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Questions and Answers
Amino acids are classified into six main classes based on their R groups.
False
All non-polar aliphatic amino acids have a branched chain structure.
False
The R group of an amino acid can affect its solubility in water.
True
Histidine is classified as a non-essential amino acid.
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Glutamine is classified as a non-essential amino acid.
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Aspartate is a polar, negatively charged amino acid.
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There are 10 essential amino acids that the human body cannot synthesize.
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Phenylalanine is an aromatic amino acid.
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Glucogenic amino acids can be converted into glucose during metabolism.
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Glucogenic amino acids are catabolised to pyruvate, α-ketoglutarate, succinyl CoA, fumarate or oxaloacetate while ketogenic amino acids produce glucose.
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Both glucogenic and ketogenic amino acids can result in the formation of fat or ketone bodies.
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Alanine is classified as a glucogenic amino acid.
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Phenylalanine is exclusively a glucogenic amino acid.
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Lysine can be classified as a glucogenic amino acid.
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4-hydroxyproline is an example of a common amino acid modification found in collagen.
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Non-protein amino acids are found in cells but not in proteins.
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Tryptophan is a glucogenic amino acid.
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Glutamate is only classified as a ketogenic amino acid.
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The degradation of amino acids leads only to seven common metabolic intermediates.
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Infants born with a complete deficiency of urea synthesizing enzymes can survive after their first protein meal.
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Citrullinemia is characterized by elevated levels of citrulline in plasma and cerebrospinal fluid.
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Patients with inherited defects in urea synthesizing enzymes can tolerate protein-rich diets without concerns.
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Hyperammonemia Type 2 is an X chromosome-linked deficiency caused by ornithine transcarbamoylase deficiency.
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The first approach to treatment for urea cycle disorders includes a high protein intake.
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Argininosuccinic aciduria is characterized by low levels of argininosuccinate in blood, cerebrospinal fluid, and urine.
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Ornithine and citrulline are products of the urea cycle.
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The average adult body contains approximately 100 kg of body protein.
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Amino acids can only be synthesized from dietary protein.
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The daily protein breakdown in adults is estimated at 300-400 g.
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Histamine is produced by the decarboxylation of glutamate.
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Energy accounts for 10-15% of the body’s daily requirement from protein metabolism.
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Decarboxylation reactions convert amino acids into carbohydrates.
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Histidine decarboxylase requires vitamin B6 as a coenzyme.
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Non-essential amino acids are synthesized mainly from dietary protein.
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The body loses approximately 30-50 g of protein daily through various processes.
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Citrulline condenses with aspartate to form argininosuccinate in the mitochondria.
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The synthesis of argininosuccinate consumes one ATP and is catalyzed by argininosuccinate synthetase.
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Ornithine is regenerated during the cleavage of arginine by arginase.
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The overall urea cycle reaction is reversible and consumes 4 ATP.
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Fumarate is produced during the argininosuccinate lyase reaction and enters the urea cycle.
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ATP generated in the TCA cycle is utilized for urea synthesis.
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Hyperammonaemia can result from defects in any one of the five enzymes involved in urea synthesis.
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Oxaloacetate can be formed from fumarate in the mitochondria.
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The urea cycle does not involve any interactions with the citric acid cycle.
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The TCA cycle utilizes CO2 produced during the urea cycle.
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Study Notes
Amino Acid Classification
- Amino acids differ based on their R group, which influences their structure, size, and electrical charge.
- The R group also affects the amino acid's solubility in water.
- Five classes of amino acids exist:
- Non-Polar Aliphatic
- Aromatic
- Polar, Uncharged
- Polar, Positively Charged
- Polar, Negatively Charged
Non-Polar Aliphatic Amino Acids
- Glycine (Gly, G)
- Alanine (Ala, A)
- Proline (Pro, P)
- Valine (Val, V) Branched Chain
- Leucine (Leu, L) Branched Chain
- Isoleucine (Ile, I) Branched Chain
- Methionine (Met, M)
Aromatic Amino Acids
- Phenylalanine (Phe, F)
- Tyrosine (Tyr, Y)
- Tryptophan (Trp, W)
Polar, Uncharged Amino Acids
- Serine (Ser, S)
- Threonine (Thr, T)
- Cysteine (Cys, C)
- Asparagine (Asn, N)
- Glutamine (Gln, Q)
Polar, Positively Charged Amino Acids
- Lysine (Lys, K)
- Arginine (Arg, R)
- Histidine (His, H)
Polar, Negatively Charged Amino Acids
- Aspartate (Asp, D)
- Glutamate (Glu, E)
Nutritional Classification
- Essential amino acids cannot be synthesized by the human body and must be obtained through diet.
