Amino Acids #2
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Which are the branched chain AAs? (3) Where are they preferentially metabolized? What do they indicate?
Isoleucine, leucine, valine
High serum levels of BCAA indicate pre-diabetes
What are sources of AA that add to AA pool? (3)
Break down of body protein (~400 g), dietary protein (~100 g), and de novo synthesis of nonessential AAs (varies)
What are fates of AA that deplete AA pool?
Substrates - 6
Random things - 5
- 1. Urea cycle
- 2. Synthesis of body protein
- 3. Glucogenic/ketogenic AAs --> glucose, CO2 + ATP, ketone bodies, FAs, steroids. (6)
- 4. Synthesis of porphyrins, creatinine, neurotransmitters, purines, pyrimidines (5)
What happens if AA intake > biosynthetic needs? What if fuel source is needed?
- 1. AAs are immediately degraded
- 2. GNG or directly into TCA cycle
What is the goal of amino acid catabolism? (2)
What happens to each part of amino acid?
- 1. Separate a-amino group from C skeleton
- 2. Shunt each into specialized metabolic pathway
NH4+ from liver & other organs --> liver to be excreted as urea.
C skeleton --> pyruvate, acetyl CoA or other TCA intermediates.
What do all transaminases need? What are they all capable of?
Vitamin B6 (pyridoxal phosphate) and all capable of reversible rxns
What is role of B6 and which forms are specifically involved in transamination?
Cofactor for transaminases - amino group is transferred via pyridoxal phosphate. B6+NH3 = pyridoxamine phosphate.
Draw rxn for how general aminotransferase works (include all 4 compounds involved)
What does AST stand for? What does it transfer a-amino group from? to? forming what? is it reversible?
AST = aspartate aminotransferase.
Transfers amino group from glutamate to oxaloacetate forming aspartate.
Yes, it is reversible.
What does the amount of ALT or AST in blood tell you? Which is more specific for liver disease? Which is more sensitive due to higher levels in liver?
ALT or AST is released into blood during normal cell turnover (cell dies and releases contents into blood). However, elevated levels indicate damage to cells high in these enzymes.
ALT is more specific for liver disease.
AST is more sensitive due to higher levels in liver.
What diseases are associated with elevated ALT or AST levels? (7) Name 5.
Liver disease, myocardial infarction, muscle disorder, extensive cell necrosis, severe viral hepatitis, toxic injury and prolonged circulatory collapse.
What do most transaminases use as an amino group accceptor? What is the product of most transamination rxns (2)?
Glutamate + a-keto acid
What are the functions of transamination rxns? (2)
1. Help maintain adequate levels of non-essential AAs required for protein synthesis.
2. Funneling amino groups from catabolized AAs to glutamate & aspartate for eventual excretion as urea.
With positive N balance, will more or less NH2 be transaminated from alanine to a-ketoglutarate? What about negative balance?
For + and - balance, will there be more or less transaminase activity?
Positive N balance --> catabolism to excrete excess amino acids --> ALT will go towards creating glutamate.
Negative N balance --> catabolism of body protein to create energy due to low ATP and GTP levels.
Either way, there is transaminase activity!
Why might BCAA implicate diabetes? What groups were studies using? (don't have to answer if you don't want to).
High levels of circulating amino acids may promote insulin resistance possibly via disruption of insulin signaling in skeletal muscle.
Obese non-diabetic Chinese/Asian-Indian insulin resistant men and obese siblings of pairs of obesity discordant monozygotic twins.
What happens after transamination to a-ketoglutarate and pyruvate and glutamate is formed? What are the 3 steps?
Oxidative deamination in liver/kidney via glutamate DH.
- 1. Glutamate is transferred to liver for oxidative deamination via glutaminase, then in liver its broken back down into glutamate and NH3.
- 2. In liver/kidney glutamate -- glutamate DH --> a-ketoglutarate + free NH3.
- 3. A-ketoglutarate is recycled.
How is glutamate dehydrogenase regulated:
Activated by: 2
Inhibited by: 2
Also depends on what else?
