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Aromatic Family: Phenylalanine and Tyrosine

by Kevin Ahern, PhD
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    So phenylalanine is an essential amino acid. That means, of course, it has to be in our diet. It is also a precursor of the amino acid tyrosine. Now phenylalanine metabolism requires the enzyme phenylalanine hydroxylase. That catalyzes the formation of tyrosine form phenylalanine in the reaction that we can see here on the right. In this reaction, phenylalanine is getting a hydroxyl group. And this is a complicated hydroxylation reaction that involves tetrahydrobiopterin and molecular oxygen. The products of that reaction being dihydrobiopterin and water, plus the amino acid phenylalanine and you can see the different between the phenylalanine and the tyrosine is the addition of that hydroxyl group. A deficiency of this enzyme, phenylalanine hydroxylase, is very, very important because this enzyme when it's deficient causes the disease known as phenylketonuria. Fortunately, phenylketonuria can be treated if it's diagnosed early and it's commonly diagnosed at birth. If it's not diagnosed and not recognized early enough, then high levels of phenylalanine in the diet damage the brain and you get sever neurological consequences. Now fortunately, as I said, it's treatable by reducing the amount of phenylalanine that a person gets in their diet, and this works fairly well. But one of the complicating factors is that the artificial sweetener, Nutrasweet, contains phenylalanine. So a person who has phenylketonuria should absolutely not consume Nutrasweet, and that's why you see the warning on cans of soda, for example, that contain Nutrasweet. Tyrosine is an amino acid whose essentialness or not essentialness depends upon whether or not phenylalanine is present. If it's present in the diet, then tyrosine is not essential because tyrosine can be made from phenylalanine. Tyrosine is important as a precursor of catecholamines. So just as tyrosine was a product of phenylalanine breakdown, the catecholamines are a product of...

    About the Lecture

    The lecture Aromatic Family: Phenylalanine and Tyrosine by Kevin Ahern, PhD is from the course Amino Acid Metabolism. It contains the following chapters:

    • Phenylalanine
    • Tyrosine

    Included Quiz Questions

    1. It can cause brain damage in high amounts.
    2. It is a non-essential amino acid
    3. It is made from tyrosine
    4. All of the answers are true.
    5. None of the answers are true.
    1. All of the answers are true.
    2. It is essential only in the absence of phenylalanine.
    3. It is important in chlorophyll.
    4. It is an important precursor of neurotransmitters.
    5. None of the answers are true.

    Author of lecture Aromatic Family: Phenylalanine and Tyrosine

     Kevin Ahern, PhD

    Kevin Ahern, PhD


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