Practice: Amino Acid Oxidation 1: Videos & Practice Problems

Serum transaminase enzymes, such as serum glutamate pyruvate transaminase (SGPT) and serum glutamate oxaloacetate transaminase (SGOT), are enzymes that facilitate the transfer of amino groups from amino acids to α-keto acids. These enzymes are crucial indicators of liver damage because they are released into the bloodstream when liver cells are damaged or die. Elevated levels of these enzymes in the blood can indicate liver conditions such as hepatitis, cirrhosis, or liver injury. Monitoring these enzyme levels helps in diagnosing and assessing the severity of liver diseases.

The urea cycle, also known as the ornithine cycle, is a series of biochemical reactions that occur primarily in the liver. Its main function is to convert toxic ammonia, a byproduct of amino acid metabolism, into urea, which is then excreted in the urine. The cycle involves both mitochondrial and cytosolic reactions. Key intermediates include carbamoyl phosphate, citrulline, argininosuccinate, and arginine. The urea cycle is crucial for detoxifying ammonia and maintaining nitrogen balance in the body. Disruptions in this cycle can lead to hyperammonemia, a condition characterized by elevated ammonia levels, which can be toxic to the brain.

Tetrahydrofolate (THF) and its derivatives are essential cofactors in the transfer of one-carbon units during amino acid metabolism. These one-carbon units are crucial for various biosynthetic processes, including the synthesis of nucleotides and certain amino acids. THF derivatives participate in reactions such as the conversion of serine to glycine and the synthesis of methionine from homocysteine. By facilitating these one-carbon transfers, THF and its derivatives help integrate amino acid metabolism with other metabolic pathways, ensuring the proper functioning of cellular processes and the maintenance of metabolic balance.

Muscles contribute to the urea cycle in the liver by transporting nitrogen in the form of amino acids, primarily glutamine and alanine. During protein catabolism, muscles release these amino acids into the bloodstream. Glutamine is taken up by the liver and deaminated to release ammonia, which enters the urea cycle. Alanine is converted to pyruvate in the liver, providing a substrate for gluconeogenesis, and the amino group is transferred to α-ketoglutarate to form glutamate, which also contributes to the urea cycle. This inter-organ nitrogen transport helps in the efficient detoxification of ammonia and supports metabolic homeostasis.

Amino acid degradation pathways often lead to the formation of key intermediates of the citric acid cycle (CAC), also known as the Krebs cycle. These intermediates include α-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate, and acetyl-CoA. For example, the degradation of glutamate produces α-ketoglutarate, while the breakdown of valine and isoleucine generates succinyl-CoA. These intermediates enter the CAC, where they are further oxidized to produce energy in the form of ATP, as well as reducing equivalents (NADH and FADH2) that are used in the electron transport chain. This integration of amino acid metabolism with the CAC is essential for energy production and metabolic flexibility.