Question 1

Amino acid nutrition — Which of the following is NOT an essential amino acid for humans?

Accepted Answer

Introduction / Context: Essential amino acids cannot be synthesized de novo in sufficient amounts by humans and must be obtained from the diet. Recognizing essential versus nonessential amino acids is fundamental in nutrition and metabolism. Given Data / Assumptions: Histidine, leucine, and methionine are typically listed as essential. Glycine can be synthesized endogenously from serine and other precursors. Focus is on human requirements. Concept / Approach: Humans require nine dietary essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Nonessential amino acids (e.g., alanine, asparagine, aspartate, glutamate, glutamine, glycine, proline, serine, tyrosine in most contexts) can be made internally. Step-by-Step Solution: Identify essentials in the list: histidine, leucine, methionine → essential.Assess glycine: synthesized from serine via serine hydroxymethyltransferase.Conclude glycine is not essential.Select “Glycine.” Verification / Alternative check: Dietary guidelines and biochemistry references concur that glycine is nonessential for healthy adults. Why Other Options Are Wrong: Histidine is essential, particularly important in hemoglobin synthesis. Leucine is essential and a key branched-chain amino acid. Methionine is essential and a methyl-group donor via S-adenosylmethionine. Common Pitfalls: Confusing “conditionally essential” states; for instance, in certain illness or rapid growth, requirements change, but glycine remains nonessential in general adult nutrition. Final Answer: Glycine

Question 2

Urea cycle — The two nitrogen atoms in urea are derived from which sources?

Accepted Answer

Introduction / Context: The urea cycle detoxifies ammonia by converting it to urea for excretion. Understanding the origin of urea’s two nitrogen atoms helps clarify the flow of nitrogen from amino acid catabolism to excretion. Given Data / Assumptions: Urea has two nitrogens and one carbon (CO(NH2)2). Ammonia arises from oxidative deamination and other reactions. Aspartate donates an amino group via transamination pathways. Concept / Approach: One nitrogen enters as free ammonia (NH3/NH4+) when carbamoyl phosphate is formed from NH3 and CO2. The second nitrogen is donated by aspartate during the argininosuccinate step, ultimately appearing in urea after cleavage of arginine by arginase. Step-by-Step Solution: CPS I reaction: NH3 + CO2 + 2 ATP → carbamoyl phosphate (first nitrogen).Aspartate + citrulline → argininosuccinate (introduces second nitrogen from aspartate).Argininosuccinate → arginine + fumarate; arginine → ornithine + urea.Thus, urea nitrogens come from ammonia and aspartate. Verification / Alternative check: Isotopic labeling shows one 15N from ammonia and one 15N from aspartate incorporated into urea in hepatocytes. Why Other Options Are Wrong: Nitrate and nitrite are not used as nitrogen donors in human urea formation. “Ammonia” alone accounts for only one nitrogen. Common Pitfalls: Confusing aspartate’s role as a nitrogen donor with its carbon skeleton fate; the carbon backbone leaves as fumarate. Final Answer: Ammonia and aspartic acid

Question 3

Urea cycle function — What does the urea cycle convert in order to protect the body from nitrogen toxicity?

Accepted Answer

Introduction / Context: Ammonia is neurotoxic at elevated concentrations. The liver converts ammonia into urea via the urea cycle, enabling safe transport in blood and excretion by the kidneys. This detoxification is central to nitrogen metabolism. Given Data / Assumptions: Ammonia arises during amino acid deamination. Urea is water soluble and relatively non-toxic. Cycle occurs primarily in hepatocytes (mitochondria and cytosol). Concept / Approach: The urea cycle consumes free ammonia and the amino group of aspartate, plus CO2 (as bicarbonate), using ATP to form urea. This process prevents accumulation of ammonia and maintains nitrogen balance, especially during high-protein intake or fasting when amino acids are catabolized. Step-by-Step Solution: Ammonia + CO2 + ATP → carbamoyl phosphate (mitochondria).Carbamoyl phosphate + ornithine → citrulline; citrulline → cytosol.Citrulline + aspartate → argininosuccinate → arginine + fumarate.Arginine → urea + ornithine (cycle continues). Verification / Alternative check: Clinical hyperammonemia in urea cycle disorders demonstrates the detoxifying role of this pathway; treatment strategies aim to reduce ammonia. Why Other Options Are Wrong: Ketoacids to amino acids describes transamination, not the urea cycle. Amino acids to ketoacids occurs in deamination, preceding urea synthesis. “None” is incorrect because the cycle clearly detoxifies ammonia. Common Pitfalls: Confusing carbon flow (fumarate production) with nitrogen disposal; the cycle’s purpose is nitrogen detoxification. Final Answer: Ammonia into a less toxic form

Question 4

Source of carbon in urea synthesis — In the hepatic urea cycle, which compound supplies the carbon atom of urea?

