PHYSICO-CHEMICAI STUDIES ON THE INTERACTION OF PROTEINS WITH POLYACIDS, METAL IONS & HYDROUS-OXIDE SOLS

Abstract

The proteins are naturally ocouring highly complex compounds composed of one or saore polypeptide chains which are the polyaraides of •c-amino acids characterised by the presence of ( -rffl-CH (R)-C -' repeating unit. They are of great biological interest and significance sinee they include some of the most important constituents of the living ceils. In fact they are to intimately bound up with life processes that each type of organism and each kind of cell possesses Its own characteristic proteins*- A first broad classification of the proteins can be made on the basis of their structure,I.e.,fibrillar and globular. The fibrillar protein consists of long peptide chains organised into submicroscopie crystalline bundles lying approximately parallel to the fibre axis or inclined at a roughly constant angle to It. These polypeptide chains are extended,but In some cases,however,the chains are apparently colled and these may be extended by the application of a force. Ihe exact nature of the •coiled* structure is not known. According to some workers the polypeptide chain is folded in a series of seven membered rings, the folds of the chain being stabilised by hydrogen bonding* The folded form of a fibrous protein Is known as the «-forra and the extended as the B-form, To fibrous proteins belong such things as, silk,wool,hairs,natls,tendons,sauscle fibres,fibrin and i ' 2 collagen* The globular or corpuscular proteins are more compact than the fibrillar proteins* In this case the peptide chains are coiled and folded In to well defined patterns. The chains probably constitute thin layers with lnterspread layers of water molecules in between when the protein Is in solution. The colled structure is maintained by definite cross linkages of the types operative in fibrous proteins. A few proteins like lnsulln,h@moglobins, pepsin and albumins etc.belong to globulsr elass of proteins. Denaturatlon of the globular proteins changes their structure more towards the fibrillar type. They are also classified on the basis of their physical properties,particularly solubility,into three main groups, via.,simple ^conjugated and derived proteins. Simple proteins usually yield amino acids or their derivat ives on hydrolysis. The conjugated proteins are character ised by the presence of some non-protein substances attached to them In the form of prosthetic group removable on careful hydrolysis. Example of conjugated proteins are nucieoproteins,chromoprotelnt, glycoproteins tphosphoproteins, lipoproteins and metalloprotelns* Derived proteins are formed from simple and conjugated proteins and are the least well defined elass of proteins. The natural proteins are exceedingly diverse In structure and function. On one hand,they include relatively very inert structures like keratin of wool and hair or collagen of tendon and on the other hand they have numerous oosipoundt of high chemical and biological reactivity,including ensysaas,viruses and hormones. The latter comprises the anterior pltutary honames,urinary and serum gonadoproteins,posterior pltutary hormones,Insulin and thyroid hormones. They are effective in minute quantities. It is assumed that they behave like ensyraes at biocatalyst and hence do not contribute energy or satter In any signifi cant amounts during the process of physiological reaction In the body. Like cellulose,the proteins possets hi#* molecular weight. Anumber of methods have been employed for molecular weight determination* In recent years the molecular weight of many proteins have been determined by Svedberg,uslng ultracentrlfuge yielding values varying from 34,500 for egg albumin to 5,000,000 for the respiratory proteins of certain marine snails. The macromolecules of proteins are characterised by the presence of a balanced proportion of catlonlo and anionic groups hence are amphoteric in nature. All of them have a definite isoelectric pH depending on the proportion of acidic and basic groups. Proteins composed predominantly of the dicarboxyllo acids,will be more acidic at they will contain more carboxyiic groups than amino groups. Likewise, those having an excess of disminoaclds will be basic. Those 4 having an excess of free e&rboxyl groups,when dittolved in water,will have a negative charge,whereas those with excessive amino groups will be positive. The following table gives the isoelectric points of various class of proteinss