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What Happens to Excess Amino Groups?

The amino group (-NH2) is a functional chemical group that is part of all amino acids. Free amino acids circulate in the blood in limited concentrations, just enough to be available to cells in the process of building new proteins and no more. When there are excess amino acids in the blood, the body breaks down amino acids and amino groups must be removed from the body.

  1. Amino Group Structure

    • The amino group (--NH2) consists of one atom of nitrogen covalently bonded to two atoms of hydrogen. The amino group has an extra valence electron at the nitrogen atom, which enables this molecular group to bond with another atom.

    Amino Acid Structure

    • Amino acids are so named because each molecule contains an acidic carboxyl (COOH) group and an amino (NH2) group. In addition, each amino acid has a side chain, known as an R group. All three groups are bonded to the same carbon atom, known as the alpha carbon. The simplest amino acid, glycine, has an R group that consists of a single hydrogen atom. Other amino acid R groups range from the simple methyl (-CH3) group in alanine to more complex hydrocarbons, acids, amides, alcohols, and amines.

    Amino Acid Catabolism

    • When amino acids undergo catabolism, the first step is the removal of the amino group (NH2) from the molecule. Once the amino group is removed, the remaining carbon skeleton of the amino acid is converted to energy or stored as fat. The amino group, on the other hand, represents excess nitrogen, which must be carried out of the body.

    Aquatic Animals

    • Ammonia is highly soluble in water, so fish and some aquatic amphibians are able to excrete nitrogen in the form of ammonia (NH3). In some species of fish, excess nitrogen is transported through the blood as part of the amide in the amino acid glutamine. In the gills, the glutamine undergoes hydrolysis to form glutamic acid and ammonia is released into the surrounding water.

    Urea Cycle

    • Most land-dwelling vertebrate animals convert excess amino groups to water-soluble urea, Urea with the chemical formula, (NH2)2CO, is a molecule with two amino groups attached to a carbonyl group (C=O). Synthesis of urea occurs in the liver cells. From the liver, urea is released into the blood stream, filtered from the blood by the kidneys, then expelled from the body in the urine.

    Uric Acid

    • Birds, reptiles and some desert-dwelling mammals, convert excess nitrogen to uric acid (C5H4N4O3) which is excreted in the feces. Synthesis of uric acid is costly in terms of metabolic energy but less water is required to remove it from the body.

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