These are hydrophilic amino acids that are positively charged at neutral pH. The imidazole group of his-tidine has a pKa just below 7, so it is weakly ionized at physiological pH, giving it some buffering capacity and making it useful at the active site of many enzymes. Histidine is also a precursor for the physiologically active amine histamine. Arginine is an intermediate in the urea cycle and a precursor for polyamine synthesis. It is also the precursor for nitric oxide, which appears to have many physiologically important properties, including that of an endothelial-derived relaxing factor and a cell signaling molecule in the coordination of the inflammatory response. Although arginine, like glutamine, is a nonessential amino acid, there is evidence that increasing the dietary supply of arginine can improve clinical outcome in critically ill patients. Lysine is the limiting amino acid in cereals and cereal-based diets.
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