Amino Acid Degradation
Bryant Miles

The carbon skeletons of amino acids are broken down into metabolites that can either be oxidized into CO2 and H2O to generate ATP, or can be used for gluconeogenesis. The catabolism of amino acids accounts for 10 to 15% of the human body’s energy production. Each of the 20 amino acids has a separate catabolic pathway, yet all 20 pathways converge into 5 intermediates, all of which can enter the citric acid cycle. From the citric acid cycle the carbon skeletons can be completely oxidized into CO2 or diverted into gluconeogensis or ketogenesis.
Glucogenic amino acids are broken down into one of the following metabolites: pyruvate, &豩-ketoglutarate, succinyl CoA, fumarate or oxaloacetate. Ketogenic amino acids are broken down into acetoacetate or acetyl-CoA. Larger amino acids, tryptophan, phenylalanine, tyrosine, isoleucine and threonine are both glucogenic and ketogenic. Only 2 amino acids are purely ketogenic they are lysine and leucine. If 2 of the amino acids are purely ketogenic and 5 amino acids are both ketogenic and glucogenic, than that leaves 13 amino acids that are purely glucogenic: Arg, Glu, Gln, His, Pro, Val, Met, Asp, Asn,Ala, Ser, Cys, and Gly.
I. Amino Acids that are Catabolized into Pyruvate.
Pyruvate is the entry point for amino acids that contain 3 carbons, alanine, serine and cysteine.
Alanine transaminase reversibly transfers the amino group from alanine to &豩-ketoglutarate to form pyruvate and glutamate. Note that enzyme requires a pyridoxal phosphate cofactor. The &豩-ketoglutarate is regenerated by glutamate dehydrogenase.

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