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Amino Acid Metabolism 2: Amino acid biosynthesis, amino acids are metabolites, metabolic genetic disorders. Bioc 460 Spring 2008 - Lecture 39 (Miesfeld). Albinism is caused by mutations in the tyrosinase gene which is required for pigment biosynthesis.

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Amino Acid Metabolism 2:Amino acid biosynthesis, amino acids are metabolites, metabolic genetic disorders

Bioc 460 Spring 2008 - Lecture 39 (Miesfeld)

Albinism is caused by mutations in the tyrosinase gene which is required for pigment biosynthesis

Roundup Ready soybeans are resistant to glyphosate and are sold by Monsanto

It was thought at one time that Count Dracula may have suffered from porphyria

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Key Concepts in Amino Acid Metabolism

  • Plants and bacteria synthesize all twenty amino acids, humans can only synthesize about half of the twenty amino acids.
  • In general, the more complex amino acids are essential amino acids in humans as they require enzymes that have been lost from the human genome over evolutionary time.
  • The herbicide glyphosate (RoundUp) is an inhibitor of the Shikimate Pathway responsible for the synthesis of aromatic amino acids.
  • Numerous genetic diseases are caused by defects in amino acid metabolic pathways; genetic diseases can be due to recessive or dominant mutations.
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Overview of Amino Acid Biosynthesis

The carbon skeletons of all twenty amino acids are derived from just seven metabolic intermediates, that together, are found in three metabolic pathways. These include:

three glycolytic pathway intermediates; 3-phosphoglycerate, phosphoenolypyruvate, and pyruvate,

two pentose phosphate pathway intermediates; ribose 5-phosphate and erythrose 4-phosphate

two citrate cycle intermediates; -ketoglutarate and oxaloacetate.

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Overview of Amino Acid Biosynthesis

Arginine is listed as an essential amino acid because humans require arginine in their diet to support rapid growth during childhood and pregnancy, even though it is made by urea cycle.

Tyrosine is also highlighted because this conditional nonessential amino acid is made in humans from the essential amino acid phenylalanine.

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In general, the structures of the essential amino acids are more complex than the nonessential amino acids which is reflected in the number of enzymatic reactions required for synthesis.

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It is critical that metabolic flux through various amino acid biosynthetic pathways be tightly regulated by feedback inhibition to provide the required proportions of each amino acid in response to cellular needs.

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Overview of Amino Acid Biosynthesis

The biosynthesis of three nonessential amino acids (alanine, aspartate and asparagine), and six essential amino acids (methionine, threonine, lysine, isoleucine, valine and leucine) in E. coli involves two interconnected pathways utilizing pyruvate and oxaloacetate.

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One of the most widely used herbicides is glyphosate, the active ingredient in Roundup®. Glyphosate is a competitive inhibitor of the enzyme EPSP synthase which is required to convert shikimate 3-phosphate to EPSP.

Since animals do not have the shikimate pathway enzymes, Roundup is an animal safe herbicide.

Do you think glyphosate works faster in the summer or winter? Explain.

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Roundup Ready Crops are Glyphosate-Resistant

Monsanto developed glyphosate-resistant crop plants so that farmers could spray their transgenic crops with Roundup (also made by Monsanto) and kill weeds that reduce crop yields without harming the crop plants.

The first glyphosate-resistant crop plant developed was a strain of soybeans marketed as Roundup Ready®soybeans.

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Amino Acids as Metabolic Precursors

Because of the nitrogen content of amino acids (the -amino group), they are also used as metabolic precursors for numerous biomolecules, including:

• Heme groups (hemoglobin and cytochromes),

• Nucleotide bases (purines and pyrimidines)

• Signaling molecules(neurotransmitters, hormones, nitric oxide).

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Amino Acids as Metabolic Precursors

Tyrosine is the precursor to several important molecules in metabolic signaling and neurotransmission, including epinephrine and dopamine.

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Tyrosine is also the precursor to pigment molecules called melanins that are produced from dopaquinone.

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Amino Acids as Metabolic Precursors

Natural loss of hair color occurs as a result of aging when melanin production shuts down in human melanocytes located near the base of hair follicles and these defective cells are not replaced as they normally are in younger individuals.

Gray hair can be colored by treating it with a mixture of hydrogen peroxide and an ammonia based solution containing artificial pigments.

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Inborn Errors of Metabolism: Genetic Disease

A genetic defect in the gene encoding phenylalanine hydroxylase is responsible for the metabolic disease phenylketonuria (PKU).

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The clinical symptoms of PKU are caused by the accumulation of phenylalanine in the blood that is 30-50 times higher than normal.

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NutraSweet contains a phenylalanine derivative

Phenylketonuriacs have to be careful to avoid processed foods and beverages containing the food additive aspartame (aspartyl-phenylalanine methyl ester).

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PKU is an autosomal recessive genetic disease

The phenylalanine hydroxylase gene is located on chromosome 12 making it an autosomal recessive genetic disease.

What would the probability be of having a PKU afflicted child if the mother was normal (PP) and the father was a carrier (Pp)?

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Type 1 albinism is an autosomal recessive genetic mutation in the tyrosinase gene

Interestingly, individuals with phenylketonuria can have light skin and hair at birth because of low levels of tyrosine.

However, phenylketonuriacs are not albinos because they obtain sufficient amounts of tyrosine in their diets to support melanin biosynthesis.

Why is PKU treatable, but albinism is not, even though both are the result of genetic mutations in enzymes?

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Congenital porphyrias effect heme biosynthesis and can be the result of dominant mutations

How might a mutant protein cause a dominant (gain of function) phenotype?

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Congenital porphyrias effect heme biosynthesis and can be the result of dominant mutations

It is thought that King George III of England may have suffered from acute intermittent porphyria based on descriptions of his health that were written at the time of the American Revolution.

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Congenital porphyrias effect heme biosynthesis and can be the result of dominant mutations

A more rare form of porphyria is congenital erythropoietic porphyria that is characterized by red urine, reddish brown fluorescent teeth, sensitive to sunlight, and otherwise sickly (anemic).