Within the cylindrical segments, the polypeptide chain is arranged in an alpha-helix. Figure 2-18 Protein Structure. The quaternary structure develops when separate polypeptide subunits interact to form a larger molecule. A single hemoglobin molecule contains four globular subunits, each structurally similar to myoglobin. In keratin and collagen, three fibrous subunits intertwine.
Keratin is a tough, water-resistant protein found in skin, hair, and nails.
Collagen is the principal extracellular protein in most organs. They are a structural element of the membrane. Peripheral Membrane Proteins - bind to the surface of the membrane. Carbohydrates - chains of sugar molecules attached to membrane phospholipids and proteins making them glycolipids or glycoproteins.