Amino Acid Metabolism 3
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- What are the branched chain amino acids (BCAA)?
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*Valine
*Leucine
*Isoleucine
-hydrophobic, major component of body protein -
Which BCAA are ketogenic and which are ketogenic?
What component do they produce? -
Valine --> purely glucogenic
Isoleucine --> both
Leucine --> purely ketogenic
Valine and Isoleucine glucogenic through SUCCINYL CoA (PROPIONYL-CoA)
Isoleucine ketogenic through ACETYL CoA
Leucine ketogenic through HMG CoA (ACETYL CoA) - What are the first two steps of BCAA metabolism?
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1. Transamination by a specific amino-transferase to make an alpha-keto-BCAA
2. Oxidative Decarboxylation, shortening chain of BCAA by one C, using a decarboxylating dehydrogenase (same for all three)--site of defect in maple syrup urine disease
*occurs largely in muscle -
Describe the modified Cori Cycle using BCAA? -
Used to take off Nitrogen from BCAA. First put N onto glutamte then alanine through transamination. Export the N from muscle tothe liver using alanine. Take alanine back to glutamate so that the N can be excreted in urea. The C-skeleton is taken to pyruvate. (Alanine is the same oxidation level as lactate) The cycle lets us do BCAA metabolism in muscle and export N group to liver for excretion. This way couples amino group transport with gluconeogenesis in liver to supply muscle with glucose. - Where does amino acid metabolism take place?
- Mainly in the liver where there is an abundance of Branched Chain Dehydrogenase, but for BCAA this mainly takes place in the muscle.
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If you have a defect in Branched Chain Dehydrogenase, what disease do you have?
Symptoms
Prevalence
Therapy -
Maple Syrup Urine Disease
inherited
S-failure to thrive, feeding difficulties, presents after first protein meal, mental development severely affected, convulsions, characteristic odor
P-low incidence, but 1/200 births among PA Mennonites
T-low protein meal, low in BCAA - How is Propionyl CoA glucogenic? and what BCAAs produce it?
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*Propionyl CoA is carboxylated using using biotin as a cofactor to form Methlmalonyl CoA
*Methylmalonyl CoA is rearranged ot succinyl CoA using Methylmalonyl CoA Mutase that requires B12 to straighten the 4 C chain to form Succinyl CoA.
*Succinyl CoA feeds TCA to form new Malate, and Glucose - What aa are ketogenic?
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*Lysine
*Leucine - What is the role of B12 in BCAA metabolism?
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It is a cofactor of Methylmalonyl CoA Mutase.
It straightens out the 4 C chain of methylmalonyl CoA.
You can get it from animal sources such as meat and eggs. - What two reactions use B12?
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1. Homocysteine --> Methionine
2. Methylmalonyl CoA --> Succinyl CoA
*both reactions have toxic substrates, methylmalonate (from MMCoA) causes demyelination