular weight, the lower is the salt required for precipitation. Serum globulins are precipitated by The amount of salt required for protein half saturation with ammonium sulfate while albumin is precipitatedby full saturation. Salting out procedure is conveniently used for separating serum albumins from globulins.

15. SALTING IN: The addition of small quantities of neutral salt s increases the solubility of proteins . This process called as salting in is due to the diminished protein-protein interaction at low salt concentration .

16. PRECIPITATION BY SALT OF HEAVY METALS Heavy metal ions like Pb2+, Hg2+, Fe2+, Zn2+ cause precipitation of proteins. These metals being positively charged , when added to protein solution ( negatively charged)in alkaline medium results in precipitate formation

17. PRECIPTATION BY ANIONIC DETERGENTS Precipitation by anionic or alkaloid reagents : Proteins can be precipitated by trichloroacetic acid, sulphosalicylic acid , phosphotungstiacid , picric acid, tannic acid, phosphomolybdic acid etc . By the addition of these acids, the proteins existing as cations are precipitated by the anionic form of acids to produce proteinsulphosalicylate , protein- tungstate , proteinpicrate

18. PRECIPITATION BY ORGANIC SOLVENTS Precipitation by organic solvents : Organic solvents such as alcohol are good protein precipitating agents. They dehydrate the protein molecule by removing the water envelope and cause precipitation

19. COLOUR REACTION OF PROTEIN The proteins give several colour reactions which are often useful to identify the nature of the amino acids present in them . BIURET REACTION; Biuret is a compound formed by heating urea to 180”C . When biuret is treated with dilute copper sulfate in alkaline medium , a purple colour is obtained . This is the basis of biuret test widely used for identification of proteins and peptides

20. Biuret test is answered by compounds containing two or more CO-NH groups i.e., peptide bonds. All proteins and peptides possessing at least two peptide linkages i.e ., tripeptides (with 3 amino acids) give positive biuret test. Histidine is the only amino acid that answers biuret test. The principle of biuret test is conveniently used to detect the presence of proteins in biological fluids .

21. The mechamism of biuret test is not clearlv known, lt is believed that the colour is due to the formation of copper co- ordinated complex , The presence of magnesium and ammonium ions interfere in the biuret test. This can be overcome by using excess alkali. The colour reactions given by proteins due to the presence of specific amino acids are given in These reactions are often useful to know the presence or absence of the said amino acids in the given protein.