PHYSIEO-CHEMICAL STUDIES ON THE INTERACTION OF PROTEINS WITH POLYACIDS, METAL COMPLEXES AND SURFACTANTS

Abstract

Proteins are naturally occurring macromolecu] es consisting of one or more than one polypeptide chains. On hydrolysis under suitable conditions they are broken down into *< -amino acids. A complete and exact classification of proteins is very difficult. In fact the variety in chemical composition, structure and configuration is so great that classifications put forward so far cover only part of this variety. However, broadly the proteins can be classified into globular (also called corpuscular) and fibrous (linear) pro teins. The former in cells and tissues serve as transoort agents or as active enzymes and harmones, while the latter serve as structural materials. The globular proteins are further classified into simple and complex proteins. In complex proteins a non-protein component is found in the form of prosthetic groups. The examples of this class of oroteins are respiratory proteins, certain enzymes and harmones, lipo proteins etc. Some simple proteins have also been found to contain prosthetic groups. It has been found that all of the fibrous proteins consist of long peptide chains organised into submicroscooic crystalline bundles lying approximately parallel to the fibre axis or inclined at a roughly constant angle to it. In globular proteins the peptide chains are coiled and folded into welldefined pattern. The coiled or rolled up structure is maintain ed by various forces operative in proteins. As a preliminary step to a more detailed knowledge of the structure of a protein, the knowledge of aminoacids 2 constituting the polypeptide chains and their sequence is important. The aminoacid analysis is generally carried out by acid hydrolysis and identification through chromotographic or ion exchange method. The real break through in determining the sequence of aminoacids in proteins was made only in late forties when Sanger(l) introduced his method of dinitrophenylation of N terminal amino acids. Methods apoeared also for determining the sequence of amino acids starting from other side of the chain i.e. -C terminal (2,3). Usually the large polypeptide chain is chopped into smaller frag ments (simpler polypeptide), which are studied separately, and the results of all fragments are combinedly put into an integrated orderU). It may be pointed out that diffe rences in aminoacid sequence has been noted in the same protein but with different origins(s). In the words of Crick(6) "Biologist should realise that before long we shall have a subject called protein taxonomy-the study of the aminoacid sequences of the proteins of an organism and the comoarision of them between species. It can be argued that these sequences are the most delicate expression possible of the phenotype of an organism and that vast amounts of evolutionary information may be hidden away within them". Besides amino acid sequence, the other impor tant aspect of the protein structure is the configuration of the peptide chains and their organisation into protein structures, which are characteristic in both properties and biological functions. It is convenient to divide the 3 protein structure into three levels of organisation: The primary structure refers to the sequence and configuration of aminoacid residues into polypeptide chains as well as to the nature of covalent cross linkages that may bind two or more chains together or hold one chain in more limited conformation(7). It has been found that in certain cases the chain assumes the shape of soiral or helix, and the configuration is stabilized by intramolecular hydrogen bonding between carbonyl and imino groups of polypeptide chain. This repre sents the so called secondary structure of protein. The helical configuration of the chains has been established in a few linear proteins such as collagen (8), while hydrogen bonding between adjacent chains has been noted in such fibrous proteins as silk(9). Some additional considerations are required for the structure and configuration of globular proteins, since the chain or chains must be folded in order to have a compact globular shape. This folding in each globular nrotein is specific and different and must have a definite pattern. What these patterns are in each case is not known. Only in the case of myoglobin it has been established by X-ray structural analysis that this protein is folded several times, but it is difficult to see the exact configuration at the points of folding, nor is it possible yet to allocate various side groups Rl, R2...(l0). The factors responsible for the stabilization of this folded structure, known as 4 tertiary structure, is assumed to be •lectroitatic(ll) as well as hydrophobic forces encountered in the protein mole cule (12,13-14). Protein molecules are delicate structures and the integration of the structure of proteins, as it exists in natural state, is dependent on the organisation of secon dary and tertiary structures (configuration). Distortion in these structures by the use of high temperature, of pH values . ' too far from neutrality, of most organic solvents, or of many quite mild reagents vathout affecting the primary struc ture is known as denaturation. Various criteria are used to ascertain denaturation. Two important ones are increase in viscosity(l5-17) and optical rotation(l8-20). Denaturation of globular proteins changes their structure more towards the fibrillar type. The high reactivity of oroteins with large number of inorganic cations and anions, detergents^ and other organic compounds has made a great impact on various industries, e.g., artificial fibres, plastics, adhesives, tanning etc. The reaction of detergents with protein unfolds the native struc ture of the latter resulting in the formation of elastic and highly double refracting fibres(2l). Fibres can be pre pared from egg albumin, zein, wheat glutamin, casein and blood albumins etc,(22-26). In general, protein fibers have pleasing appearance and good affinity for dyes. The reaction of formaldehyde with proteins is of immense importance. It converts casein, soybean and a number of other proteins into inert horny masses which are 5 produced commercially as plastics, '//hen it reacts with a protein molecule, it forms methylene bridges between chains and thus alters the mechanical properties of the protein. Various patents, from time to time, have apoeared in the literature for preparing casein fibres (27), casein wool and casein filaments