Introduction and Properties of Protein

Introduction

The name protein ( proteins means holding the first place ) was suggested by Berzelius (Swedish chemist) to the group of the organic compound that are almost important to life .Mulder (Dutch chemist) used the term protein for the high molecular weight nitrogen rich and almost abundant substances presents in animals and plants.

Proteins are the most abundant macromolecules present in the cell because of their very high molecular weight.They constituent half of the dry weight of most organisms.They occupy the central position in the architecture functioning of living matter.

They are entirely connected with all phases of physical and chemical activities of cells.They differ in their molecular weight ,number of amino acids residue and types of amino acids.Most naturally occur polypeptide contains less than 2000 amino acid residue .Some proteins contain a single polypeptide eg; enzyme,ribonuclease whereas some protein contains more than one type of polypeptide chain as hemoglobin which contains two∝-peptide chain and 2β- peptide chain.

Simply proteins are the long chain of amino acids and are the most abundant and functionally diverse molecules in the living system. Virtually , every life process depends on this class of molecules For example ;enzymes and polypeptide hormones directs and regulate metabolism.where as contractile protein in muscles permit movement.Hence in this chapter we study how the amino acid joined to form the protein that has unique three-dimensional structures, making them capable of performing specific biologic functions.

Properties of proteins

The overall properties of proteins can be discussed under two headings;

1) physical property

2) Chemical property

1) Physical properties of protein:

It includes followings properties

a) Colour and taste:Proteins are usually colorless and tasteless.

b) Shape and Size:The shape of proteins consists of simple spherical structure to long fibrous structure.

• Globular proteins: They have spherical shape occur mainly in plants,seeds, and leaves.
• Fibrous proteins: They have the thread like and ribbon-like structure and generally present in animal muscles.

c)Molecular weight : The protein has generally larger molecular weight ranging between 5*10³ K Da( dalton) to 1*10⁶ KDa . But smaller amino acids are predominant in most proteins.So the average molecular weight becomes 128KDa. During the formation of peptide bond one molecule of water is released ,hence the average molecular weight becomes 128-18 = 110 KDa .

approximate number of an amino acid residue=total molecular weight of protein /Total molecular weight of 20 amino acids

d) Colloidal nature: Because of their larger size ,the protein exhibits many colloidal properties such as

• their diffusion rates are extremely low
• they produce the light scattering effect (Tyndall effect)

e) Denaturation of protein: Denaturation refers to the structural changes in the protein ,which causes the loss of biological activity (property ).It is a total loss or randomization of a 3D structure .In this process, a secondary ,tertiary,and quaternary structure of protein molecule are lost but the primary structure is not affected .In many instances,the process of denaturation is followed by coagulation(process where denaturated protein molecule tend to form large aggregate and precipitate from solution.)

It was suggested byWu (1931) that denaturation leads mainly to the unfolding of the peptide chain.Hence, causing disorganization of an internal structure of protein .When the peptide chains are unrolled certain bonds and new sides of bundles are exposed to the action of certain proteolytic enzymes causing hydrolysis under certain condition.Denaturation is reversible which cause the protein refolds into its normal or original structure when the denaturating agents are removed.eg trypsin;If trypsin is exposed to the temperature of 80°-90°C it denaturated and the solution is cooled at 37°C the solubility and activity of an enzyme are regained.

The process of regaining normal protein property by a denaturated protein is called as renaturation or refolding. However , when the protein is denaturated they are permanently disorder. Denaturated proteins are often insoluble and therefore precipitated from solution.

^{Effect of denaturation:}

The effects of denaturations of proteins are listed below:

1. Decrease in their solubility
2. Cessation of biochemical activity such as enzymatic or hormonal activity.
3. Decrease in size and shape of the molecules.
4. Alteration of surface tension and loss of antigenicity.

^{Cause of denaturation:}

Different types of denaturating agents are involved for the denaturation of protein which can be categories into two groups:

_{A) Physical agent:}

1. Mechanical mixing
2. Heat treatment
3. Radiation
4. High hydrostatic pressure
5. Ultrasound

_{B) Chemical agent} :

1. Organic solvent ( acetone ,alcohol, and ether)
2. Urea
3. Acids
4. Alkalies
5. Some detergents
6. Salicylates

Significance of denaturation:

1. After the denaturation of protein, their solubility decreases and hence can be separated from blood serum by filtration ,and centrifugation process for the clinical analysis of other molecules in serum.
2. Denaturated proteins are used for the analysis of amino acid sequence on polypeptide chain.
3. Denaturated proteins are also used in the research laboratory .

f) The solubility of protein:Solubility of protein decreases at an isoelectric point and increases with increase in acidity and alkalinity. The solubility is significantly influenced by pH, temperature, and types of solvents.

1) Salting in effect: Globulin proteins are slightly soluble in water but their solubility is enhanced by the addition of salt like NaCl. This phenomenon is commonly called as salting in -effect.

2) Salting out effect: The proteins are precipitated from aqueous solution by the used of high concentration of neutral salt such as Magnesium salt ,Sodium sulphate .The phenomena are commonly called as salting out effect.

g)Optical activity: All protein solution rotates the plane-polarized light to the left ie.Leave-rotatory.

h) Amphoteric property; Like amino acid proteins are also amphoteric in nature.

i) Zwitterionic effect:Several free amino groups and the carboxyl group are present in proteins. These can be ionized and exist in hybrid form.

j) Absorbance: The proteins have the maximum absorbance at a wavelength of 280nm in UV region.

k) Ion binding capacity:Proteins when combined with different ions, there is the formation of insoluble salts. These different ions are used as precipitating agents for proteins. Anions of acid like trichloroacetic acid and picric acid form insoluble salts with proteins.

l) Mobility in an electric field; Proteins are either positively charged ,negatively charged or neutral. Hence,on the basis of the charge, they are migrated towards cathode and anode .The protein can be separated by the technique of electrophoresis.

B) Chemical Properties of proteins:

The protein produced colour in certain chemical reactions. These reactions are not specific for protein molecules but this is due to characteristics of particular amino acids present in given protein.

1) Xanthoproteic test: Amino acids containing an aromatic nucleus (Tyrosine ,Tryptophan and Phenylalanine) form yellow nitro derivatives or heating with conc.HNO₃ . The salts of these derivatives are orange in colour .Protein containing this amino acid also give the positive response to this test. Phenylalanine gives negative or weakly positive reactions though these amino acids contain aromatic nucleus because it is difficult to nitrate under normal condition.

2) Sakaguchi test: This test is specific for arginine because this reaction is given by Guanidinium .The Argentium reacts withα-napthol and on oxidising agents such as bromine water or sodium chloride to give a red colour product.

3) Biuret Reaction: This test is a general test for compounds having a peptide bond. Since proteins contain peptide bond ,It is given by all proteins. Alkaline copper sulphate reacts with the compound containing two or more peptide bond to give a violet colour product which is due to the formation of coordination complex of cupric ions with unshared electron pairs of peptide nitrogen and oxygen of water.

Biuret reaction

4) Millions test:Protein containing Tyrosine and tryptophan gives this reaction . When protein is treated with million reagents (10 percent mercurous chloride with H₂SO₄) ,the ppt is formed and after the addition of NaNo3,the ppt change into pink-red colour.