Secrets of protiens

Structural Uses of Globular Proteins:
Some structural proteins are globular. For example, microtubules, which function in a variety of ways inside the cell, are made up of globular proteins. They are long hollow tubes-so long that their entire length can seldom be traced in a single microscopic section.
They apparently act as internal skeletons, stiffening parts of the cell body. They also may serve as tracks along which substances can move inside the cell.
The formation of a new cellulose cell wall in a plant can be predicted by the appearance at the site of large numbers of microtubules; when cellulose fibrils are being laid down outside a plant cell membrane, as a cell wall forms or grows, it is possible to detect microtubules inside the cell aligned in the same direction as the fibrils outside.
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Chemical analysis shows that each microtubule consists of a very large number of subunits, each of which is a globular protein made up of two polypeptide chains. The two polypeptides fit together because of their complementary configurations, forming approximately spherical subunits.

The subunits assemble themselves into tubules, adding on length as required. When their job is over, they separate. Assembling requires an input of energy but just how it is triggered is not known.
Proteins are colloidal in nature but many of them can be crystallised. They are generally soluble in water, weak salt solutions, dilute acids and alkalies. Each protein has got a particular isoelectric point at which it is precipitated. These precipitates again dissolve when the reaction of the medium is shifted from isoelectric point or pH.
During precipitation proteins do not undergo any intramolecular change. They simply separate out because the medium is not favourable for solution. Most of the proteins undergo coagulation by heat or acid. Coagulation involves intramolecular change. There is another kind of change which the proteins undergo, called denaturation.
This may be done by many kinds of chemical or physical treatment, such as shaking, change of temperature, change of reaction, addition of neutral salts, etc. The exact nature of change undergone during denaturation is not known. It is probable that the molecular arrangement alters. All proteins show the characteristic properties of colloids.
Proteins not only differ from one another in the number and variety of amino acids they contain but also in chemical structure, physical and physiological properties. There are infinite varieties of proteins, each one differing from the rest in certain characteristics. The proteins of a particular tissue of an animal are quite different from those of the same tissue of another species.
Moreover, in the same species, each type of tissue contains proteins which are distinct from those found in the other tissues. No two proteins are found to be exactly the same in their physiological properties. This is another characteristic of proteins as a class, which are not found with the carbohydrates or fats. Vegetable proteins differ from animal proteins in the fact that the former is generally poorer in essential amino acids.