Classification of proteins

• Based on structure • Based on composition • Based on functions

• Fibrous proteins • Globular proteins • Intermediate proteins

• Simple Proteins • Conjugated proteins

• Structural • Enzymes • Hormones • Respiratory pigments • Transport proteins • Contractile proteins • Storage proteins • Immunoglobulins • Toxins

• Linear • Tough & strong • Long parallel polypeptide chains cross linked at regular intervals • Perform structural functions in the cells • Usually do not have a (complex) tertiary structure • Insoluble in water • Most important structure: secondary structure

• Collagen • Elastin • Myosin • Keratin • Silk

• Spherical/globular shape • Polypeptide chain tightly folded into spherical shapes • Physically softer • Most proteins in cells are globular proteins • Function: form enzymes, antibodies and some hormones • Readily soluble in water • Most important structure: Tertiary Structure

• insulin • hemoglobin • albumin • DNA polymerase and RNA polymerase

• Intermediate shape more or less linear • Soluble in water

Blood clotting proteins

• Composed of only amino acids, relatively simple structure

• Myosin • Collagen • Keratin • Insulin

• Complex, contains one or more non-amino acid component called prosthetic groups, essential for biological function. • Usually globular and soluble in water • Most enzymes are conjugated proteins • Further classification based on the nature of the prosthetic groups