Activated by ADP and GDP allosterically (low energy)
Inhibited by ATP and GTP
Also depends on concentrations of rxns and products: relative concentrations of glutamate:a-ketoglutarate, ammonia, and oxidized to reduced coenzymes.
What direction does glutamate dehydrogenase rxn strongly go towards?
Goes strongly towards production of a-ketoglutarate and free NH3 (catabolism)
What are the effects of high-protein meal vs starvation?
Which will have AA degradation? Ammonia formation? AA synthesis?
What is the effect of increased protein on glutamate production for ALT?
Starvation --> low ATP:ADP and low GTP:GDP, so need high AA degradation to get energy production from C skeletons, thus increased synthesis of a-ketoglutarate for TCA cycle or for GNG.
Also get amino acid degradation and ammonia production. In liver, rxn proceeds in direction of AA degradation and formation of ammonia to get rid of excess protein. BUT CAN ALSO GET AA SYNTHESIS FROM CORRESPONDING A-KETO ACIDS.
Increased protein --> increased glutamate
How is NH3 transported to liver/kidney (2) (most tissues vs. ? Why are NH3 levels so highly regulated? (3)
In most tissues: glutamate + NH3 + ATP -- glutamine synthetase --> glutamine (nontoxic transport form of 2 ammonias) --> liver.
- Muscle (glucose alanine cycle):
- Transamination of pyruvate to form alanine. Alanine is transported to liver via blood and is converted to pyruvate and glutamate --> GNG.
What is the main purpose of urea cycle? How many ATP does it require? Where do its 2 nitrogens (Ns) come from (which compounds) and what do they combine with?
Main purpose: major route for NH3 disposal.
ATP - 3
2 Ns: NH3 (oxidative deamination) and from aspartate combine with CO2.
What does BUN stand for? When will it be high? What can it measure?
BUN = blood urea nitrogen
Will be high with faulty urea synthesis (faulty waste disposal).
Measures nitrogen balance
What is the mechanism of urea synthesis pathway?
How is urea cycle regulated? (4)
Activated by high protein diet, high protein meal containing arginine, N-acetylglutamate, and starvation.
1. Long-term: Activated by high protein diet (increased use of protein for energy/catabolism --> increased urea) also activated by starvation (use of body protein as energy) upregulates autophagy and expression of urea cycle enzymes.
2. Short-term: N-acetylglutamate - allosteric activator of carbamoyl phosphate synthetase 1 (CPS1)
N acetylglutamate is formed from acetylCoA + glutamate and its synthesis is increased by arginine (in protein rich meal).
What inhibits urea synthesis? (don't really need to know).
high levels of alanine apparently
Will you see higher protein degradation during positive N balance or negative N balance?
During negative N balance, even though protein degradation does occur during positive N balance.
What is hyperammonemia generally caused by? Name 5 symptoms of hyperammonemia?
Caused by defect in oniline enzyme.
Vomiting, cerebral edema, coma, irritability, excessive sleepiness
What are possible reasons for hyperammonemia? (3) outside of enzyme def
- 1. Altered metabolism of important compounds (pyruvate, lactate, glycogen, glucose)
- 2. Changes in NMDA and GABA receptors
- 3. Free radical production & induction of membrane permeability transition phenomenon leading to mitochondrial dysfunction.
What is defective process? What is defective enzyme? Symptom?
Melanin synthesis from tyrosine.
Lack of pigment
CPS-deficiency (other urea cycle defects)
What is defective process? What is defective enzyme? Symptoms (3)
Lethargy, convulsions, early death
Maple Syrup Urine
What is defective process? What is defective enzyme? Symptom? (3)
Branched chain AA degradation.
Branched chain a-keto acid dehydrogenase
Mental disease, retardation, early death
What is the defective process? Defective enzyme? Symptoms? (1) What is special about PKU?
Conversion of Phe --> Tyr
Easiest to treat, can be treated via diet. (low protein diet + tyrosine supplements) Pregnant women must continue treatment to prevent birth defects.
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