Proteins and their Characterisation

Views:
 
Category: Education
     
 

Presentation Description

No description available.

Comments

Presentation Transcript

PowerPoint Presentation:

Presented By: Mohammad Ovais Dar Class: B.pharma (3 rd year ) Roll no : 105

Introduction::

Introduction: The term ‘ protein ’, derived from Greek word proteios meaning first , was first of all used by Mulder in 1839. Proteins are defined as complex nitrogenous substances which are found in protoplasm of all animal and plant cells.

PowerPoint Presentation:

Actually, the proteins are biopolymers containing large number of amino acids joined to each other by peptide bonds .

Biological Importance of Proteins::

Biological Importance of Proteins: Proteins are important for many cellular functions . Some of these are listed as follows:- Proteins constitute the chief structural units of protoplasm . Proteins participate in such diverse and necessary processes as respiration , muscular contraction , active transport of cellular constituents , electrical transmission , etc

PowerPoint Presentation:

Proteins function as hormonal regulators of metabolism and as storage depots for certain molecules. Antibodies are complex proteins. These serve as a weapon in the defence arsenal of the organism. Proteins are good buffers and play an important role in control of pH in biological systems.

PowerPoint Presentation:

Proteins present in diet serve as the primary source of amino acids , the building blocks for cellular proteins. Viruses are composed of two types of molecules: proteins and nucleic acids . The protein portion serves as an enclosure which is occupied by the nucleic acids, the infective agent.

Characteristics of Proteins::

Characteristics of Proteins: Molecular weight : Proteins are macromolecules which are constructed by the repetition of one or more structural elements, called the monomers . In case of proteins, the monomers are a group of about 20 amino acids. These are linked together to form chains of varying lengths. One arbitary convention designates proteins as those molecules which are generally considered to have minimal chain length of about 100 amino acids corresponding to molecular weights – 10,000 .

PowerPoint Presentation:

Some representative molecular weights of proteins are Ribonuclease 13,700 Trypsin 23,800 Haemoglobin 68,000 Fibrinogen 450,000 Thyroglobulin 630,000 and viruses, a few to several millions. Molecular weights of proteins have been successfully investigated by methods such as ultracentrifugal sedementation , X-ray diffraction , light scattering effects , molecular sieves(gel filtration) and by chemical analysis .

PowerPoint Presentation:

2) Denaturation: All the proteins are optically active and precipitated from aqueous solution by heating , by keeping in acid or alkaline solution, by treatment with organic solvents such as ethanol or acetone , by high concentration of urea , by detergents , and by physical means such as X-ray, light and shaking. Proteins in this precipitated state are said to be denatured and the process of reaching this state is called denaturation which occurs most readily near the isoelectric point.

PowerPoint Presentation:

Denaturation of proteins is usually irreversible . However, examples are known in which denaturation has been found to be reversible. This reversal of denaturation is generally called renaturation or refolding . For example, haemoglobin can be denaturated in acid solution and the process is reversed by neutralisation under the correct conditions. Denaturation is now considered to be the result of changes in conformation or unfolding of the protein molecule, i.e., the secondary and tertiary structures are lost in denaturation without any break in the primary structure

PowerPoint Presentation:

An egg provides a good example of how heat can change the structure of proteins. Proteins are found in both the egg white and yolk. Example of Irreversible denaturation (Boiling of Egg):

PowerPoint Presentation:

Lets consider the protein in an egg white. In their native, uncooked stage, the proteins are chains of amino acids that fold into complex shapes. Several types of bond form between amino acids in the chain, stabilizing the globular structure of the protein.

PowerPoint Presentation:

When the egg is cooked, the heat makes the molecules vibrate. The bonds linking the amino acids in chains are strong covalent bonds that withstand the heat. However, most of the bonds that stabilize the 3D structure of the protein are weak bonds- typically hydrogen bonds- which break, allowing the protein to unravel.

PowerPoint Presentation:

When two unfolded proteins make contact, they form new bonds with each other . A large net of interconnected proteins forms, and the egg hardens. Light can no longer penetrate the egg, which is why the clear portion of the egg turns white.

PowerPoint Presentation:

Isoelectric Point: As each protein contains one α-amino acid and one free-end carbonyl group for each protein chain, most of the ionisable groups must come from the R-groups . Due to the presence of these ionisable groups in the protein chains, the proteins also have some definite isoelectric pH at which they do not migrate in an electric fields.

PowerPoint Presentation:

At the isoelectric pH, the number of positive charges is equal to the negative charges , giving a net charge of zero . At the isoelectric pH, the physical properties of a protein are at a minimum . For example, mobility in an electric field, osmotic pressure, s welling capacity, viscosity , and solubility are minimal at this isoelectric pH. Proteins are cations at pH values lower than the isoelectric pH and anions at pH values higher than the isoelectric pH.

PowerPoint Presentation:

4) Salting in and out of Proteins : The solubility of many proteins is increased in the presence of small concentrations of various neutral salts . This is referred to as salting in of proteins. The phenomenon of salting in of proteins is probably caused by forces of attraction between salt and protein at low salt concentration, leading to increased solubility.

PowerPoint Presentation:

As the concentration of the neutral salts is increased , the solubility increases to maximum and then starts decreasing and finally the protein is precipitated . This is referred to as salting out . The phenomenon of salting out of proteins is probably caused due to the competition for water between the protein and the salt at high concentrations. The salting out of proteins is an effective method of purification of proteins

PowerPoint Presentation:

Colour reactions: Certain colour reactions like Millon , xanthoproteic and ninhydrin reactions as well as the Hopkin-cole reactions ( Glyoxylic acid test) are specific tests for various amino acids. The biuret reaction specific to peptide linkages is also used as a test for the presence of proteins. Some of these tests for proteins are outlined as follows:-

PowerPoint Presentation:

Biuret test: When an alkaline solution of protein is warmed with a dilute solution of copper sulphate , there occurs the formation of a red or violet colour. This reaction indicates the presence of the grouping - CO –NH –CHR –CO –NH – present in protein.

PowerPoint Presentation:

However, dipeptides do not respond to this test because at least two peptide linkages (-CONH ) must be present which are absent in dipeptides. The structure of the coloured product formed at a section of the protein chain is as follows:

PowerPoint Presentation:

b) Xanthoproteic test: When proteins are warmed with concentrated nitric acid , they produce a yellow colour which becomes orange when the solution is made alkaline.

PowerPoint Presentation:

This test is due to the nitration of the benzene ring in phenylalanine , tyrosine and tryptophan . The yellow colour in xanthoproteic acid is the same that is formed on the skin when the latter comes in contact with concentrated nitric acid.

PowerPoint Presentation:

c) Millon’s reaction: Millon’s reagent is prepared by dissolving mercuric nitrate in nitric acid containing a trace of nitrous acid.

PowerPoint Presentation:

When Millon’s reagent is added to a protein solution, a white precipitate which turns red on heating indicates the presence of protein. However, this reaction is characteristic of phenols and is, therefore, given by proteins containing tyrosine .

PowerPoint Presentation:

d) Ninhydrin reaction: Proteins and peptides respond to this test but the colours obtained from these are different from that of amino acids.

authorStream Live Help