Protein and Amino Acid Metabolism Quiz

See Modification of Amino Acids. Option D is correct. To use an amino acid for energy, the body first has to remove its amino group so that only the carbon skeleton is left. One way to do this is transamination, which swaps the amino group of an amino acid for a keto group (the C=O group of a keto acid). Two enzymes do most of this work: glutamic-pyruvic transaminase (alanine aminotransferase, ALT) and aspartate aminotransferase (AST), and the reactions are reversible. Glutamic-pyruvic transaminase (ALT) moves the amino group from alanine to alpha-ketoglutarate, producing pyruvate and glutamate. The pyruvate can then be used for energy or to make glucose, and the glutamate carries the nitrogen onwards. This links amino acid metabolism with the central pathways of carbon metabolism. The other way to remove an amino group is deamination, where it is released as free ammonia, which is eventually turned into urea in the urea cycle.

• Transamination: It swaps the amino group of an amino acid for a keto group.
• Enzymes: ALT and AST do most of this work and the reactions are reversible.
• Example: ALT moves the amino group from alanine to alpha-ketoglutarate, giving pyruvate and glutamate.

A helpful aid is to match the first letters: ALT starts from Alanine, and AST starts from Aspartate.