76 Questions
Where does the urea cycle primarily take place?
Liver
Which molecule is formed when ammonia reacts with carbon dioxide and ATP?
Carbamoyl phosphate
Which enzyme is involved in the reaction where carbamoyl phosphate is formed?
Carbamoyl phosphate synthetase I
Which metabolic pathway is responsible for removing ammonia from the body?
Urea cycle
What is the primary function of the urea cycle in the body?
To remove ammonia
Which enzyme is primarily responsible for the conversion of glutamate and ammonia into glutamine?
Glutamine synthetase
Which of the following is NOT a source of ammonia in the body?
Carbohydrate breakdown
What is the main transport form of ammonia in the circulation?
Glutamine
Hyperammonemia is most directly caused by:
Compromised liver function
Which symptom is NOT associated with ammonia intoxication?
Hyperactivity
Which of the following correctly describes the fate of urea in the body?
Excreted by the kidneys in the glomerular filtrate
Which condition can lead to hyperammonemia due to genetic defects?
Urea cycle disorders
What is the primary mechanism for the removal of ammonia in the brain?
Formation of glutamine
Which of the following processes does NOT contribute to the production of the amino acid pool?
Direct participation of amino acids in energy metabolism
What enzyme mediates the oxidative deamination of glutamate to α-ketoglutarate and NH3?
Glutamate dehydrogenase
What regulates protein turnover involving the amino acid pool?
PEST sequences
Which molecule is formed as a result of the activity of aminotransferases?
Aspartate
Which of the following is a direct consequence of inherited enzyme deficiencies related to nitrogen metabolism?
Hyperammonemia
Accumulation of what substance can be an indicator of liver damage?
Ammonia
Where does the degradation of body protein typically occur?
Proteasome
What nitrogen-containing substance is incorporated into urea along with CO2?
NH3
What is the primary route for disposing of nitrogen from the body?
Urea
What is a common cause of hyperammonemia in adults?
Liver disease
Which enzyme deficiency is most commonly associated with congenital hyperammonemia?
Ornithine transcarbamoylase
How is liver cirrhosis related to hyperammonemia?
It forms collateral circulation around the liver
Which inheritance pattern do most urea cycle disorders follow?
Autosomal recessive
What compound is phenylbutyrate converted to after oral administration?
Phenylacetate
What is the result of shunting portal blood directly into systemic circulation in liver disease?
Impaired conversion of ammonia to urea
Which of the following amino acids is associated with less severe symptoms in urea cycle disorders due to its ability to contain two waste nitrogens?
Arginine
Which enzyme system is responsible for ATP-independent degradation of proteins in cells?
Lysosomal acid hydrolases
What is a treatment strategy for urea cycle defects that has improved survival rates?
Administration of compounds binding covalently to amino acids
What form do fatty acids take when stored in adipose tissue?
Neutral TAGs
Why do TAGs provide concentrated stores of metabolic energy?
They are highly reduced and largely anhydrous
How much energy is yielded from the complete oxidation of fatty acids to CO2 and H2O?
9 kcal/g fat
How does the energy yield from fatty acids compare to that from proteins or carbohydrates?
Fatty acids yield 3 kcal/g more than proteins or carbohydrates
What characteristic of TAGs makes them an efficient form of energy storage in the body?
Their highly reduced state and anhydrous nature
Which factor increases the melting temperature of a fatty acid?
More carbons in the chain
Which fatty acid is represented by the structure 18:3(9,12,15)?
α-Linolenic acid
What characterizes a saturated fatty acid chain?
Contains no double bonds
What is the precursor of ω-6 arachidonic acid?
Linoleic acid
Which fatty acid deficiency can lead to scaly dermatitis and neurologic abnormalities?
Linoleic acid and α-Linolenic acid
Which statement is true about an ω-3 fatty acid?
Has a double bond three carbon atoms away from the methyl-terminal end
Which fatty acid has the structure 20:4(5,8,11,14)?
Arachidonic acid
Which fatty acid is not found in significant quantities in milk?
Formic acid
In the transamination reaction involving oxaloacetate and glutamate, what amino acid is formed from oxaloacetate?
Aspartate
Which of the following statements about the urea cycle is correct regarding the nitrogen sources?
The nitrogen sources are ammonia and aspartate.
An increase in which of the following would most strongly indicate a defect in the enzyme argininosuccinate lyase?
Argininosuccinate
Which enzyme's deficiency would most likely result in elevated urinary orotic acid levels?
Ornithine transcarbamylase
In a patient with argininosuccinate synthetase deficiency, which substrate would accumulate?
Citrulline
The hydrolysis of which amino acid directly produces urea in the urea cycle?
Arginine
Which one of the following pairs is not correctly matched in terms of amino acid and corresponding α-keto acid?
Glutamate, arginine
Which enzyme in the urea cycle is associated with both the mitochondria and the cytosol?
Ornithine transcarbamylase
What role do plasma free fatty acids primarily serve?
Transport energy substrates to tissues
At physiologic pH, the terminal carboxyl group of a fatty acid ionizes to become which functional group?
-COO⁻
What percentage of fatty acids found in the plasma are in the form of fatty acid esters?
