Amino Acid Metabolism Flashcards
(33 cards)
Question 1
Which amino acid can be directly deaminated to form a keto (oxo) acid?
A. Alanine
B. Aspartate
C. Cysteine
D. Glutamate
E. Glutamine
D
✅ Correct! The amino acid that can be directly deaminated to form a keto acid is Glutamate (D).
Glutamate is unique because it can undergo oxidative deamination directly — it releases its amino group as free ammonia (NH₃) without needing to transfer it to another molecule first.
Question 2
What is the main function of glutamine in inter-organ nitrogen transport?
A. It stores glucose in the form of glycogen
B. It transports ketone bodies from liver to brain
C. It safely carries ammonia to the liver and kidneys
D. It forms acetyl-CoA for fatty acid synthesis
E. It converts to pyruvate directly in muscle
C
✅ Correct again! Glutamine (C) is the body’s main way of safely carrying excess nitrogen (as ammonia) through the bloodstream — especially from muscle to the liver and kidney — for excretion or pH regulation.
Question 3
Which is the only mammalian amino acid that can be directly deaminated to a keto (oxo) acid?
A. Alanine
B. Aspartate
C. Cysteine
D. Glutamate
E. Glutamine
D
You’re spot on again — the correct answer is D. Glutamate ✅
This is a key concept:
Glutamate is unique among amino acids because it can undergo oxidative deamination directly, releasing ammonia (NH₃) and forming α-ketoglutarate — a TCA cycle intermediate.
What are the end products of nitrogen metabolism?
A. Urea, creatinine, uric acid, ammonia
B. Glucose, lactate, ketone bodies, urea
C. Urea, ammonia, fatty acids, alanine
D. Pyruvate, ammonia, creatinine, cholesterol
E. Bicarbonate, urea, amino acids, glucose
A
✅ Correct — the answer is A. Urea, creatinine, uric acid, and ammonia.
These are the four major nitrogenous waste products the body produces during protein and amino acid metabolism:
Urea → from the breakdown of amino acids (main nitrogen excretion form)
Creatinine → from the breakdown of creatine phosphate in muscle
Uric acid → from purine (DNA/RNA) metabolism
Ammonia (NH₃ / NH₄⁺) → toxic, converted to urea or excreted by the kidneys for pH control
What is the main site of transcription of the genes responsible for ribosomes?
A. Cytoplasm
B. Histones
C. Nucleolus
D. Nucleosome
E. Nucleus
The nucleolus, a dense structure inside the nucleus, is where ribosomal RNA (rRNA) is transcribed and where ribosome subunits are assembled.
Ribosomes are essential for translating amino acids into proteins, so this ties back to your understanding of how amino acids are used after digestion and absorption.
Which metabolic pathway predominates within the liver and kidney during prolonged fasting?
A. Fatty acid synthesis
B. Gluconeogenesis
C. Glycolysis
D. Ketogenesis
E. Pentose phosphate shunt
B
During prolonged fasting, your body needs to maintain blood glucose levels, especially for organs like the brain and red blood cells, which depend heavily on glucose.
The liver and kidney take over glucose production by converting non-carbohydrate sources (like amino acids, lactate, and glycerol) into glucose.
This process is called gluconeogenesis
Gluconeogenesis: 🟢 Active in fasting — maintains blood sugar
Ketogenesis: ✅ Also occurs, especially in liver, but not in kidney — makes ketone bodies, not glucose
Fatty acid synthesis & glycolysis: 🔴 Fed state pathways
Pentose phosphate pathway: Supplies NADPH, not major in fasting
Gluconeogenesis: 🟢 Active in fasting — maintains blood sugar
Ketogenesis: ✅ Also occurs, especially in liver, but not in kidney — makes ketone bodies, not glucose
Fatty acid synthesis & glycolysis: 🔴 Fed state pathways
Pentose phosphate pathway: Supplies NADPH, not major in fasting
Which of the following best explains why dietary protein is required daily?
A. The body loses amino acids in sweat
B. There is no dedicated storage form of protein in the body
C. Protein is needed to provide ketone bodies
D. All amino acids are synthesised in the body
E. Protein is stored in muscle and released when needed
B
✅ Correct! The answer is B. There is no dedicated storage form of protein in the body.
Unlike fat or carbohydrates, the body cannot store amino acids — so continuous dietary intake is essential to replace losses from turnover, repair tissues, and support metabolism.
Which enzyme catalyses the transfer of an amino group from an amino acid to 2-oxoglutarate, forming glutamate?
A. Urease
B. Alanine dehydrogenase
C. Aminotransferase
D. Glutaminase
E. Transketolase
C
✅ Correct again! The answer is C. Aminotransferase.
These enzymes — like ALT and AST — catalyse transamination, the process of transferring an amino group from an amino acid to 2-oxoglutarate, forming glutamate and a corresponding keto acid.
Which of the following amino acids is exclusively ketogenic?
A. Isoleucine
B. Tyrosine
C. Lysine
D. Phenylalanine
E. Tryptophan
C
🧠 Explanation:
Lysine (along with leucine) is exclusively ketogenic, meaning it is broken down into acetyl-CoA or acetoacetate — precursors for ketone bodies, not glucose.
The others (like isoleucine and phenylalanine) are both glucogenic and ketogenic.
Which of the following processes occurs primarily in the liver to detoxify free ammonia?
A. Beta-oxidation
B. Gluconeogenesis
C. Urea cycle
D. Glycogenolysis
E. Transamination
C
The urea cycle, also called the ornithine cycle, is a liver-specific pathway that safely converts toxic ammonia (NH₃) into urea, which is then excreted by the kidneys.
