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Example Questions
Example Question #1 : Protein Catabolism Enzymes
The urea cycle is vital to the excretion of ammonia, a harmful byproduct of amino acid breakdown.
What enzyme in the urea cycle converts L-ornithine to L-citrulline?
Ornithine synthetase
Argininosuccinate lyase
Ornithine transcarbamylase
Argininosuccinate synthetase
Ornithine transcarbamylase
Ornithine transcarbamylase catalyzes the reaction between the substrates ornithine and carbamoyl phosphate to form citrulline and phosphate. This process occurs primarily in the hepatic mitochondria, and to a lesser extent in the mitochondria of the renal cells.
Example Question #1 : Protein Catabolism Enzymes
What enzyme is deficient in Lesch-Nyhan syndrome?
Adenine phosphoribosyltransferase (APRT)
Uridine kinase
Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)
Uridine phosphorylase
Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)
Lesch-Nyhan syndrome is an x-linked deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT). HGPRT plays a central role in the generation of purine nucleotides through the purine salvage pathway. Cell breakdown products cannot be reused, and are therefore degraded. This gives rise to increased uric acid, a purine breakdown product. This build-up of uric acid is associated with severe gout and kidney problems, poor muscle control and mental retardation, usually in the first year of life. In the second year of life a common sign is self-mutilating behaviors.
Example Question #3 : Protein Catabolism Enzymes
Which of the following classes of enzymes could possibly breakdown collagen, the most common protein in the human body?
Lipases
Proteases
Ligases
Isomerases
Polymerases
Proteases
Because collagen is a protein, it must be broken down by a protease. Proteases exert their effects by hydrolyzing peptide bonds. The specific enzyme that breaks down collagen, predictably, is called collagenase.
The other classes of enzymes listed have actions unrelated to protein breakdown. An isomerase rearranges bonds to form an isomer, a polyermase adds nucleotides to DNA, a lipase breaks down fats, and a ligase creates a chemical bond.
Example Question #724 : Biochemistry
Which of these describe(s) the role of aminotransferases in protein catabolism?
I. They are present in the liver and muscle
II. They transfer the amino group from one amino acids to alpha-ketoglutarate
III. In the muscle, these enzymes move the amino group to glutamate, which can be further converted to glutamine which is a excess nitrogen carrier from tissues
IV. They catalyze irreversible reactions and release amino groups as free ammonium
II and III
I and II
I, II, and III
I, II, III, and IV
II and IV
I, II, and III
Aminotransferases do not release amino groups, but rather transfer them to other amino acids. The reactions catalyzed by aminotransferases are reversible. Their blood concentrations can be used as clinical indicators for damage to liver or muscle.
Example Question #725 : Biochemistry
Which of the following describes the role of glutamine synthetase in protein catabolism?
I. Glutamine synthetase converts glutamate and ammonia to glutamine using one molecule of ATP
II. The reaction catalyzed by glutamine synthetase is reversible
III. The reaction catalyzed by glutamine synthetase removes excess nitrogen (in the form of toxic ammonia) from protein degradation
IV. Glutamine synthetase converts glutamine to glutamate
I and III
I, II, and III
III only
I and IV
IV only
I and III
Glutaminase synthetase is present predominately in the brain, liver and kidneys.The importance of the reaction catalyzed by glutamine synthetase is that excess nitrogen (in the form of toxic ammonia) from protein degradation can be removed from tissues, especially the brain. The reaction catalyzed by glutamine synthetase is irreversible. It converts glutamate to glutamine.
Example Question #1 : Catabolic Pathways And Metabolism
Which of the following regarding mitochondrial carbamoyl phosphate synthetase are true?
I. Mitochondrial carbamoyl phosphate synthetase forms intermediates in the urea cycle
II. Mitochondrial carbamoyl phosphate synthetase utilizes adenosine triphosphate (ATP)
III. A decrease in normal levels of the enzyme leads to a decrease in ammonium ions in the blood
IV. Mitochondrial carbamoyl phosphate synthetase converts ammonium ions to carbamoyl phosphate
I, II, and IV
I, II, and III
I and IV
I and III
II and IV
I, II, and IV
Mitochondrial carbamoyl phosphate synthetase is important in excretion of ammonium ions as urea. Without it there is an increase in blood ammonium levels (which are toxic to the brain), as well as decreased blood urea levels and increased levels of glutamine (the transporter of ammonium in the blood).
Example Question #1 : Protein Catabolism
Which of these describe the role of ornithine transcarbamoylase in protein degradation?
I. Ornithine transcarbamoylase converts carbamoyl phosphate to citrulline in the presence of ornithine
II. Ornithine transcarbamoylase is present in the mitochondria
III. Deficiency of the enzyme leads to a decrease of ammonium ions in the blood
IV. Deficiency of the enzyme leads to an increase in urea in the blood
I and III
I only
II and III
I and II
III and IV
I and II
Ornithine transcarbamoylase is part of the urea cycle; it converts carbamoyl phosphate to citrulline in the presence of ornithine. It participates in the pathway that transforms toxic ammonium ions released in amino acid degradation to non-toxic urea that can be eliminated in urine. Ornithine transcarbamoylase is a mitochondrial membrane and its deficiency not only affects urea production, but also leads to accumulation of nucleotide bases such as uracil in the blood and urine. The levels of urea in the blood decrease (not increase) in ornithine transcarbamoylase deficiency.
Example Question #1 : Protein Catabolism
Which of the following is a possible consequence of an arginase deficiency?
No change to the levels of arginine, ornithine, and urea
Excess urea production
Excess ornithine production
Arginine deficiency
Hyperammonemia
Hyperammonemia
Arginase catalyzes the conversion of arginine to ornithine and urea. Therefore, if there is a deficiency in arginase, there will be a buildup of arginine, and a deficiency in ornithine and urea. Without the ability to form urea, the ammonium waste from protein and amino acids degradation will not be able to be converted to urea safely. The buildup of ammonium will cause hyperammonemia, which results in some unfavorable physiological scenarios.
Example Question #9 : Protein Catabolism Enzymes
The enzyme responsible for the reaction that produces ammonia during amino acid deamination is ___________.
glutamate dehydrogenase
pyridoxal-5’-phosphate
carbamoyl phosphate synthetase
aminotransferase
cathepsin
glutamate dehydrogenase
The correct answer is "glutamate dehydrogenase." Glutamate is the only amino acid that has an enzyme that removes its amino group as ammonia, rather than just transferring the amino group to a α-keto acid and forming a new amino acid. The transfer of amino groups, which may form glutamate, is performed by aminotransferases that require pyridoxal-5’-phosphate as a cofactor. Cathepsins are a type of proteases contained in lysosomes, and they break down proteins into amino acids rather than metabolize the amino acids themselves. Carbamoyl phosphate synthase is not involved in deamination, but rather condenses ammonia with bicarbonate to enter the urea cycle.
Example Question #1 : Catabolic Pathways And Metabolism
Phenylketonuria is a genetic disorder resulting in a lack of phenylalanine hydroxylase activity. During the first step of phenylalanine metabolism, phenylalanine is hydroxylated into what amino acid?
Alanine
Threonine
Serine
Tryptophan
Tyrosine
Tyrosine
The first step in phenylalanine metabolism is conversion of phenylalanine to tyrosine. Tyrsoine is simply a hydroxylated version of phenylalanine.
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