Download Exam IV answers

ANSWER KEY
1. (2 points for each part; 38 points total) Consider each statement. If it is true, write TRUE in
the margin. If it is false, change the underlined word(s) to make it true. (If the statement is already true, do NOT change it!)
The uptake of dietary amino acids by enterocytes is coupled to potassium export by these cells.
Sodium
Non-essential amino acids are ones that in a healthy young adult are not needed for protein synthesis.
Can be synthesized in sufficient amounts to maintain homeostasis
The transamination of lysine is coupled to the conversion of oxaloacetate to aspartate.
Any amino acid except lysine or threonine (though serine, histidine and methionine
are normally deaminated directly) / alpha-ketoglutarate to glutamate
In an acidotic adult, a major fate of leucine exported from skeletal muscle is oxidative deamination in the kidney.
Glutamine
In humans a major reaction that directly generates free ammonia is the conversion of branched
chain amino acids to alpha-keto acids.
Any one of reductive deamination of glutamate; hydrolysis of glutamine or asparagine; deamination of serine, threonine or histidine; deamination of cystathionine;
deamination of AMP or dAMP; deamination of guanine; deamination of CMP
A newborn child lacking the enzyme needed to synthesize N-acetyl glutamate would accumulate
toxic levels of phenylalanine in his or her blood.
ammonia
Ketogenic amino acids are ones that are catabolized to yield pyruvate.
acetyl CoA, acetoacetyl CoA, or acetoacetate
As an otherwise healthy adult fasts for an extended period (weeks), urea output gradually falls.
True
Cholesterol biosynthesis occurs only in the liver.
Predominantly in the liver, but to some extent in nearly all cells (not RBC)
All of the carbon atoms for cholesterol biosynthesis are derived from acetyl CoA.
True
Statins competitively inhibit the LDL receptor.
HMG CoA reductase
1
A drug that blocked cholesterol transport from the cytosol to the mitochondrial matrix would be
expected to stimulate steroid hormone synthesis.
Inhibit
Ibuprofen irreversibly inhibits Cox I.
Reversibly (competitively) inhibits
Most eicosanoids act on target cells physically distant from the cells in which they are synthesized.
Close to the synthesizing cell, or the synthesizing cell itself
Most eicosanoids bind to receptors in the nuclear hormone receptor family of proteins.
Trimeric G protein receptor
The synthesis of dUDP and the other products of ribonucleotide reductase is regulated such that
all four dNTPs are present in low concentrations (micromolar).
True
A condition such as glucose 6-phosphatase deficiency that leads to elevated cellular levels of ribose 5-phosphate would tend to increase a person’s risk of developing gout.
True
In people whose ability to convert hypoxanthine to IMP is reduced due to an enzyme defect, uric
acid synthesis would be reduced.
Increased
Most neonatal jaundice is due to mutations in the gene encoding bilirubin glucuronyl transferase.
Delayed expression of the gene
2. (5 points) A patient in the burn ICU unit is receiving optimum nutrition via TPN (total parenteral nutrition; a synthetic mixture of nutrients is infused via a catheter that is inserted into a
large vein). The patient is found to be in negative nitrogen balance, so the first year resident, Dr.
Donough Good, doubles the amount of amino acids. The next day, the resident is surprised to
find the patient suffering due to a combination of very high BUN (blood urea nitrogen) and negative nitrogen balance. What is happening here?
The stress of the injury is causing CRH release from hypothalamus, which in turn is
stimulating ACTH release from the pituitary the ACTH is stimulating synthesis of
StAR cholesterol transporter protein in the adrenal Zona fasciculata and Zona
glomerulosa, allowing increased conversion of cholesterol to pregnenolone and
hence to cortisol. Cortisol (a glucocorticoid) is stimulating muscle breakdown and
conversion of the resulting carbon skeletons to glucose, which also generates elevated levels of amino groups that must be disposed of as urea. These regulatory
processes are going on independent of the patient’s fed / fasting state, as part of
the patient’s response to trauma. Increasing the amounts of amino acids given to
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the patient will not influence these regulatory processes, but will add to the burden of nitrogen that the body must dispose of.
3. Cortisone is sometimes administered systemically to treat severe inflammation (e.g., in the
management of autoimmune diseases). Cortisone is a drug that works by the same molecular
mechanism as cortisol.
A) (5 points) Outline the events that lead from an injection of cortisone to the inhibition of
cyclooxygenase that is already present in lymphocytes. For full credit, your outline MUST include ALL of the steps of the pathway.
The limiting step in synthesis of pro-inflammatory eicosanoids in lymphocytes is
the cleavage of a membrane phospholipid by phospholipase A2 to yield free cytosolic arachidonic acid. High levels of cortisol/cortisone bind to a nuclear receptor in
lymphoid cells, causing its activation and movement to the nucleus where it activates transcription of lipocortin. The lipocortin protein inhibits phospholipase A2,
thus reducing the supply of the starting material for eicosanoid biosynthesis. The
cortisol/cortisone – receptor complex also represses transcription of COX II enzyme, thus decreasing the synthesis of eicosanoids that mediate pain and inflammation.
