In the human body, it is estimated that there are more than 250,000 unique proteins that fulfill a variety of biological functions.6 Examples of biological functions that proteins fulfill within mammalian cells are as follows:
• Structural: proteins that provide the scaffold for tissues, cells, and sub-celluar organelles (e.g., skin, muscle, bone, blood vessels, cytoskeleton). Examples are collagen, a-keratin, actin and myosin, fibronectin, etc.
• Regulatory: protein hormones that carry messages from one part of the body to the other to help maintain homeostasis. Examples are insulin, thyrotropin, somatotropin, follicle-stimulating hormone, etc.
• Osmotic: proteins help regulate osmotic and pH balance in biological fluids. Examples are plasma albumins, immunoglobulins, lipoproteins, etc.
• Metabolism: protein enzymes catalyze a multitude of chemical reactions within cells. Examples are proteases that break down proteins, polymer-ases (which catalyze the synthesis of DNA and RNA), ATPases (which hydrolyze ATP, providing energy to support cellular reactions), etc.
• Transport: proteins that transport substances (lipids, vitamins, oxygen, etc.) throughout the body and into and out of cells. Examples are hemoglobin (which transports oxygen in the blood) and transferrin (which carries iron in the blood to various body tissues).
• Defense: coagulation proteins (which prevent blood loss) and immuno-globulins (which defend against invading pathogens such as viruses and bacteria). Examples are fibrin (which prevents blood loss following injury to the vascular system), and immunoglobulins and interferon (which protect the body against bacterial or viral infection).
• Motor function: motile proteins that allow cells to move, contract, or change shape; and permit muscle contraction, movement of chromosomes during cell division, and nerve axon transport. Examples are actin and myosin (which are involved in the contraction of muscle tissue), kinesin, and dynein motor proteins involved in movement of chromosomes and flagella.
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