Jane is a college freshman who has PKU, an autosomal recessive genetic disorder. She is taking a biology course where she is learning about genetics, and the information is helping her to better understand her condition. Her typical diet consists of fruit, vegetables, low-protein breads and pasta. She also drinks a phenylalanine-free formula every day that contains all the amino acids except phenylalanine, plus vitamins and minerals. If she goes on a “normal” diet, she says she, “just can’t think straight. I can’t remember anything, I can’t concentrate, I can’t even pay attention in class. Biology was impossible when I tried eating a “normal” diet.’ Her lunch today consists of a baked potato, broccoli, and the phenylalanine-free formula.
- What enzyme is missing for Jane that prevents her from consuming phenylalanine?
She is missing phenylalanine hydroxylase which converts phenylalanine to tyrosine.
- Where is the gene for that enzyme found? (i.e., what chromosome, etc.?)
PKU is an autosomal recessive genetic disorder PAH gene , located on chromosome 12, bands 12q22-q24.1.
- What is the actual cause of the genetic abnormality? (Gene mutation? Gene deletion? Some other issue?)
- If Jane drinks a diet beverage containing aspartame, she will see a warning on the label due to the phenylalanine content of the artificial sweetener. How will the body digest and absorb the phenylalanine in the soda? (Where does digestion of amino acids begin? Where is the phenylalanine likely to be absorbed? When it makes it through the gut cell, where will it go?)
- Under normal circumstances, phenylalanine would most likely not be used for protein synthesis, but will be transaminated. There are two primary transamination pathways for metabolism of phenylalanine; what are the two most common products of those transaminations?
- If the phenylalanine cannot be normally transaminated, what happens to the phenylalanine that builds up in the blood? (i.e., how and why are ketones produced from the excess phenylalanine?)
- What are the symptoms of PKU, and can you identify a biochemical mechanism that causes those symptoms?
- Phenylalanine is considered an essential amino acid; what role(s) does it play in the body, and is it possible to obtain phenylalanine from any place other than diet?
- What are the byproducts of the body’s attempt to metabolize phenylalanine in PKU? (i.e., what are the metabolites?)
- The disease is known as phenylketonuria, not phenylketonemia. Is this an accurate description of the disease? Why or why not?
- What vitamins (or other nutrients) are required for the normal conversion of phenylalanine to its common metabolites?
- What treatment options are available for PKU patients?
- Why is PKU not treated with an oral dose or an injection of phenylalanine hydroxylase?
- Is phenylalanine a part of the structure of any enzymes, hormones, or other bioactive compounds, and if so, which ones?
- What is meant by the term “autosomal recessive disorder,” and is there a gene therapy that will correct the disorder?
- Define the terms “glucogenic” and “ketogenic” with regard to amino acids. Which term would you use to describe phenylalanine?
- Complete the following chart:
|Hormone||Tissue location in the gut||Primary physiological actions|
- If a PKU patient like Jane were to decide to eat steak, in what form will that protein be absorbed? Will anything change about the digestion and absorption of the protein because of her condition?
- If phenylalanine can be converted to tyrosine in the normal way, and tyrosine needs to be deaminated, with what substance will tyrosine react to form glutamate?
- Once the amine group has reached the liver cell in the form of ammonia, what substance must be created in order to go through the urea cycle?
- How many ATP are required to create that substance?
- How many more high-energy phosphates will be required to form urea?