90%
Which intracellular molecules attach to fatty acids to enhance the ability of proteins to associate with membranes?
Intracellular proteins
What component is NOT mentioned as part of the triacylglycerol synthesis and degradation pathway?
Pyruvate
During fasting, where can substantial amounts of free fatty acids be found?
Plasma
What term best describes the nature of a long-chain fatty acid?
Amphipathic
Which of the following serves as a major energy reserve in the body?
Triacylglycerols
Which component provides the nitrogen atom that is incorporated into urea during the urea cycle?
Aspartate
What is the immediate precursor of both ammonia and aspartate nitrogen in the urea cycle?
Glutamate
Which enzyme requires N-Acetylglutamate as an essential activator in the urea cycle?
Carbamoyl phosphate synthetase I
How many high-energy phosphate bonds are consumed in the synthesis of each molecule of urea?
Four
Following the ingestion of a protein-rich meal, what happens to the intrahepatic concentration of N-acetylglutamate?
It increases due to increased substrate and activator availability.
Which metabolic intermediates link to produce ammonia, essential for the movement of nitrogen from peripheral tissues to the liver?
Transdeamination of aminotransferase and glutamate dehydrogenase
What is the role of renal glutaminase in the metabolism of ammonia?
Excreting ammonia as NH₄⁺ into urine
Which of the following correctly represents the overall stoichiometry of the urea cycle?
Aspartate + $NH_3$ + $CO_2$ + 3 ATP + $H_2O$ → urea + fumarate + 2 ADP + AMP + 2 P + PP
What initiates the hydrolytic release of fatty acids from triacylglycerol (TAG)?
Hormone-sensitive lipase
Which enzyme phosphorylates hormone-sensitive lipase to activate it?
3'5'-cyclic AMP (cAMP)-dependent protein kinase
What molecule binds to receptors on the adipocyte cell membrane to activate adenylyl cyclase?
Epinephrine
Why can't glycerol released during TAG degradation be metabolized by adipocytes?
Adipocytes lack glycerol kinase
Which metabolic pathway can dihydroxyacetone phosphate (DHAP) participate in?
Glycolysis and gluconeogenesis
Which tissue types cannot use plasma free fatty acids (FFA) for fuel?
Erythrocytes and brain
Which process reduces plasma free fatty acids (FFA) to mitigate insulin resistance?
Glyceroneogenesis
What do long-chain fatty acids (LCFA) convert to in the cytosol after entering a cell for beta-oxidation?
Fatty acyl CoA derivatives
Study Notes
Metabolism of Ammonia
- Ammonia is a by-product of protein metabolism, produced in the body from the breakdown of amino acids, purines, and pyrimidines.
- Ammonia is detoxified in the liver to urea and excreted by the kidneys.
- Sources of ammonia include:
- Intestinal glutamine metabolism
- Bacterial action in the intestine
- Amino acids
- Purines and pyrimidines
- Transport of ammonia in the circulation:
- Urea is the main disposal route for ammonia, traveling from the liver to the kidneys.
- Glutamine serves as a non-toxic storage and transport form of ammonia.
Hyperammonemia
- Occurs when the capacity of the hepatic urea cycle is exceeded, leading to elevated serum ammonia levels.
- Can also occur when liver function is compromised, such as in liver disease or genetic defects of the urea cycle.
- Hyperammonemia can cause a direct neurotoxic effect on the CNS.
- Symptoms of ammonia intoxication include:
- Tremors
- Slurring of speech
- Somnolence
- Vomiting
- Cerebral edema
- Blurring of vision
- At high concentrations, ammonia can cause coma and death.
Urea Cycle
- The urea cycle is a metabolic pathway that removes ammonia from the body.
- It takes place in the liver and is a cyclic process.
- The overall stoichiometry of the urea cycle is:
- Aspartate + NH3 + CO2 + 3 ATP + H2O → urea + fumarate + 2 ADP + AMP + 2 P + PP
- Regulation of the urea cycle involves N-acetylglutamate, which is an essential activator for carbamoyl phosphate synthetase I.
Fatty Acid and Triacylglycerol Metabolism
- Fatty acids exist in a free and unesterified form or as fatty acyl esters in more complex molecules.
- Fatty acids are stored in adipose tissue in the form of neutral triacylglycerols (TAGs).
- Mobilization of stored fat requires hydrolytic release of fatty acids and glycerol from their TAG form.
- Release of fatty acids from TAG is initiated by hormone-sensitive lipase.
- Activation of hormone-sensitive lipase involves phosphorylation by a 3'5'-cyclic AMP (cAMP)-dependent protein kinase.
B-Oxidation of Fatty Acids
- The major pathway for catabolism of fatty acids is a mitochondrial pathway called B-oxidation.
- In B-oxidation, two-carbon fragments are successively removed from the carboxyl end of the fatty acyl CoA, producing acetyl CoA, NADH, and FADH2.
- Transport of long-chain fatty acids (LCFA) into the mitochondria involves conversion to their CoA derivative by long-chain fatty acyl CoA synthetase (thiokinase).
Ammonia is a by-product of protein metabolism, produced in the body from amino acids, purines, and pyrimidines, and detoxified in the liver to urea and excreted by the kidneys.
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