which of the following urea cycle enzymes catalyze
which of the following enzymes is likely to be act
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UNE Medical Bio-Chem
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UNE Medical Bio-Chem FINAL Verified Questions
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Which of the following urea cycle enzymes catalyzes the reaction hydrolyzing the
release of urea from arginine? ✅Arginase
Arginase releases urea from arginine through a hydrolytic cleavage.
Which of the following enzymes is likely to be active in both the fed and fasted states?
✅Glutaminase
Glutaminase in the liver is responsible for freeing ammonia from glutamine. In the fasted
state, glutamine will be in excess due to protein catabolism while in the fed state, amino
acids will come from dietary intake.
Several amino acids are intermediates of the urea cycle, having side ammonia groups
that join with free carbon dioxide (CO2) and ammonia to produce net excretion of
ammonia as urea. Which of the following amino acids has a nitrogen in its side chain
and is an intermediate of the urea cycle? ✅Arginine
Arginine is an intermediate of the urea cycle and is therefore a non-essential amino
acid.
Which of the following reactions of the urea cycle takes place in the mitochondria?
✅The synthesis of carbamoyl phosphate
The synthesis of carbamoyl phosphate takes place in the mitochondria and is combined
with ornithine to generate citrulline which is shuttled out of the mitochondria.
Which of the following non-essential amino acids is synthesized as part of the urea
cycle? ✅Arginine
Arginine is synthesized in the urea cycle and therefore it is non-essential.
Your patient presents with a deficiency of the enzyme that catalyzes synthesis of N-
acetylglutamate (NAGs). Which of the following would be a consequence of this
deficiency in the patient? ✅Increased amino acids in the blood
NAGs is an essential activator of carbamoyl phosphate synthetase I which catalyzes the
first step in the urea cycle. Loss of this activator will decrease urea cycle activity
resulting in an increase in amino acids in the blood.
The urea cycle takes place in which of the following subcellular locations? ✅The
cytosol and the mitochondria
,The urea cycle spans both the cytosol and the mitochondrial compartments.
A newborn infant was resuscitated at birth and was found to be neurologically
compromised, with marked flaccidity (lack of muscle tone), and cardiac insufficiency.
Glutamine was largely absent from the serum and ammonia levels were excessively
elevated and urea production was otherwise not impaired. Based on this information
which of the following enzymes is likely deficient in this infant? ✅Glutamine synthetase
Which of the following compounds or substrates is required for the synthesis of
tyrosine? (+ indicates the compound is required; - indicates the compound is not
required)
Tetrahydrofolate (THF) Tetrahydrobiopterin (BH4) Tryptophan Phenylalanine
1. + + + - 2. - + + - 3. + - - + 4. - + - + ✅4
Deficiency of tetrahydrobiopterin (BH4) would impair phenylalanine metabolism to
tyrosine and result in phenylketonuria.
Homocystinuria can be caused by several different enzyme or vitamin deficiencies. An
elevation of homocysteine and a decrease in cysteine would most likely present due to
which of the following deficiencies? ✅Loss of function of cystathionine-β synthase
Homocysteine can be metabolized to cysteine by cystathionine-β synthase. A loss of
this enzyme would result in an elevation of homocysteine with a decrease in cysteine
production.
An individual is suspected of having a deficiency in branched chain keto acid
dehydrogenase. Supplementation of which of the following vitamins or cofactors may
help resolve this issue? ✅Thiamine
Much like the pyruvate dehydrogenase complex, the branched chain keto acid
dehydrogenase also requires the cofactor thiamine.
In an individual with a deficiency in the branched chain ketoacid dehydrogenase
complex, metabolism of which of the following amino acids would not likely be
impaired? ✅Tyrosine
This is a described presentation of Maple Syrup Urine disease which would result in the
inability to metabolize branched chain amino acids; Valine, leucine and Isoleucine are
all branched chain amino acids.
A 52-year- old male sought medical help complaining about progressive appearance of
hyperchromic (hyper pigmented) papules on the edge of the second finger of both
hands. He also complained about darkening of urine. Incisional biopsy of one of the
spots on his hands and urine sample tested positive for or homogentisic acid. Based on
, these findings a diagnosis of alkaptonuria is made. This diagnosis is consistent with a
deficiency in which of the following enzymes? ✅Homogentisate oxidase
Alkaptonuria is consistent with a deficiency in homogentisate oxidase.
A 55-year old man presents to his primary care physician with bouts of dizziness and
confusion. He is diagnosed with hyperhomocysteinemia due to a deficiency of
cystathionine-ß synthase. In this individual which of the following amino acids could
potentially become essential? ✅Cysteine
Cysteine is synthesized from homocysteine through the described metabolic pathway.
You and a friend are being chased by a bear. Under these conditions levels of
epinephrine are increased due to metabolism of which of the following amino acids?
✅Tyrosine
Tyrosine is metabolized to epinephrine and norepinephrine in neural cells.
Phenylketonuria can be caused by an enzymatic deficiency or it can present due to a
deficiency in which of the following required metabolic cofactors?
✅Tetrahydrobiopterin (BH4)
Deficiency of tetrahydrobiopterin (BH4) would impair phenylalanine metabolism to
tyrosine and result in phenylketonuria.
The conversion of ribose bases into deoxy-bases will be occurring under which of the
following conditions? ✅When cells are proliferating
The conversion of ribose bases to deoxy bases will only occur when they are needed
for the process of DNA synthesis. This is most likely occurring when cells are
proliferating.
Methotrexate is a potent anticancer agent that starves dividing cells of
deoxyribonucleotides through direct inhibition of which of the following processes?
✅Metabolism of folic acid
Methotrexate is an inhibitor of dihydrofolate reductase. This enzyme is needed to
reduce a methyl group from N5,N10
A 47-year-old man complains of pain in the joints of his left big toe, which are obviously
swollen and tender. The pain has been chronic but became intolerable the day after
Thanksgiving when he had a large meal and several glasses of red wine. Based on this
information a diagnosis of gout is made. Which of the following is involved in the
pathophysiology of this patient's condition? ✅Elevated uric acid
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