top of page

Amino Acid Catabolism

image.png

Protein and AA Metabolism

  • major role of dietary protein is to replace body protein

  • ​protein is digested to amino acids (AA) which are a source of​

    • essential (EAA)​

    • non-essential (NEAA)

  • body is constantly degrading and synthesizing and replacing its proteins

  • new proteins are synthesized from 

    • "old" AA from protein breakdown​

    • "new" AA from dietary protein

  • amount of protein in adults is relatively fixed

    • loss can be due to illness or sarcopenia​

    • gain with training

    • each day we lose the same about of nitrogen that we take in

      • N in = N out (N balance)​

      • presence of N is what makes protein different from CHO and fat

    • the skeleton is a source of C,H and O for the TCA cycle

  • energy source is proteins secondary role

    • must remove the NH2 group​

      • transamination

        • transfers NH3​

      • deamination​

        • removes NH3​

        • AA transaminates to glutamate

    • remaining C skeleton used in metabolism

Transamination

  • transfer of amino group from an AA to a keto acid, produces a new AA and a new keto acid

    • this process allows for the synthesis of NEAA from keto acids​

    • the production of keto acids are used for energy needs and gluconeogenesis)

  • the most active aminotransferases are: 

    • alanine aminotransferase (ALT)​

      • in serum used as clinical indication of myocardial infarction, myocardial damage​

    • aspartate aminotransferase (AST)

      • in serum used as a clinical indication of liver damage​

      • used to monitor patients on drugs that might damage liver

  • after the NH2 is removed, the carbon skeletons enter metabolic pathways

    • pyruvate --> acetyl CoA or OAA​

    • OAA --> TCA

    • α-KG --> TCA

    • other AA carbon skeletons enter at acetyl CoA directly

    • energy derived from these C skeletons is derived from the processes of TCA and ETS

  • transamination is also the means for disposing excess AA

  • also means using body's own protein for fuel

​

Screenshot 2023-11-11 at 3.56.52 PM.png
Screenshot 2023-11-11 at 3.57.32 PM.png
Screenshot 2023-11-11 at 3.58.31 PM.png
image.png

Deamination & Urea Cycle

Screenshot 2023-11-11 at 4.09.26 PM.png

  • Removal of nitrogen via urea cycle

    • NH4 cannot be free in cells as it is toxic​

    • processed in the liver by urea cycle

      • urea is non toxic​

  • carbamoyl-p enters urea cycle like acetyl CoA enters TCA (by combining with the last compound in the cycle)

1. carbamoyl-P + ornithin --> citrulline

  • so far only 1 N is involved

  • we are still in the mito​

2. citrulline + aspartate --> argininosuccinate

  • citrulline leaves mito and goes to cytosol

  • we added one more N (aspartate is the N donor)

  • requires input of ATP

  • occurs in the cytosol

3. argininosuccinate --> arginine + succinate --> fumarate (TCA)

  • succinate comes off and is converted to fumarate

4. arginine + H2O --> ornithine (urea made from this step)

  • cycle begins again with another NH4

image.png
image.png
image.png
image.png

©2023 by Syracuse University Dr.Margaret Voss

bottom of page