This process is vital because free ammonia is neurotoxic, especially to the brain.
Which amino acid acts as a safe carrier of two ammonia equivalents in the blood?
A. Glutamate
B. Alanine
C. Aspartate
D. Glutamine
E. Ornithine
D
Glutamine safely carries two ammonia equivalents — one on the amide side chain and one on the alpha-amino group.
It transports nitrogen from muscle and other tissues to the liver (for urea formation) and to the kidneys (for pH buffering).
Ornithine, while important in the urea cycle, is not the main nitrogen transporter in the blood.
Which of the following amino acids is both glucogenic and ketogenic?
A. Leucine
B. Lysine
C. Tyrosine
D. Valine
E. Glutamate
C
Tyrosine is both glucogenic (can form glucose via fumarate) and ketogenic (can form acetoacetate).
Valine (D) is only glucogenic.
Leucine and lysine are exclusively ketogenic.
What is the main amino acid product formed when alanine is transaminated in the liver?
A. Pyruvate
B. Oxaloacetate
C. Glutamine
D. Acetyl-CoA
E. Fumarate
A
When alanine undergoes transamination, it donates its amino group to 2-oxoglutarate, forming glutamate and leaving behind pyruvate as the carbon skeleton.
This pyruvate can then enter gluconeogenesis during fasting.
Oxaloacetate (B) is formed from aspartate, not alanine.
Which of the following is the most direct fate of glutamate during oxidative deamination in the liver?
A. Alanine
B. Urea
C. Ammonia and α-ketoglutarate
D. Glutamine
E. Aspartate
C
In oxidative deamination, glutamate is converted back into α-ketoglutarate, releasing free ammonia (NH₃) in the process. This reaction primarily occurs in the liver, and the ammonia goes straight into the urea cycle for detoxification.
This step is essential for safely removing nitrogen from the body.
Which organ is the only site where the urea cycle takes place?
A. Kidney
B. Intestine
C. Liver
D. Muscle
E. Pancreas
C
The urea cycle is exclusive to the liver — specifically in hepatocytes. It’s the only tissue equipped to fully detoxify ammonia by converting it into urea, which is then excreted by the kidneys.
Other organs (like the kidney or muscle) can handle ammonia to some extent (e.g., for pH buffering), but they do not run the urea cycle.
Which amino acid is used by the kidney to release ammonium ions (NH₄⁺) for acid-base balance?
A. Alanine
B. Glutamine
C. Glutamate
D. Aspartate
E. Arginine
BIn the kidney, glutamine is broken down to release ammonium ions (NH₄⁺).
These ions are excreted in urine to help buffer excess acid, playing a key role in maintaining blood pH — especially during metabolic acidosis or fasting.
What is the immediate nitrogen source for urea synthesis in the urea cycle?
A. Glutamine and arginine
B. Ammonia and aspartate
C. Alanine and pyruvate
D. Glutamate and glutamine
E. Citrulline and fumarate
B
The two nitrogen atoms in urea come from:
Free ammonia (NH₃) — released by oxidative deamination of glutamate
Aspartate — donates the second nitrogen via a transamination reaction
Citrulline and fumarate are intermediates in the urea cycle, not nitrogen donors themselves.
Which of the following conditions is most directly associated with a defect in the urea cycle?
A. Ketoacidosis
B. Hyperammonaemia
C. Lactic acidosis
D. Hypoglycaemia
E. Hyperuricaemia
B
A defect in the urea cycle (whether genetic or due to liver failure) leads to an inability to detoxify ammonia (NH₃).
This results in hyperammonaemia — a toxic buildup of ammonia in the blood.
It can cause confusion, coma, and even death if not managed quickly.
Which of the following is a semi-essential amino acid, particularly important during childhood growth?
A. Leucine
B. Glutamate
C. Arginine
D. Alanine
E. Valine
C
Arginine is considered semi-essential — it’s usually synthesised by the body, but not in sufficient amounts during periods of rapid growth, such as childhood.
It’s also important in wound healing and immune function.
Which of the following best describes trans-deamination?
A. Direct deamination of all amino acids
B. Conversion of amino acids to glucose
C. Transamination followed by oxidative deamination of glutamate
D. Synthesis of amino acids from keto acids
E. Glutamine formation from glutamate
C
🔬 Explanation:
Trans-deamination is a two-step process:
Transamination: Transfers the amino group from most amino acids to 2-oxoglutarate, forming glutamate.
Oxidative deamination: Glutamate then loses the amino group as free ammonia, which enters the urea cycle.
It’s how the body collects and disposes of nitrogen from multiple different amino acids.
What is the fate of creatinine in nitrogen metabolism?
A. It is reused to regenerate creatine
B. It is converted to ketone bodies
C. It is excreted in the urine
D. It enters the TCA cycle
E. It donates nitrogen to the urea cycle
C
Creatinine is a waste product formed from the breakdown of creatine phosphate in muscle.
It is not reused — the body cannot convert it back to creatine.
It is filtered by the kidneys and excreted in urine.
Creatinine levels are often used as a marker of kidney function in blood tests.
What is nitrogen balance?
The difference between nitrogen intake and excretion; reflects body protein status (positive = building, negative = breaking down).
What causes negative nitrogen balance?
Starvation, illness, cancer, or trauma — leads to muscle loss and can be fatal if prolonged.
What are the two main protein degradation pathways?
Ubiquitin-proteasome system and autophagy-lysosome pathway.
