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Classification of proteins

There is no satisfactory classification of proteins. In the past they have been classified on the basis of their molecular shapes. Through it is difficult to define proteins precisely enough, we can consider several classes of proteins on the basis of solubility.

Albumins  These are soluble in water and dilute salt solution, and precipitate from solution at high ammonium sulphate concentration without denaturation.

Globulins  Sparingly soluble in water but dissolve easily in salt solution of neutal salts, and can be procipitated when ammonium sulphate solution reaches half saturation.

Protamines and histones.  These are basic proteins, highly soluble, low molecular weight and stable to heat treatment. They easily form salts with mineral acids and are generally associated with nucleic acid of the cell nucleus.

Glutelins They are insoluble in water and alcohol, but dissolve in dilute acid or bases. They are present in many seeds.

Prolamines    These are insoluble in water but soluble in dilute acid or bases

Scleroproteins they are fibrous proteins and insoluble in water and other solvents.

Complex proteins these are a special class of proteins which have a prosthetic or non- proteins component in addition to a protein part. These included metalloproteins, glycoprotins, phosphoproteins, lipoproteins,nucleoproteins and chromoproteins.   

Solubilty behaviour of proteins

The behaviour of  proteins in solution has been of significant help in isolation and separation of variety of proteins. Globular proteins are generally soluble in aqueous media, to different extents. Solubility behaviour is influced by the nature of solvent, pH, ionic strength, properties of solvent and temperature.
Salts may increase or decrease solubility of proteins, and if increasing solubility is observed, the phenomenon is called salting-in effect. Solubility also depends on the ionic strength of the salt solution and is expressed by the formula

           μ = 1/2∑CiZi2 

Where the ionic strength μ is equal to half the  sum of concentration of each ion (Ci ) multiplied by the square of its charge (Zi).

Proteins can be easily precipitated in aqueous solution by adding trichloracetic acid or perchloric acid. Salts of heavy metals, such as phosphotuyngstic acid and phosphomolybdic acid are commonly used in laboratory practice to precipitate proteins. This is called salting out. Similarly cations like Zn2+ and Pb+ also help precipitation.

Salting –out of proteins

A common salting out method is the use of neutral salts. At low concentration proteins are more soluble, but increasing concentration decrease solubility and allow precipitation. Albumins are highly soluble in distilled water, dilute salt solutions, dilute acids and bases, and hence can easily be precipitated with saturated solutions of ammonium sulphate or sodium sulphate , the former being more effective. Globulins are, soluble in dilute salt solutions (5% NaCI) and are precipitated by adding saturated ammonium sulphate solution.

Another factor that influences solubility is the nature or organic solvent. Because of low dielectric constant, ether, ethanol and acetone reduce their solubility and help in precipitation of proteins at low temperature (0o  - 5o). Proteins are temperature sensitive and so is their solubility, but beyond 50oC they become unable and undergo denaturation .The condition of pH also affect solubility and this criterion is employed for selective precipitation . At isoeletric pH proteins are either insoluble or sparingly soluble.

lonic behaviour   

Like amino acids, proteins also possess ionizable groups and behave as ampholytes. Constituent amino acids have a number of ionizable groups and their ionic behaviour can be studied by acid base titration. Their isoelectric pH can be determined by the number of pK values, any pH above isoelectric point shows negative charges and below in separating and analyzing protein mixture by eletrophoresis and ion-exchange chrokmatography.

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