Every living cell must have a supply of amino acids constantly available for diverse synthetic processes, especially protein synthesis. These amino acids may be derived by synthesis from simple substances (Glucose as a carbon source & ammonium ion as a nitrogen source (or) by absorption from the surrounding environment).
The biosynthetic pathways leading to amino acids and nucleotides share a requirement for nitrogen. Because soluble, biologically useful nitrogen compounds are generally scarce in natural environments, most organisms maintain strict economy in their use of ammonia, amino acids, and nucleotides.
Indeed, as we shall see, free amino acids, purines, and pyrimidines formed during metabolic turnover of proteins and nucleic acids are often salvaged and reused. We first examine the pathways by which nitrogen from the environment is introduced into biological systems.
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