Accepted Answer

Introduction / Context: Urea contains one carbon atom and two nitrogens. While the nitrogens derive from ammonia and aspartate, the carbon originates from CO2 (as bicarbonate). Recognizing this source clarifies how carbon and nitrogen metabolism intersect in hepatic detoxification. Given Data / Assumptions: Carbamoyl phosphate synthetase I (CPS I) uses CO2 and NH3. The carbon in urea is the carbonyl carbon. Process requires ATP and occurs partly in mitochondria and partly in cytosol. Concept / Approach: CO2 (bicarbonate) condenses with ammonia to form carbamoyl phosphate, introducing the carbon that persists through citrulline and arginine intermediates to appear in urea. Neither arginine nor aspartate donates the carbonyl carbon of the final urea molecule. Step-by-Step Solution: CPS I: NH3 + HCO3− + 2 ATP → carbamoyl phosphate (provides urea carbon).Carbamoyl phosphate + ornithine → citrulline → argininosuccinate (with aspartate).Argininosuccinate → arginine → urea + ornithine.The carbonyl carbon traces back to CO2/HCO3−. Verification / Alternative check: Isotopic 13C bicarbonate labeling appears in the urea carbonyl during hepatic perfusion studies, confirming the source. Why Other Options Are Wrong: Arginine is a cycle intermediate cleaved to release urea; it does not supply the carbon atom de novo. Aspartate provides a nitrogen, not the carbonyl carbon. Glucose is not directly used as the carbon donor in urea formation. Common Pitfalls: Assuming the nitrogen donor aspartate also donates carbon; in fact, its carbon skeleton exits as fumarate. Final Answer: Carbon dioxide

Question 5

Stoichiometry of urea synthesis — What are the inputs required for one complete turn of the urea cycle (to produce one urea)?

Accepted Answer

Introduction / Context: The urea cycle has a defined stoichiometry because it converts toxic nitrogen into urea. Understanding its inputs is crucial for biochemistry, nutrition, and clinical medicine (e.g., hyperammonemia management). Given Data / Assumptions: One urea molecule is produced per cycle turn. Two nitrogens are required: one from free ammonia, one from aspartate. Energy is supplied by three ATP molecules (four high-energy phosphate bonds because one ATP → AMP + PPi). Concept / Approach: Key reactions: CPS I uses NH3, CO2, and 2 ATP to form carbamoyl phosphate. Later, argininosuccinate synthetase uses aspartate and 1 ATP (to AMP) to add the second nitrogen. Overall, one NH3, one aspartate, and one CO2 are consumed, with 3 ATP hydrolyzed (equivalent to 4 phosphoanhydride bonds), yielding one urea and one fumarate. Step-by-Step Solution: Account for nitrogen: NH3 (first) and Asp (second).Account for carbon: CO2 provides the carbonyl carbon of urea.Energy: 2 ATP in CPS I + 1 ATP in argininosuccinate synthetase = 3 ATP.Products: urea + fumarate, with ornithine regenerated. Verification / Alternative check: Metabolic flux analyses and textbook conventions consistently list these inputs for one urea molecule produced. Why Other Options Are Wrong: Option B mixes inputs and products incorrectly. Option C lists products rather than the correct reactants and confuses ATP with AMP accounting. “None” is incorrect because Option A is accurate. Common Pitfalls: Confusing “3 ATP” with “3 high-energy bonds”; the cycle actually consumes 4 high-energy bonds due to one ATP → AMP. Final Answer: 1 aspartic acid, 1 ammonia, 1 carbon dioxide, and 3 ATP

Question 6

Biological nitrogen fixation — How many ATP molecules are hydrolyzed by nitrogenase to reduce one N2 to two molecules of ammonia (2 NH3)?

Accepted Answer

Introduction / Context: Nitrogen fixation converts atmospheric nitrogen (N2) into ammonia (NH3), making nitrogen bioavailable. The enzyme complex nitrogenase catalyzes this energetically demanding reduction, consuming large amounts of ATP. Understanding the ATP cost is key to appreciating the bioenergetic burden of fixation. Given Data / Assumptions: The overall stoichiometry for Mo–Fe nitrogenase is commonly summarized as requiring at least 16 ATP to reduce 1 N2 to 2 NH3, with H2 as a byproduct. Reaction occurs in diazotrophs under reducing conditions, often protected from oxygen. ATP hydrolysis occurs at the Fe protein during electron transfer cycles. Concept / Approach: Nitrogenase operates via repeated cycles of electron transfer from a reduced electron donor (such as ferredoxin) through the Fe protein to the MoFe protein, where N2 is bound and reduced. Each electron transfer step is coupled to ATP hydrolysis, and multiple electrons are required to fully reduce N2 to NH3. The canonical minimal requirement is 16 ATP per N2 reduced to 2 NH3. Step-by-Step Solution: Overall reaction (simplified): N2 + 8 e− + 8 H+ + 16 ATP → 2 NH3 + H2 + 16 ADP + 16 Pi.Electron delivery steps each couple to ATP hydrolysis in the Fe protein.Summing ATP across required electron transfers yields 16 ATP per N2.Therefore, to form two NH3 molecules, 16 ATP are hydrolyzed. Verification / Alternative check: Biochemical and genetic studies of Mo–Fe nitrogenase consistently report the 16 ATP minimal requirement, though actual cellular costs may be higher due to ancillary processes. Why Other Options Are Wrong: 10, 5, or 15 ATP underestimate the energy cost of the nitrogenase reaction. Common Pitfalls: Confusing the urea cycle’s ATP use with nitrogen fixation; these are distinct processes. The urea cycle uses 3 ATP per urea, not per ammonia formation from N2. Final Answer: 16