- Non-essential amino acids can be synthesized by the body.
Metabolic Classification
- Amino acids are categorized based on their metabolic intermediates produced during catabolism.
- Glucogenic amino acids are catabolized to pyruvate, α-ketoglutarate, succinyl CoA, fumarate, or oxaloacetate, which can form glucose.
- Ketogenic amino acids are catabolized to acetyl CoA or acetoacetate, which can form fat or ketone bodies.
Uncommon Amino Acids
- Aside from the 20 common amino acids, uncommon amino acids are found in small amounts in proteins and non-protein compounds.
- These unusual amino acids are often modifications of common amino acids – derived amino acids.
- Some examples include 4-hydroxyproline and 5-hydroxylysine found in collagen, and 6-N-methyllysine found in myosin.
Non-Protein Amino Acids
- Some amino acids are found in cells but not in proteins.
- Ornithine and citrulline are examples, acting as intermediates in arginine biosynthesis and the urea cycle.
Amino Acid Pool
- The body contains a pool of ~100g of amino acids.
- This pool is maintained through dietary intake, protein breakdown, and synthesis.
- Amino acids are utilized for protein synthesis, non-protein compound synthesis, and energy production (10-15% of daily requirements).
- Excess dietary protein is converted to carbohydrates and fat.
General Reactions of Amino Acids
- Amino acids undergo various reactions, including decarboxylation, transamination, and deamination (oxidative and non-oxidative).
Decarboxylation Reaction
- Amino acids are decarboxylated to form amines.
- Histidine is decarboxylated to histamine by histidine decarboxylase, requiring pyridoxal phosphate as a coenzyme.
Histamine
- Histamine is involved in various physiological roles, including gastric acid secretion and neurotransmission.
Urea Synthesis (Urea Cycle)
- The urea cycle is a metabolic pathway that removes ammonia from the body.
- Step III: Synthesis of argininosuccinate: Citrulline condenses with aspartate to form argininosuccinate, catalyzed by argininosuccinate synthetase. Requires one ATP.
- Step IV: Cleavage of argininosuccinate: Argininosuccinate is cleaved into fumarate and arginine by argininosuccinate lyase.
- Step V: Formation of urea: Arginine is cleaved by arginase to yield urea and ornithine.
Overall Urea Cycle Reaction
- The cycle is irreversible and consumes 4 ATP.
- NH4+ + CO2 + Aspartate + 3 ATP → Urea + Fumarate + 2 ADP + 2 Pi + AMP + PPi
Link Between Ornithine/Urea and Citric Acid Cycles
- The two cycles are linked in several ways:
- Fumarate produced in the urea cycle is transported to the mitochondria and ultimately converted to oxaloacetate.
- Oxaloacetate can be used in the TCA cycle, gluconeogenesis, or transamination to aspartate, which is a nitrogen donor in the urea cycle.
- CO2 liberated from the TCA in mitochondria is used in the urea cycle.
- The TCA cycle generates 12 ATP, while the urea cycle consumes 4 ATP.
Inherited Enzymatic Defects of the Urea Cycle
- Defects in any of the five enzymes in the urea cycle lead to hyperammonemia.
- Symptoms include lethargy, stupor, convulsions, CNS impairment, and, in severe cases, coma and death.
- Infants with complete deficiencies in urea synthesizing enzymes die shortly after their first protein meal.
- Partial deficiencies may allow survival into adulthood if managed properly.
- Patients with deficiencies require low-protein diets and may benefit from α-keto analogues of essential amino acids.
- Hyperammonemia Type 1: Carbamoyl phosphate synthase I deficiency.
- Hyperammonemia Type 2: Ornithine transcarbamoylase deficiency (X-linked).
- Citrullinuria/Citrullinemia: Argininosuccinate synthetase deficiency.
- Argininosuccinic aciduria/acidemia: Argininosuccinase deficiency.
- Hyperargininemia: Arginase deficiency.
Treatment of Urea Cycle Defects
- Low protein diets are essential.
- Avoid prolonged fasting.
- α-keto analogues of essential amino acids can be provided.
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Description
Test your knowledge of amino acid classification based on their R groups. This quiz covers various classes including non-polar, polar, and aromatic amino acids, as well as their characteristics and examples. Understand the structure and properties of different amino acids in biological systems.
