1. As enzymes. Many proteins function as enzymes to catalyse specific chemical reactions. For eg trypsin, pepsin etc.
2. As carriers. Some proteins act as carriers which bind and transport specific molecules across a membrane or in a body fluid. GLUT-4 enables glucose transport into cells.
3. Nutrient and storage proteins. Seeds of many plants store nutrient proteins required for the growth of embryonic plant as in wheat, corn. Ovalbumin is the protein of egg and casein is the milk protein.
4. Contractile or motile proteins. Proteins actin and myosin are filamentous proteins found in the contractile system of skeletal muscles which help in the movement.
5. Structural proteins. Some fibrous proteins form supporting filaments. Tendons are formed of collagen proteins and ligaments of elastin; keratin is found in hair, nails and feathers.
6. Defence proteins. Immunoglobins or antibodies form immune system of the body. These recognize bacteria, viruses or foreign proteins.
7. Regulatory proteins. Certain hormones or antibodies are globular proteins (such as insulin). They regulate cellular or physiological functions.
8. Receptor proteins. Receoptors proteins help in the reception of smell, taste. hormone etc.
9. Blood clotting. Proteins like fibrinogens help in blood clotting.
There are three types on the basis of composition :
1. Simple proteins 2. Conjugated proteins 3. Derived proteins.
1. Simple proteins. Simple proteins consist of only amino acids or their derivatives.
2. Conjugated proteins. These consist of simple proteins in combination with some non-protein component. The non-protein groups are called prosthetic groups.
3. Derived proteins. These are derived from pre-existing proteins either by partial hydrolysis or by coagulation.
Characters |
A-DNA |
B-DNA |
Z-DNA |
1. Base pair per turn of the helix. 2. Tilt of base pairs 3. Axial rise (h) 4. Helical diameter (A) 5. Rotation of the double helix DNA. |
11 20.2 Å 2.56 Å 23 Å right handed |
10 6.3 Å 3.37 Å 20 Å right handed |
12 7 Å 3.7Å 18Å left handed |
According to Watson and Crick model of DNA :
1. The Double Helix consists of two polynucleotide chains, which are coiled like a rope in helical (spiral) fashion.
2. The two strands of polynucleotides are run in the opposite direction and are antiparallel.
3. The backbone is formed by the sugar-phosphate-sugar chain.
4. The nitrogen bases lie perpendicular to this
backbone but face inside.
5. A always pairs with T and G pairs with C. A and T are joined by two hydrogen bonds while the G and C pair have three bonds.
6. A base pair represents each step of ascent of the spiral. At each step the strand turns 36.
7. There are ten base pairs in a complete turn which has ten steps.
8. The pitch is 34Å. the rise per base pair is 3.4Å.
The Watson-Crick model of DNA.