Proteins are the complex organic molecules containing carbon, hydrogen, oxygen and nitrogen. However, some proteins also contain phosphorus and sulphur. The fundamentals unit in the case of proteins are amino acids. Proteins are macromolecules with molecular masses generally more than 1000 a.m.u.
There are about 20 amino acids that combine in different ways to form different types of proteins. Amino acids links through peptide linkage to form protein molecules. Most of the proteins which are required to perform different functions in our body are synthesized from the unbonded amino acids. However, there are certain proteins which we directly obtain from food.
Every species of animals has its characteristic proteins. It is the proteins that give each species its specific characters and uniqueness. Plants can synthesize all the amino acids they need, from simple inorganic chemical compounds, but animals are unable to do this.
Therefore, in order to obtain the amino acids necessary for building proteins they must eat plants or other animals which have in their turn lived on plants. When food is digested, the proteins present in it are broken into constituent amino acids molecules. During the digestion the peptide linkage that joins the amino acids in proteins gets hydrolysed. Digestion of proteins takes place in the small intestine and the amino acids produced in the process are absorbed from the intestine by the blood. These amino acids are then regrouped to from specific proteins in the cells of our body.
Essential Amino Acids
The human body has certain limited powers of converting one amino acid into another. This is achieved in the liver under the influence of certain enzymes. However, there are several amino acids which our body cannot make for itself and so must obtain from the diet. These amino acids are known as essential amino acids. Thus essential amino acids are the amino acids which our body cannot synthesize by itself. There are 8 essential amino acids.
Functions of Proteins
Proteins are the basis of protoplasm and hence they occur in all living organisms. Some of the different forms of proteins and their important functions are given below:
1. Proteins as muscle, skin, hair, and other tissues constitute the bulk of body’s non-skeletal structure. For example, the protein keratin is present in hair and nails.
2. Some proteins as hormones regulate many body functions. For example, the hormone insulin is a protein. It regulates sugar level in the blood.
3. Some proteins as enzymes catalyse or help in biochemical reactions. For example, pepsin and tripsin.
4. Some proteins act as antibodies, and protect the body from the effect of invading species or substances.
5. Transport proteins carry different substances in the blood of different tissues. For example, haemoglobin is a transport protein. It carries oxygen from lungs to the tissues and carbon dioxide from tissues to the lungs.
6. Contractile proteins help in contraction of muscle and other cells of our body. Myosin is an example of contractile protein.