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Table 1 Functions of proteins

From: Proteins and amino acids are fundamental to optimal nutrition support in critically ill patients

  Proteins are the major components of muscles, required for muscle dynamics and function
  Enzymes are proteins. Therefore, proteins are essential for intermediary metabolism and energy production. Similarly, all cell carriers are proteins
  Some proteins are involved in specific immunity (that is, immunoglobulins) and in nonspecific immunity (for example, opsonins)
  Proteins contribute to the architecture and structure of organs and tissues. A typical example is collagen, which has a major architectural role, for example, in bone and skin
  Proteins secreted into the blood by the liver are carriers of lipid-soluble molecules: hormones (for example, transthyretin for thyroxin), vitamins (for example, retinol binding protein for vitamin A), nutrients (for example, albumin for free fatty acids and tryptophan), and a number of drugs
  Proteins in the blood, especially albumin, are involved in the control of oncotic (colloid osmotic) pressure
  Proteins contribute physiologically to energy expenditure (12 to 15% of total daily expenditure) in the postabsorptive state, through release of amino acids following proteolysis. This may occur directly (for example, branched-chain amino acids in the muscles) or indirectly (through glucose (gluconeogenesis) or ketone body (ketogenesis))