Nature of Protein and Its Classification
Proteins are highly complex molecules which contain the elements of carbon, hydrogen, nitrogen, and usually sulfur or sulphur. They are synthesized by living cells and are an essential part of the structure of the cell and its nucleus. The plant proteins are more easily isolated in crystalline form. Proteins are stored in plants in the form of aleurone grains. They are required for animals as the source of nitrogen in food.
The molecular weight of most proteins can be estimated only approximately by centrifugal sedimentation methods for soluble proteins, which vary from few thousand to many millions. Other methods are osmotic pressure measurements, X-ray diffraction, light scattering effects, gel filtration and chemical analysis.
Soluble proteins form colloidal solutions which are generally viscous and may form gel. Many of them (e.g. egg albumin) are coagulated by heat or action of acids, alkalis or certain chemicals or by U.V. light. This process is known as 'denaturation' in which solubility of the protein i.e. decreased.
Proteins are hydrolyzed to form simpler substances and ultimately amino acids. The hydrolytic process takes place during digestion by proteolytic enzymes or in laboratory by hot acids or alkalis:
Protein —> Polypeptide -4 Peptide —> Amino acids
Proteins are built up from amino acids linked together in chains or rings. The carboxylic group of one molecule is condensed with an ammo group of the adjacent molecule with the formation of the amide linkage commonly known as peptide linkage. The hemolytic nature of experimental protein deprivation (PD) anemia has been established.
Natural compounds formed in this way are called as 'polypeptide' if they have molecular weight below 10,000. Molecules with molecular weight above - 10,000 are known as proteins. In general, proteins and polypeptides differ in chemical and physical properties. Both type of compounds often exhibit physiological activity as in case of enzymes and hormones.
Recently, studies on nitrogen balance provided more accurate ways to measure the body's protein requirements. Joel Fuhrman, M.D. in his book Eat to Live writes that an easy way to calculate your own daily protein requirement according to the U.S. RDA is to multiply 0.36 (grams) by your body weight. That translates to about 44 grams for a 120-pound woman and 54 grams for a 150-pound male. In metric terminology the RDA is 0.8 grams per kilogram of body weight.
Nearly every function in the body is controlled or promoted by some type of protein. Excluding proteins called antibodies the human body is capable of making about 31,000 different types of proteins. What makes a protein unique is its chemical properties .
Proteins may be classified as :
(i) Fibrous proteins which serve as structural materials for animals, e.g., collagen, horn, feathers, nails, fibroin (silk), and
(ii) Globular proteins which are soluble in water and dilute acids and alkalis.
In another classification proteins are divided as:
(i) Simple proteins, which yield only amino acid on hydrolysis, e.g., protamines, albumin, globulins, etc.,
(ii) Conjugated proteins, which contain a non-protein group. known as prosthetic group, e.g., nucleoproteins. chromoproteins, glycoproteins, phosphoproteins, lipoproteins, and metalloproteins, and
(iii) Derived proteins, which are the degradation products obtained by the action of acids, alkalis or enzymes on proteins. For example, denatured proteins, metaproteins, secondary proteoses, peptones, polypeptides, simple peptides and amino acids are the derived proteins.
But when a particular type of protein is present in a cell, there usually will be millions of copies of it. Think of the population of proteins in a cell as a small army of workers, each of which is equipped to do exactly the same job.
Since proteins are present in all living organisms, they are of great importance in biochemistry. They form an important class of food. e.g. meat, fish and egg are important source of animal proteins. Cereal grains, e.g.. wheat, pulses, etc.. are plant protein foods. Whole glandular products, oil-bearing plant seeds, antitoxins, serums, and globulins contain proteins in combination with other biochemical substances. These products possess therapeutic activity. Allergens are usually proteinaceous materials producing allergic reactions.
Certain proteins are highly poisonous. Among them are plant toxalbumins, ricin from castor beans, robin from locus: bark, abrin from jequirity seeds, hemolysins from salamanders and various toxins, e.g. neurotoxins from snake venom.
Researchers compared organic molecules preserved in the T. rex fossils with those of living animals, and found they were similar to chicken protein. A US team of researchers have published the finding in Science journal.The team says their technique could help reveal evolutionary relationships between other living and extinct organisms.