B) (4 points) Long-term treatment with cortisone can result in Cushing syndrome. This syndrome
is associated with elevated blood glucose levels and muscle weakness. What causes these symptoms?
Meanwhile, elsewhere in the body, the high levels of cortisone are mimicking the
effects of cortisol on skeletal muscle to cause breakdown of muscle proteins and
use of the derived carbon skeletons to synthesize high levels of glucose. These effects are responsible for the “Cushingoid” symptoms seen in patients treated systemically with high doses of corticosteroids.
4. Pregnenolone is marketed by health food stores (about $18 for 100 10mg capsules) as a “Product for Mental Clarity, Mental Energy, and
Mental Strength”. One ad claims that “Pregnenolone is taken to en-
Prgnenalone is simply ducky … or is it
just quacky?
hance mental alertness and awareness, improve long-term
memory, improve mood and reduce symptoms of PMS, arthritis, and stress*
… *The above statements have not been evaluated by the FDA”.
Each of the following statements, made by one marketer of pregnenolone, has true and false
components to it. Based on your MBM knowledge, what is true and what is false? Explain your
answers.
A) (4 points) “There is no negative feedback loop. Once produced, pregnenolone leaves the mitochondria so it cannot inhibit its own synthesis.”
True: The side-chain cleavage enzyme that converts cholesterol to pregnenolone is
not inhibited by its product.
False: The pregnenolone, whether endogenously synthesized or provided from
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the pills, is then converted to various end products (cortisol, aldosterone, sex
steroids) which will, via the pituitary, repress StAR synthesis in the appropriate
cell type, down-regulate the synthesis of the corresponding steroid, and thus restore near-normal levels despite the ingested pregnenolone.
B) (4 points) “The cytochrome P-450 enzyme system is stimulated by the addition of pregnenolone. This enzyme system is important in neutralizing cellular toxins.”
True: The synthesis of cytochrome P450 enzymes in the liver is stimulated by
steroids, so elevated pregnenolone levels way well induce synthesis of specific cytochrome P450 enzymes (and associated hepatic heme biosynthesis! – why might
this be a problem for some people?)
False: The “toxins” these cytochrome P450’s will hydroxylate, to facilitate their
clearance from the body, will include the pregnenolone itself. It’s not clear what
other foreign chemicals these P450 enzymes will also hydroxylate.
C) (4points) “Complete hormone balance. It [pregnenolone] does everything DHEA does and
more [note: DHEA is dehydroepiandrosterone, an immediate precursor to androstenedione].”
True: Pregnenolone is a precursor to mineralocorticoids, glucocorticoids, and sex
steroids, while DHEA cannot be converted back to such a general-purpose precursor, but can only be metabolized “forward” to sex steroids.
False: But meanwhile, to the extent that the exogenous pregnenolone gets converted to these end products, the end products will act via normal feedback loops
to down-regulate their own synthesis, counteracting the effect of the exogenous
pregnenolone.
5. (4 points) Mice have been bred that have no prostacyclin receptor (IP-/- mice). They survive to
adulthood, but have a major physiological abnormality. Based on your MBM knowledge, explain
what their problem might be and whether or not you would expect Vioxx to help.
Normally, prostacyclins produced by endothelial cells act on IP receptors on nearby
platelets, setting off a signaling cascade in the platelets that represses platelet
aggregation and clot formation. Without the receptor, the signaling cascade would
fail and platelets would be abnormally likely to aggregate. The mice should thus be
susceptible to thrombophlebitis. Vioxx inhibits COXII, the major cyclooxygenase
of endothelial cells. As the platelets are already unreceptive to prostacyclin signaling from endothelial cells, Vioxx should have no further effect, one way or the
other on that interaction. However, platelet aggregation is not the only process
targeted by endothelial cell prostacyclin, so Vioxx treatment might indeed have an
effect on the mice.
6. (7 points) Your patient is anemic but otherwise healthy. He has been on a vegan diet (no animal products) for the past 20 years. A blood sample that was sent to the hematology lab
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produced a report stating that there are an unusually large number of erythroid precursor cells
and that these cells are unusually large in size. You deduce that there is a deficiency that is similar to one caused by an autoimmune attack on the parietal cells of the stomach. You know that at
least two of the following will alleviate your patient’s anemia, but only one of them is a good
treatment. For each of the following state one reaction in which it participates and state whether
or not it will help your patient.
A) Vitamin B-6 (precursor to pyridoxal phosphate)
A cofactor in reactions that require the transfer of an amino group. Examples are
transamination reactions (amino acid-1 + alpha-ketoglutarate <-> alpha-ketoacid-1 +
glutamate).
