Dietary protein is the main source of amino acids.  Amino acids can be used as fuel, but usually more important roles for them are as building blocks for proteins, and as a source of carbon and nitrogen for biosynthesis of other biochemicals.

        In the process of digestion, proteins are broken down to free amino acids in the gastrointestinal tract.  They are then absorbed and pass into the circulation, and are transported to liver where -NH2 groups are removed by transamination.  The resulting alpha-keto acid is then used as fuel, or as a biosynthetic intermediate.

        Amino acids are not stored in the body like fats or carbohydrate; there are no specialized cells in the body to maintain a reservoir.  Of course, amino acids are ubiquitous, being present in structural proteins, enzymes, transport proteins, etc.  Some of these proteins (notably serum albumin) can be degraded under conditions of fasting or starvation, to release free amino acids.

        Adult humans are unable to synthesize all twenty amino acids needed for protein synthesis; those which cannot be synthesized and which must then be acquired via the diet are referred to as essential.  The ten which the body can synthesize on its own are nonessential.

The ten essential amino acids for adults are*
*A useful mnemonic for this list is to read the first letter of each amino acid (highlighted), which gives "Pvt. Tim Hall".

        Dietary intake of amino acids is typically not balanced to exactly match the body's demands for various amino acids.  Amino acids taken via the diet must be chemically modified and rearranged to provide adequate levels of all the amino acids needed.  There are a large number of pathways in the body for balancing the pool of amino acids, both for synthesis and for degradation.  The number of enzymes involved creates a great potential for genetic diseases.  Furthermore, disruption (by mutation of just one enzyme) in the metabolism of only one amino acid can have profound consequences for growth and development; some of the genetic diseases are fatal.


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