Problem Set 21-Phenylalanine and Tyrosine



Please read sections 21.1-21.5 of CBM 2nd Ed. before proceeding.


a. Phenylalanine is an essential  amino acid, but tyrosine is not.

b. Phenylalanine is converted to fumaric acid and acetoacetic acid.

c. Infants with the genetic disease, alkaptonuria, excrete homogentisic acid and accumulate abnormal levels of 4-hydroxypyuvic acid and 4-hydroxyphenyllactic acid.

1.  Construct tentative map for catabolism of phenylalanine and tyrosine.  HINT: Start by working backwards.

2. There are two possible reactions for decarboxylating 4-hydroxyphenylpyruvic acid. One requires TPP;  the other is catalyazed by an oxidase.

a. Write a mechanism for the oxidase reaction.

b. Do either of these enzymes exist and if so, in which organisms?

c. What conclusions can you draw?

3. The widely distributed enzyme, 4-hydroxphenylpoyrvate dioxygenase (EC, catalyzes the conversion of 4-hydroxyphenylpruvate to homogentisic acid without releasing any intermediates.  Write a mechanism for the reaction Both oxygen atoms must end up in homogentisic acid.

4. Construct a map for catabolism of phenylalanine in Homo sapiens

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