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17 Cards in this Set

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How do you get Nitrogen into Urea?
*17 of 20 aa can transfer N to @-ketoglutarate through transamination to form Glutamate
*Glutamate can do either of two things:
*Do another transamination with Oxaloacetate to form Asparate which goes into the Urea Cycle or
*Using GDH form ammonia that goes into the Urea Cycle
*Urea gets its 2 Nitrogen
What three amino acids cannot donate nitrogen through transamination/aminotransferase reactions to an acceptor @-keto-acid?
Lysine
Proline
Leucine?
What do you get from an aminotransferase/transamination reaction?
*The transfer of an amino group from an amino acid to an @-keto acid to form a new amino acid and a new @-keto acid
*almost always @-ketoglutarate and glutamate are the substrate/product pair
*Glutamate is a resevoir compound for N or incoming aa
*PLP is an essential cofactor for all transaminases (formed in an ATP dependent reaction from vit B6)
What are the characteristics of aminotransferases?
*Freely reversible: N goes in both directions
*Need PLP as a cofactor from Vit B6
*An AT for nearly every amino acid
*Most use @-ketoglutarate/glutamate as one substrate/product pair
What does ALT and AST in serum mean?
It means that tissue lysis has occured because aminotransferases are intracellular. Indicator for hepatitis, heart or muscle damage.

ALT = alanine AT Ala -> Pyr
AST = asparate AT Asp -> OAA
What enzyme comes from Vit B6?
Pyridoxal Phosphate (PLP)

*Which aminotransferaces need as a cofactor
*If absent in enfant formula,causes convulsions b/c neurotransmitters not maed.
What is special about the Glutamate Dehydrogenase Reaction?
It's an oxidative deamination that helps to move N around the urea cycle.

*It's a reversible reaction that releases NH4+ as glutamate goes to @-KG, and can take up nitrogen from the system in the reverse reaction.
Name 6 ways to produce ammonia.
Liver
1. Glutamate to @-KG by GDH
2. Glutamine to Glutamate by glutaminase
3. Asparagine to Aspartate by asparaginase
4. Histidine to Urocanate by histidase
Gut
5. Produced by bacteria that cleaves urea
Muscle
6. Purine nucleotide cycle, AMP to IMP
In a transaminase/aminotransferace reaction, where does the nitrogen and carbon skeleton come from?
*N comes from the aa substrate
*C-skeleton comes from the @-keto-acid (and can now make glucose during fasting and FA during the fed state)
What are the forms of Nitrogen that enter the Urea Cycle? What do they form?
*NH4+ forms carbomyl phosphate in the mitochondria using CPSI, 2 ATP and CO2

*Aspartate ultimately leaves as fumarate, and nitrogen says in arginine
(cytoplasm)
5 Steps of Urea Cycle
1. activation of NH4+, reacts w/CO2, 2 ATP using carbamoyl phosphate synthetase I (CPSI) to form CARBAMOYL PHOSPHATE
2. carbamoyl phosphate reacts w/ORNITHINE to make CITRULLINE with enzyme ORNITHINE TRANSCARBOXYLASE (mitochondria)
3.-5. add ASPARATE to CITRULLINE in cytoplasm
-4 C of aspartate taken off as fumarate and N used to make ARGININE
-cleave off amidino of Arginine to make UREA and regenerate ORNITHINE
Why does urea production increase during fasting?
During fasting, your body runs out of glycogen and needs to keep the blood glucose level up. So the body breaks down intracellular proteins to scavenge for aa to make glucose. The carbon skeletons are feed into gluconeogenesis and the nitrogen is excreted as urea. As the body adapts to fasting, tissues shift from glucose usage to ketone bodies.
How can we control the flow of nitrogen in to the urea cycle?
*transcriptional repression
*control at CPS I level
-required allosteric effector NAG, that positively affects CPS I
-NAG is made from glutamate and acetyl CoA, whose reaction if positively controlled by ARGININE
*TEST*
What are the activators of the Urea Cycle?
*Arginine
*NAG
*Glutamate
*Ammonia - substrate availability
What happens during liver failure?
What can be done?
*No urea made
*Ammonia builds up
*Toxicity occurs
*Coma, death

*Activate urea cycle
*Transplant
what happens during kidney failure?
What can be done?
*Urea builds up
*Later ammonia builds up
*Death

*Dialysis
*Low protein diet
In hereditary disorders, how is nitrogen excreted?
*Serine -> Glycine -> Hippurate (that is excretable)