Amino Acid Catabolism and Transamination

• Amino acid catabolism involves removing the amino group, then metabolizing the remaining carbon skeleton.
• Transamination is generally the first step, where the α-amino group is removed.
• During transamination, the amino group transfers from an amino acid to α-ketoglutarate.
• Results in an α-keto acid (derived from the original amino acid) and glutamate
• Aminotransferases (transaminases) catalyze transamination.
• They are present in the liver and many other tissues.
• Pyridoxal phosphate (PLP), derived from vitamin B6, is required as a coenzyme by aminotransferases.
• PLP is covalently bound to the ε-amino group of a specific lysine residue in the enzyme's active site.
• α-ketoglutarate is used as one of the amino group acceptors by all aminotransferases.
• Alanine aminotransferase (ALT) facilitates the transfer of the amino group from alanine to α-ketoglutarate.
• Forms pyruvate and glutamate.
• Aspartate aminotransferase (AST) facilitates the transfer of the amino group from aspartate to α-ketoglutarate.
• Forming oxaloacetate and glutamate.
• Aminotransferases are typically reversible.
• They are used in both amino acid degradation and biosynthesis.
• The net deamination of amino acids does not occur because of aminotransferase reactions.
• Instead, they collect amino groups from various amino acids into glutamate.

Oxidative Deamination

• Primarily occurs in the liver and kidneys.
• Glutamate is oxidatively deaminated by glutamate dehydrogenase during oxidative deamination.
• Glutamate loses its amino group as ammonia (NH3) in this reaction.
• α-ketoglutarate is regenerated which can then participate in further transamination reactions.
• Glutamate dehydrogenase utilizes either NAD+ or NADP+ as a coenzyme.
• Glutamate dehydrogenase's reaction is reversible.
• Playing a role in both amino acid catabolism and synthesis.
• Glutamate dehydrogenase is allosterically regulated.
• GTP and ATP are allosteric inhibitors.
• ADP is an allosteric activator.

Ammonia Transport in the Blood

• Ammonia is toxic and blood concentrations must be kept low.
• Peripheral tissues produce ammonia.
• It is then transported to the liver for conversion to urea.
• Two main mechanisms transport ammonia:
• Glutamine Synthesis: In most tissues, glutamine synthetase combines ammonia with glutamate to form glutamine.
• Glucose-Alanine Cycle: Pyruvate can be transaminated to alanine in muscle.
• Alanine is transported to the liver and converted back to pyruvate.
• Pyruvate can be used for gluconeogenesis.
• Glucose produced is sent back to the muscle (glucose-alanine cycle).

Urea Cycle

• The primary method for disposing of nitrogen from amino acids.
• It occurs in the liver.
• The liver produces urea.
• It is transported in the blood to the kidneys and excreted in the urine.
• The overall stoichiometry: NH3 + CO2 + Aspartate + 3 ATP + 2 H2O → Urea + Fumarate + 2 ADP + AMP + 4 Pi
• Consists of five enzymatic reactions.
• Two occur in the mitochondria and three in the cytosol.

Reactions of the Urea Cycle

• Formation of Carbamoyl Phosphate:
  • The first reaction, which occurs in the mitochondrial matrix.
  • Carbamoyl phosphate forms from ammonia, carbon dioxide, and ATP.
  • Carbamoyl phosphate synthetase I (CPS-I) catalyzes this reaction.
  • N-acetylglutamate is required as an allosteric activator.
• Formation of Citrulline:
  • Carbamoyl phosphate and ornithine react to produce citrulline.
  • The enzyme ornithine transcarbamoylase catalyzes this reaction.
  • The mitochondrial matrix is also where this reaction takes place.
  • Citrulline is then transported to the cytosol.
• Formation of Argininosuccinate:
  • Citrulline and aspartate react to form argininosuccinate.
  • Argininosuccinate synthetase catalyzes this reaction.
  • This reaction, which takes place in the cytosol, requires ATP.
• Cleavage of Argininosuccinate:
  • Argininosuccinate is cleaved to form arginine and fumarate
  • The enzyme argininosuccinase catalyzes this reaction.
  • The cytosol is where the reaction occurs.
  • Fumarate is an intermediate in the citric acid cycle.
• Cleavage of Arginine:
  • Arginine is cleaved to form urea and ornithine.
  • The enzyme arginase catalyzes this reaction.
  • The reaction occurs in the cytosol.
  • Ornithine is transported back to the mitochondria to begin another cycle.