No help. Although pyridoxal phosphate is a cofactor for ALA synthase, and a deficiency of B6 lso required for heme synthesis, the defficency results in a microcytic anemia due to lowered rate of heme production. Our patient has macrocytic
anemia due to impaired DNA replication.
B) Folic acid (precursor to tetrahydrofolate)
Folate is required for many one-carbon transfers. Reactions are summarized in the
Amino acids handuout.
Folate will alleviate the macrocytic anemia, but IT IS NOT A GOOD TREATMENT.
In the absence of vitamin B12 (the patient’s underlying defect), tetrahydrofolate
is sequestered as N5-methyl-FH4, so exogenous folic acid allows the body to replenish its FH4 supply. However, the other reaction that requires B12 (methylmalonyl-CoA mutase) will still be adversely impacted, leading to a build up of neurotoxic compounds derived from propionyl-CoA and methylamonyl-CoA.
C) Methotrexate
Competitive inhibitor of dihydrofolate reductase. It will not help.
D) Vitamin B-12
Cofactor required for the conversion of N5-methyl-FH4 to FH4, and a cofactor for
methylmalonyl-CoA mutase (propionyl-CoA metabolism).
YES. This patient is suffering from a vitamin B12 deficiency.
E) Methionine
The conversion of Homocysteine + Methyl-vitamin B12 to methionine + plain vitamin
B12, catalyzed by cystathionase. Maybe methionine would help a bit, like the folate
in part B, but, like that folate, it would do nothing to treat the underlying problem.
F) Homocystine
Reaction as in part E. No help. Homocysteine in fact is toxic at high levels and
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needs to be disposed of, e.g., by conversion to methionine.
G) Vitamin B1 (precursor to thiamine pyrophosphate)
A cofactor required for alpha-keto acid dehydrogenases (branched-chain alphaketo acid dehydrogenase, pyruvate dehydrogenase and α-ketoglutarate dehydrogenase). No help
7. Acute intermittent porphyria (incidence is about 1/10,000) is due to a defect in porphobilinogen deaminase. In this disorder, porphobilinogen is synthesized, but its conversion to further
heme precursors is defective.
A) (4 points) These people are fine most of the time, but are subject to periodic attacks that include, among other symptoms, psychotic episodes. What causes the psychotic episodes, and why
are they acute as opposed to chronic?
The PBG and ALA that accumulate because of the enzyme defect are neurotoxic.
The reason that these intermediates accumulate is given in part B.
B) (3 points) Even during an attack, production of heme by the liver in these people is nearly
normal. Explain.
The hepatic heme synthesis is regulated by feedback inhibition by heme of the
first step of the pathway. If the rate of heme synthesis is low because of a partial
loss of one of the enzymes (e.g. haploinsufficiency) in the pathway, then this negative regulation will not occur to the usual extent, concentrations of intermediates
will rise to drive the pathway forward by mass action, and the liver will be able to
maintain near-normal heme output. The cost will be accumulation of the intermediates that are inefficiently metabolized due to the enzyme defect.
C) (2 points) AIP is inherited with an autosomal dominant pattern of inheritance. What is the
molecular basis of this pattern of inheritance (e.g. dominant negative, loss of heterozygosity etc)?
Haploinsufficiency
D) (2 points) What is the allele frequency?
1/(20,000 X penetrance).
The mutant allele “q” is dominant (and in fact homozygosity for the defect is fatal)
so all affected individuals are heterozygotes, who should occur with frequency
2pq X penetrance. p is almost 1, so q = 1/(20,000 X penetrance).
8. Porphyria cutanea tarda (PCT) is due to a severe reduction in the activity of uroporphyrinogen
III decarboxylase activity that is brought on by liver damage (e.g. hepatitis C infection, excessive
alcohol consumption).
A) (3 points) These patients are sensitive to sunlight but do not have neurological symptoms.
Provide a molecularly-based explanation for this.
The mutation causes accumulation of a porphrinogen. When present in high concentrations, the porphyinogen is converted, by an uncatalyzed reaction, to a
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porphyrin. Porpyrins that reach the skin cause photosensitivity because they absorbs UV light from the environment and fluoresce. The neurotoxic compounds
PBG and ALA are converted to porphrinogens and so do not build up.
B) (3 points) These patients generally have elevated total serum bilirubin. Do you think that this
would be direct or indirect bilirubin? Explain.
The elevated serum bilirubin is probably not due to the specific enzymatic defect
in heme biosynthesis, but rather to the liver damage and thus a shortage of the
liver enzymes needed for bilirubin glucuronidation and efficient clearance from the
body. The excess bilirubin would thus be unconjugated (“indirect” bilirubin).
9. A) What was the subject of your genetic disease paper?
B) (4 points) Choose a genetic disease presented in one of the other papers in your conference
session, for which an effective treatment strategy is available for affected adolescents or adults.
Name the disease, identify the treatment strategy, and describe its molecular basis. If none of the
diseases discussed in your section can be managed in this way, you may choose another disease
to answer this question.
THE END
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