The huge variety of different proteins imparts to each species its own specific characteristics, and each organ within an animal differs in the nature of it? protein content.
All the enzymes without which the metabolic processes cannot go on, are protein bodies. Proteins are also present in the form of hormones which play vital roles in intracellular chemical processes.
The phenomena of muscular contraction are also connected with proteins, myosin and actin.
The blood contains oxygen carriers, they are also protein in nature. In the red blood cells, the conjugated protein haemoglobin is the means of transporting oxygen to the cells and helps in regulating acid-base balance in the body.
Proteins of blood plasma serve a number of purposes including regulation of water, electrolyte balance, providing antibodies. The proteins of the body, like carbohydrates and lipids, serve as a source of energy.
Proteins are highly specific. Each organism and each tissue have proteins which differ from those that form part of other organisms and other tissues.
The main significance of proteins is that they provide the material from which the cells and intercellular substances are built and substances which take part in the regulation of physiological functions, are synthesized.
However, proteins are in some measure used along with carbohydrates and fats to cover the energy output. This shows that proteins are essential compounds from the biological point of view.
During the process of digestion proteins contained in the food are hydrolyzed to a-amino acids under the influence of proteolytic enzymes found in the digestive tract. Phosphoric acid and nucleic acid (subject to further break down) may also be formed, but the a-amino acids are the primary upits liberated.
The requirement for proteins is fundamentally the requirement for amino acids because proteins are rebuilt from the amino acids.
It should be possible to express the protein’s requirement of an animal in terms of the amount and kind of ammo-acids rather than the protein itself.
The a-amino acids derived from the digestion of protein are soluble in water in the ionic form.
They are normally absorbed through the lumen of the small intestine and carried directly to the liver via the portal blood stream where the necessary proteins can be synthesized from the a-amino acids and sent, via the blood stream, as plasma protein to whichever part of the body needs them.
There is also evidence that a-amino acids circulate as such in the blood : stream and may go in this simple form to the various tissues.
By a process of rigid selection, the various parts of the body take or synthesize the proteins of particular shape and composition that they need and incorporate them as tissue.
Amino acids like carbohydrates and fats are not stored in the body. However, after a rich protein meal at the end of a fast, temporary storage of amino-acids may take place in the liver and in other tissues and is followed by a limited increase in protein in the tissues which may be regarded as restoration of protein lost during fasting.
Amino acids, which are not required for growth, tissue repair or the construction of important secretions and enzymes pass into the fuel system so that they may be metabolized to liberate energy.
Their fate depends upon the amount of protein in the diet, the age of the animal and the other sources of fuel available to it.
In emergencies amino acids may be withdrawn from the protoplasm of the cell and thus during starvation the cellular protein content declines.
Different tissues synthesize their respective proteins from the amino acids available in blood stream.
A certain proportions is utilized towards the renewal of worn-out protoplasm where as the surplus amino acids are simplified into excretable nitrogenous wastes after undergoing deamination and transamination processes.
They are broken down to (i) the non-nitrogenous part and (ii) the nitrogenous part. The non-nitrogenous part enters the Krebs cycle and is oxidized to COa and H2O, thus furnishing energy to the body or is converted into body fat where as the nitrogenous part goes to form urea or uric acid or ammonia which is excreted in the form of urine in animals.
During the metabolism of amino acids there is an increase in the production of heat by the body which Rubner referred to as specific dynamic action.
It is apparently due to reactions which the amino acids undergo in the liver, or to the stimulation of other reactions in the cells by products formed in the liver.
The phenomenon of specific dynamic action does not occur after removal of the liver.