Regulation of the Urea Cycle

• The urea cycle is regulated at several levels:
• Substrate Availability:
• Increased levels of ammonia in the liver stimulate the urea cycle.
• Allosteric Activation of Carbamoyl Phosphate Synthetase I (CPS-I):
• N-acetylglutamate activates CPS-I.
• Arginine stimulates N-acetylglutamate synthesis.
• Arginine levels increase when amino acid levels are high.
• Enzyme Synthesis:
• Long-term regulation includes changes in the synthesis of urea cycle enzymes.
• High-protein diets or prolonged starvation lead to increased synthesis of urea cycle enzymes.

Urea Cycle Disorders

• Genetic deficiencies in any of the urea cycle enzymes can lead to hyperammonemia
  • Hyperammonemia is a condition with elevated ammonia levels in the blood.
• Neurological damage, coma, and death can result from hyperammonemia.
• Treatment for urea cycle disorders typically involves:
• Dietary management: limiting protein consumption
• Administration of drugs that remove ammonia from the blood

Examples of Urea Cycle Disorders

• Carbamoyl Phosphate Synthetase I (CPS-I) Deficiency:
  • A rare disorder that results in the complete or partial deficiency of CPS-I activity.
• Ornithine Transcarbamoylase (OTC) Deficiency:
  • The most common urea cycle disorder.
  • It is an X-linked disorder.
• Argininosuccinate Synthetase Deficiency (Citrullinemia):
  • This disorder results in the accumulation of citrulline in the blood and urine.
• Argininosuccinase Deficiency (Argininosuccinic Aciduria):
  • Results in the accumulation of argininosuccinic acid in the blood and urine.
• Arginase Deficiency (Argininemia):
  • The rarest urea cycle disorder.
  • Results in the accumulation of arginine in the blood and cerebrospinal fluid.

Fate of the Carbon Skeletons

• After the amino group is removed, the carbon skeletons of amino acids are metabolized to form:
  • Glycogenic amino acids: Amino acids that are degraded to pyruvate or citric acid cycle intermediates.
  • These can be used to synthesize glucose.
  • Ketogenic amino acids: Amino acids that are degraded to acetyl CoA or acetoacetyl CoA.
  • These can be used to synthesize fatty acids or ketone bodies.
  • Some amino acids are both glycogenic and ketogenic.

Glucogenic Amino Acids

• Amino acids converted to pyruvate:
  • Alanine, Cysteine, Glycine, Serine, Threonine, Tryptophan.
• Amino acids converted to α-ketoglutarate:
• Arginine, Glutamate, Glutamine, Histidine, Proline.
• Amino acids converted to succinyl CoA:
• Isoleucine, Methionine, Threonine, Valine.
• Amino acids converted to fumarate:
• Phenylalanine, Tyrosine.
• Amino acids converted to oxaloacetate:
• Aspartate, Asparagine.

Ketogenic Amino Acids

• Amino acids converted to acetyl CoA:
  • Leucine.
• Amino acids converted to acetoacetyl CoA:
• Lysine, Phenylalanine, Tryptophan, Tyrosine.
• Amino acids that are both glucogenic and ketogenic:
• Isoleucine, Tryptophan, Tyrosine, Phenylalanine.

Explore amino acid catabolism, focusing on transamination as the initial step. Learn how aminotransferases, with the help of pyridoxal phosphate (PLP), transfer amino groups from amino acids to α-ketoglutarate, producing α-keto acids and glutamate. Understand the roles of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) in these processes.