BIOM 3200 Lecture Notes - High-Protein Diet, Protein Catabolism, Urea Cycle
28 Apr 2018
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Protein Overview
• What is the fate of NH3 (NH4+) from AA catabolism?
- NH4+ is toxic = converted into something safe or transported out of the
body
1) N2 from muscle bypassing liver (starvation mode (glutamine))
kidney Nitrogen gets excreted directly as NH4+ in the kidney
2) N2 from muscle (fed state: glutamine & alanine) liver urea
cycle kidney
- Glutamine + alanine = inter organ amino carriers
• What is the difference between the fasted and fed state in relation to
NH4+?
- 3 differences:
1) Fed state involves the formation of both glutamine + alanine
2) Fed state involves both the liver + kidney
3) Fed state involves the excretion of NH3 as urea
4) Fasted state involves the excretion of NH4+ directly
• What happens in the fed state?
- The fed state encourages Alkalosis: high consumption of protein
increases AA catabolism = increase in HCO3- (b/c H+ is used up). This
can increase pH ~7.8
- AA catabolism = NH3 release + a-keto acid (c-skeleton)
- a-ketoacid catabolized = HCO3- ; bicarbonate is a weak base that reacts
with a H+ (if this happens, no pH change)
• Why don’t we die when we eat high protein diet? (i.e. fed state)?
- 1) The liver converts amino group to urea in a process that uses HCO3-
- 2) Metabolism of S-containing AA produces a bit of H2SO4 to neutralize
pH
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• What happens in the fasted state?
- The fasted state encourages Acidosis: Starvation = minor protein
amounts catabolized (primary source of energy is fat – TAGs)
breakdown of TAGs = acidic ketone bodies
Products of TAG breakdown (long hydrocarbon chains) are not very water
soluble, so the liver converts these long hydrocarbons into small soluble
ketone bodies. pH can drop ~7
• Why don’t we die during a longer-term fast?
- AAs are catabolized in a fasted state amino group secreted directly by
kidney (thus bypassing urea cycle where HCO3- is used up). This allows
HCO3- to be used to neutralize the weak acidosis state caused by
ketones
• What are the important AAs in nitrogen metabolism and their jobs?
- Glutamate: a common end product of transamination reactions
a-ketoacid
a-ketoglutarate
- Aspartate: provides an amino group to the urea cycle
a-ketoacid
oxaloacetate
- Alanine: inter organ nitrogen carrier (goes to liver + kidney)
a-ketoacid
pyruvate
- Glutamine: most abundant, inter organ nitrogen carrier (liver + kidney),
can donate NH3 to other reactions
• What are the 4 reactions that move nitrogen from catabolized protein
between organs for excretion?
- 1) Transamination
- 2) Oxidative deamination
- 3A) Glutamine production
- 3B) Glutamate regeneration
- 4) Urea cycle
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• What is the process of transamination?
- It is the transfer of an amino group of an AA c-skeleton catalyzed by
‘aminotransferase’
- All AA undergo transamination (except: Lys, His, Thr)
- It’s a bidirectional reaction, active in all tissues, always produces AA
(usually glutamate) & a-ketoacid
AA
a-ketoacid (accept NH3 from AA)
- At least 1 transaminase exists for each AA, with each using glutamate/ a-
ketoacid (no matter the direction) as one of the pairings
- Most abundant aminotransferases in the liver: ALT & AST
- liver health tests look for those levels in the blood (shouldn’t be there)
a-ketoglutarate + alanine
ALT
Glutamate + pyruvate
a-ketoglutarate + aspartate
AST
Glutamate + oxaloacetate
• What is the process of oxidative deamination?
- Glutamate = main AA to under O.D b/c main product of transamination
- Releases NH3 from glutamate backbone
- Reaction favors the formation of a-ketoglutarate
- Process that is very active in all tissues in the body
glutamate
glutamate dehydrogenase – ATP
a-ketoglutarate + NH3
(NAD+
NADH)
• What are the uses of free NH3 from the reaction above?
- NH3 at body pH = NH4+ = ammonia = toxic
1) In EHT, can be used for glutamine synthesis
2) In the liver, can be used for urea synthesis
3) In kidenys, can be excreted directly as NH4+
Pyridoxal phosphate coenzyme
that holds NH3 group during transfer
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find more resources at oneclass.com
Document Summary
Glutamine + alanine = inter organ amino carriers: what is the difference between the fasted and fed state in relation to. The fed state encourages alkalosis: high consumption of protein increases aa catabolism = increase in hco3- (b/c h+ is used up). Aa catabolism = nh3 release + a-keto acid (c-skeleton) 1) the liver converts amino group to urea in a process that uses hco3- The fasted state encourages acidosis: starvation = minor protein amounts catabolized (primary source of energy is fat tags) breakdown of tags = acidic ketone bodies. Aas are catabolized in a fasted state amino group secreted directly by kidney (thus bypassing urea cycle where hco3- is used up). Glutamate: a common end product of transamination reactions a-ketoacid a-ketoglutarate. Aspartate: provides an amino group to the urea cycle a-ketoacid oxaloacetate. Alanine: inter organ nitrogen carrier (goes to liver + kidney) a-ketoacid pyruvate. It is the transfer of an amino group of an aa c-skeleton catalyzed by.
