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UNE medical biochem FINAL |108 Questions | With Complete Solutions.
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UNE medical biochem FINAL |108 Questions | With Complete Solutions. 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 ...

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UNE medical biochem FINAL |108 Questions | With
Complete Solutions.
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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