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10: Nitrogen metabolism and the urea cycle

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    • 10.1: Amino Acid Metabolism
      The pathways for the synthesis and degradation of amino acids used in proteins are the most varied among the reactions synthesizing biological building blocks. We start with some terms. First, not all organisms can synthesize all the amino acids they need. Amino acids that an organism cannot synthesize (and therefore must have in their diets) are called essential amino acids. The remaining amino acids that the body can synthesize are called non-essential.
    • 10.2: Amino Acid Catabolism
      Breakdown of glutamine by glutaminase is a source of ammonium ion in the cell. The other product is glutamate. Glutamate, of course, can be converted by a transamination reaction to alpha-ketoglutarate, which can be oxidized in the citric acid cycle.
    • 10.3: Urea Cycle
      Yet another cyclic pathway important in cells is the urea cycle (Figure 7.5.1). With reactions spanning the cytoplasm and the mitochondria, the urea cycle occurs mostly in the liver and kidney. The cycle plays an important role in nitrogen balance in cells and is found in organisms that produce urea as a way to excrete excess amines.

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