The proteins are all remarkably similar in structure insofar as they contain amino acids. But as little is known so far regarding their structure, a classification based totally on this criterion is not possible. However, a few systems based on one or the other criterion are given below.
CLASSIFICATION BASED ON THE SOURCE OF PROTEIN MOLECULE
Since long, the proteins have been traditionally divided into two well-defined groups: animal proteins and plant proteins. Animal proteins are the proteins derived from animal sources such as eggs, milk, meat, and fish. They are usually called higher-quality proteins because they contain (and hence supply) adequate amounts of all the essential amino acids. On the other hand, plant proteins are called lower-quality proteins since they have a low content (limiting amount) of one or more of the essential amino acids.
The four most common limiting amino acids are methionine, lysine, threonine, and tryptophan (Table 10–1). Although plant proteins have limiting amounts of some (but not all) amino acids, it should not be construed that they are poor protein sources
CLASSIFICATION BASED ON THE SHAPE OF PROTEIN MOLECULE
On the basis of the shape of the protein molecule, the proteins have been grouped under two categories: globular and fibrous.
1. Globular or Corpuscular Proteins.
These have an axial ratio (length: width) of less than 10 (usually not over 3 or 4) and, henceforth, possess a relatively spherical or ovoid shape. These are usually soluble in water or in aqueous media containing acids, bases, salts or alcohol, and diffuse readily. As a class, globular proteins are more complex in conformation than fibrous proteins, have a far greater variety of biological functions and are dynamic rather than static in their activities. Tertiary and quaternary structures are usually associated with this class of proteins. Nearly all enzymes are globular proteins, as are protein hormones, blood transport proteins, antibodies, and nutrient storage proteins.
A simple functional classification of globular proteins is not possible because of 2 reasons :
(a) Firstly, these proteins perform a variety of different functions.
(b) Secondly, many widely-differing globular proteins perform almost similar functions.
However, Conn and Stumpf (1976) have classified globular proteins as follows:
- Cytochrome C
- Blood proteins
- Serum albumin
- Globular Antibodies (= Immunoglobulins)
- Proteins Hemoglobin
- Hormones Enzymes
- Nutrient proteins
2. Fibrous or Fibrillar Proteins.
These have axial ratios greater than 10 and, henceforth, resemble long ribbons or fibers in shape. These are mainly of animal origin and are insoluble in all common solvents such as water, dilute acids, alkalies, and salts and also in organic solvents. Most fibrous proteins serve in a structural or protective role. The fibrous proteins are extremely strong and possess two important properties which are characteristic of the elastomers. These are:
(a) They can stretch and later recoil to their original length.
(b) They have a tendency to creep, i.e., if stretched for a long time, their basic length increases and equals the stretched length but, if the tension on the two ends of the fibril is relaxed, they creep to their shorter and shorter length. A large scar, for example, creeps to a smaller size if there is no tension on the scar.
On the contrary, if the scar is in a region of high tension, the scar becomes larger and larger as happens in the skin of a person gradually becoming obese. It is a heterogeneous group and includes the proteins of connective tissues, bones, blood vessels, skin, hair, nails, horns, hoofs, wool, and silk. The important examples are
- I. Collagens. These are of mesenchymal origin and form the major proteins of white connective tissues (tendons*, cartilage) and of bone. More than half the total protein in the mammalian body is collagen; acted upon by boiling in water, dilute acids or alkalies to produce the soluble gelatins; unique in containing high contents (12%) of hydroxyproline; poor in sulfur since cysteine and cystine are lacking.
- II. Elastins. Also of mesenchymal origin; form the major constituents of yellow elastic tissues (ligaments, blood vessels); differ from collagens in not being converted to soluble gelatins.
- III. Keratins. These are of ectodermal origin; form the major constituents of epithelial tissues (skin, hair, feathers, horns, hoofs, nails); usually, contain large amounts of sulfur in the form of cystine– human hair has about 14% cystine.
- IV. Fibroin. It is the principal constituent of the fibers of silk; composed mainly of glycine, alanine and serine units.
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