Proteins

Proteins are the main structural components of living organisms. These are mainly made up of C, H, and O. Some contain P, and S, while few contain Fe, I, and Mg. Proteins make the 50% of the dry weight of the cell. These are present in all parts of the cell and in all cells.   

Structure of proteins:

            Proteins are chemically the polymers of amino acids or one or more polypeptide chains. The amino acids are the building blocks of the amino acids. 170 types of amino acids are found in cells. Only 25 out of them are used in making proteins. However, most proteins are made up of 20 types of amino acids.  The number of amino acids in a protein varies from a few to 3000 or even more in some proteins. All the amino acids are built in a common plane. Proteins are formed by the condensation of the amino acids to form dipeptides and then tripeptides and so on under the instruction of mRNA during the translational process.  A simple amino acid has a nitro group as well as a carboxyl group.

 The central carbon is called the alpha carbon

Organization of proteins:

  Every protein has a specific sequence of amino acids and a specific shape in which its molecules are bent. There are four levels of organization of proteins:

• Primary structure:  This level determines the sequence and the number of amino acids in a polypeptide chain. Every protein has shown this structure at the time of its synthesis on the ribosomal surface. After its synthesis, it does not remain in its primary structure. for example, F-Sanger determined the sequence of the amino acids of insulin after his ten-year careful work. he concluded that insulin is made up of 51 amino acids in two chains which are held together through a disulfide bridge. One chain is made up of 21 amino acids while the second chain is made up of 30 amino acids. Hemoglobin is made up of 574 amino acids with four chains two beta and two beta chains. Each alpha chain is made up of 141 amino acids while each beta chain consists of 146 amino acids. The size of the protein is determined by the number and type of amino acids.
• Secondary structure:   The chains of amino acids in proteins molecule usually do not lie in straight chains. These usually coil into alpha-helix, beta-pleated sheets, or into some other configurations. Actually, the secondary structure is formed by forming hydrogen bonds. The common secondary structure is the alpha helix which is the spiral formation of amino acids in a uniform geometric shape with 3.6 amino acids in each turn of the helix. The beta-pleated sheet is formed by folding the backs of the polypeptides.
• Tertiary structure:  Usually polypeptide chain folds itself again and again by establishing the ionic bonds and disulfide bonds to form a globular shape. This tertiary confirmation of the proteins. for example, in an aqueous solution, the most stable tertiary confirmation is that in which hydrophobic amino acids are buried inside while hydrophilic amino acids are on the surface of the molecule.

• Quaternary structure:  In many highly complex protein molecules, territory polypeptide chains are aggregated and held together by hydrophobic interactions with the addition of hydrogen and ionic bonds. This is a quaternary structure that is exhibited by antibodies and hemoglobins.

Classification of the proteins:

            Proteins are classified into two groups:

•    Fibrous proteins: The protein molecules have one or more polypeptide chains in the form of fibral or filaments. The secondary structure is important in fibrous proteins. These are insoluble in water or in aqueous media, non-crystalline, and elastic in nature. These proteins perform a structural role in cells and in organisms. for example, collagen, silk fiber from silkworms and spiders' webs, actin and myosin in muscle cells, fibrin in blood clotting, and keratin in nails and hair.
•    Globular proteins: These proteins are globular or spherical or ellipsoidal in shape due to multiple folding of the polypeptide chains. Tertiary structure is most important in them. These are soluble in aqueous media, crystalline in nature, and non-elastic in nature. These are disorganized with the changes in the physical and physiological environment. for example, antibodies, enzymes, hormones, channel proteins, and hemoglobin.  

The function of the proteins:

• Proteins build many structures of the cell.
• All enzymes are proteins and in this way, these control the whole metabolism
• As the hormones and proteins regulate the metabolic process.
• Movement of organs, organisms, and chromosomes during anaphase is caused by proteins.
• Some proteins act as carriers and these transport the substance from one place to another in the body. for example, hemoglobin transports oxygen, lipids, and metal ions.
• Some Proteins perform the structural role in the body. for example, Elastin ( tendons, and ligaments) provide support to the connective tissue.
• Some Proteins perform a functional role in the body. for example, Ovalbumin is found in egg whites parts and casein is a milk-based protein. Both of them are involved in the storage of amino acids.

Significance of the sequence of the amino acids:

               The sequence of amino acids in a protein is highly specific for the functioning of the protein. If any of the amino acids do not occupy its proper place, then the protein fails to perform its function. The sequence of the amino acid is the characteristic feature of the primary structure.  The sequence is determined by the nucleotide sequence of the DNA. For example, in sickle cell anemia, which is a hereditary disease,  only one of the amino acids does not occupy its proper place, and The glutamic amino acid in beta chains of the hemoglobin at the position number "6" is replaced by valine due to point mutation in the beta-globin gene. and the shape of red blood cells becomes like a sickle or crescent and hemoglobin fails to carry oxygen leading to the death of the patient.