Question 7

Urea cycle (hepatic ornithine cycle) — identify the correct set of net products formed per complete turn of the cycle, including the nitrogenous end product and the nucleotide byproducts generated from ATP utilization.

Accepted Answer

Introduction / Context: The urea (ornithine) cycle detoxifies ammonia by converting it to urea in hepatocytes. Beyond urea, this cycle produces additional metabolites and nucleotide byproducts due to ATP consumption. Knowing the full set of products is essential for biochemistry, clinical metabolism, and interpreting lab tests in hyperammonemia. Given Data / Assumptions: The cycle incorporates 2 nitrogen atoms (one from free ammonia via carbamoyl phosphate and one from aspartate) into urea. Carbamoyl phosphate synthetase I and argininosuccinate synthetase use ATP. Fumarate emerges from cleavage of argininosuccinate and links to the TCA cycle. Concept / Approach: The energetic bookkeeping shows the cycle costs 3 ATP equivalents per turn: 2 ATP are hydrolyzed by CPS I, and 1 ATP is hydrolyzed to AMP + PPi by argininosuccinate synthetase. Tracking the nucleotide forms produced identifies AMP and ADP among the products. Step-by-Step Solution: Carbamoyl phosphate formation: NH3 + CO2 + 2 ATP → carbamoyl phosphate + 2 ADP + Pi.Argininosuccinate formation: citrulline + aspartate + ATP → argininosuccinate + AMP + PPi.Cleavage: argininosuccinate → arginine + fumarate (fumaric acid).Final split: arginine + H2O → urea + ornithine (ornithine recycled). Verification / Alternative check: Counting outputs per full turn yields urea + fumarate as carbon skeleton products, and nucleotide byproducts AMP (from the ATP → AMP step) plus two ADP (from CPS I). This matches the option containing urea, fumaric acid, AMP, and ADP. Why Other Options Are Wrong: Option a incorrectly lists ammonia and ATP as products; ammonia is consumed, not produced, and ATP is hydrolyzed. Option c wrongly shows aspartate and ammonia as products; both are inputs. Option e ignores fumarate and nucleotide byproducts. Common Pitfalls: Confusing ATP → AMP (which counts as two high-energy bonds) with ATP → ADP; forgetting fumarate’s appearance and its anaplerotic link to the TCA cycle. Final Answer: 1 molecule of fumaric acid, 1 molecule of urea, 1 molecule of AMP, and 2 molecules of ADP.

Question 8

Nitrogen metabolism — nitrate reduction capability: Which organisms can carry out biological nitrate reduction (nitrate → nitrite) under physiological conditions?

Accepted Answer

Introduction / Context: Nitrate reduction (NO3− → NO2−) is a key step in the nitrogen cycle, enabling assimilation and energy generation in diverse organisms. Understanding who performs this reduction informs agriculture, ecology, and environmental biotech. Given Data / Assumptions: Plants possess assimilatory nitrate reductase in the cytosol. Many bacteria and some fungi carry assimilatory or dissimilatory nitrate reductases. Animals generally cannot reduce nitrate to nitrite enzymatically for assimilation. Concept / Approach: Identify taxa with nitrate reductase enzymes. Plants reduce nitrate for amino acid synthesis; microorganisms do so for assimilation or anaerobic respiration (denitrification pathways). Step-by-Step Solution: Check plants: assimilatory nitrate reductase reduces nitrate for nitrogen assimilation.Check microbes: bacteria (and some archaea/fungi) perform assimilatory/dissimilatory reduction.Animals lack this pathway for assimilation; thus eliminate options including animals only.Select “Plants and microorganisms.” Verification / Alternative check: Crop physiology texts and microbial metabolism sources consistently list plant nitrate reductase and bacterial nitrate reductases as canonical examples. Why Other Options Are Wrong: “Only microorganisms” ignores plant nitrate assimilation; “Only plants” omits microbes; “None” is incorrect; “Animals and fungi only” is false. Common Pitfalls: Confusing dietary nitrate handling in animals with enzymatic nitrate reduction for assimilation. Final Answer: Plants and microorganisms.

Nitrogen Metabolism Questions