Proteins (Greek proteios, “primary” or “of first importance”) are biochemical molecules consisting of polypeptides joined by peptide bonds between the amino and carboxyl groups of amino acid residues.
Proteins are the building blocks of life and there are estimated to be almost one million different proteins in a normal cell. Each protein has very different and unique physical properties.
*Proteins are colorless and usually tasteless. These are homogeneous and crystalline.
*Size. Proteins are very large polymers of amino acids with molecular weights that vary from 6000 amu to several million amu. Proteins are too large to pass through cell membranes, and are contained within the cells where they were formed unless the cell is damaged by disease or trauma.
*Structure. The proteins range in shape from simple crystalloid spherical structures to long fibrillar structures.
A. Globular proteins- These are spherical in shape and occur mainly in plants, esp., in seeds and in leaf cells. These are bundles formed by folding and crumpling of protein chains. e.g., pepsin, edestin, insulin, ribonuclease etc.
-dissolve in water or form stable suspensions.
-not found in structural tissue but are transport proteins, or proteins that may be moved easily through the body by the circulatory system
-e.g., hemoglobin and transferrin.
B. Fibrillar proteins- These are thread-like or ellipsoidal in shape and occur generally in animal muscles. Most of the studies regarding protein structure have been conducted using these proteins. e.g., fibrinogen, myosin etc.
-insoluble in water
-major components of connective tissue, elastic tissue, hair, and skin
-e.g., collagen, elastin, and keratin.
*Aggregation. Physical protein aggregation results from the association of unfolded proteins. According to the Lumry-Eyring model, the native protein is transformed in a transitional protein species that is prone to associate and to form protein aggregate. The protein aggregation pathway occurs in three steps:
(1) protein unfolding;
(2) association of unfolded monomers in oligomers; and
(3) nucleation, growth and condensation in aggregates
*Denaturation. Denaturation refers to the changes in the properties of a protein. In other words, it is the loss of biologic activity. In many instances the process of denaturation is followed by coagulation— a process where denatured protein molecules tend to form large aggregates and to precipitate from solution.
*Amphoteric. Like amino acids, the proteins are amphoteric, i.e., they act as acids and alkalies both. Depending on pH, they can exist as polyvalent (cations, anions or zwitter ions). These migrate in an electric field and the direction of migration depends upon the net charge possessed by the molecule. The net charge is influenced by the pH value. Each protein has a fixed value of isoelectric point (pl) at which it will move in an electric field.
By definition the net charge is zero and the total charge is maximal at the isoelectric point. Lowering or raising the pH tends to increase the net charge towards its maximum, while the total charge always becomes less than at the isolectric point.
*Solubility. The solubility of proteins is influenced by pH. Solubility is lowest at isoelectric point and increases with increasing acidity or alkalinity. This is because when the protein molecules exist as either cations or anions, repulsive forces between ions are high, since all the molecules possess excess charges of the same sign. Thus, they will be more soluble than in the isoelectric state.
Two different types of protein solubility are distinguished. True or thermodynamic solubility refers to the concentration of the protein in solution, which is in equilibrium with a crystalline solid phase of that protein. In contrast, apparent solubility corresponds to the concentration of protein in a solution, which is in equilibrium with a solid amorphous precipitate of the same protein.
*Optical activity. The optical activity of proteins is due not only to asymmetry of amino acids but also to the chirality resulting from the arrangement of the peptide chain.
General physical properties of protein
