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CORE II Block 1 > Amino acid metabolism > Flashcards

Amino acid metabolism Flashcards

1
Q

What are the essential AA?

A

PVT TIM HLL (phenylalanine, valine, threonine, tryptophan, isoleucine, histidine, leucine, lysine)

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2
Q

What are the storage molecules?

A

glycogen, TAG. there is no protein storage molecule for the purpose of storing energy

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3
Q

What is the first step of AA catabolism?

A

transamination of amine group to alpha ketogluterate, generating glutamate. The leftover amino acid is called an alpha keto acid. this is done by transaminases with PLP as a coenzyme prosthetic grouup.

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4
Q

What is the role of PLP in transamination?

A

PLP = pyridoxal phosphate

  • derivative of vitamin B12
  • covalently linked to lysine at site of aminotransferase
  • picks up amino group from amino acid and transfers to alpha-keto gluterate (2 step rxn)
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5
Q

How are aminotransferases named?

A

according to the donor amino acid

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6
Q

What is the aminotransferase for catbolism of alanine, and what is its a-keto acid?

A

alanine aminotransferase. pyruvate

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7
Q

What is the aminotransferase for catbolism of aspartate, and what is its a-keto acid?

A

aspartate aminotransferase. OAA

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8
Q

Why are ALT and AST diagnositc of liver disease?

A

they help break aa’s into key intermediate metabolites. if there is too much of these enzymes in the liver, it suggests liver damage.

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9
Q

What is the second step of amino acid catabolism?

A

oxidative deamination. glutamate dehydrogenase removes the amino group as ammonia. This generates alpha ketogluterate

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10
Q

What is unique about the glutamate dehydrogenase (GDH) enzyme?

A

It can use both NADH and NADPH. It can go in both directions, but can only go in the reverse when NH3 is high.

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11
Q

How is glutamate dehydrogenase regulated?

A

The mitochondrial enzyme is activated by ADP but inhibited by GTP. The reaction is pulled forward, because alpha ketogluterate that is produced fills into the TCA cycle.

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12
Q

What are the two mechanisms that keep NH3 levels low in the blood?

A
  1. transport as glutamine and alanine
  2. conversion to urea by the urea cycle in the liver (primary mechanism of detox)
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13
Q

Describe the transport of NH3 as glutamine.

A

GLutamine synthase adds circulating ammonia to glutamate, forming glutamine. The detoxifying function of glutamine accounts for much of the glutamine in the blood. This reaction is reversed by glutaminase.

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14
Q

Where is glutamine synthase expressed?

A

brain, muscle, liver

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15
Q

Where is glutaminase expressed?

A

in the liver in kidney, so that glutamine can be converted back to glutamate

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16
Q

What is the overall rxn of the urea cycle?

A

urea, CO2, and aspartate react with the breaking of four phosphate bonds to form urea and fumerate. The resulting urea contains nitrogen from both urea and fumerate.

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17
Q

What is the rate limited and regulated step of the urea cycle?

A

CPSI adds ammonia from glutamate to bicarbonate with ATP in the mitochondria, generating carbamoyl phosphate. CPSI is allosterically activated by NAG.

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18
Q

How is the UC and the TCA cycle connected?

A

Malate aspartate shuttle

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19
Q

How can the energy cost of UC be recouped?

A

NADH generated can be reduced at the ETC to generate ATP. (can feed citrate/malate into TCA to get NADH)

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20
Q

Describe how the UC is a true cycle.

A

Ornithine is regenerated.

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21
Q

Why is there no net synthesis of arginince in the UC?

A

Arginine made by the UC is immediately cleaved by argininase to make urea and ornithine, so there is no net synthesis of arginine

22
Q

How is the UC regulated as a whole?

A
  • changes in substrate concentration
  • changes in enzyme concentration (e.g. during starvation the UC enzymes have higher expression)
  • CPSI is allostericlaly activated by NAG, made by NAG synthetase.
23
Q

In general, why are liver enzymes organized the way they are?

A

To limit te amount of ammonia escape from the liver into the bloodstream

24
Q

What is the most common UC enzyme defficiency? How is it treated?

A

OTC, the only X-linked UC enzyme. Those with this problem have hyperammonemia.

  • It can be treated with a-keto acid analogs of the essential amino acids, and the rest of the calories are just carbs and fats.
  • Patients can also be treated with N-scavenging drugs such as benzoate and phenylacetate.
  • antibiotics can also prevent metabolism of gut bacteria which produce a lot of ammonia
25
a keto acids are made from amino acid catabolism. What is the fate of these molecules?
aa are classified as ketogenic, glucogenic, or glucoketogenic depending on where the carbons of their a-keto acids end up - ketogenic: LL (leucine, lysine) - become acetyl coA - glucoketogenic: PITT (phenylalanine, isoleucine, tryptophan, tyrosine) become pyruvate which can enter TCA, gluconeogenesis, or become acetyl coA for ketone body formation - the rest of the AA are glucogenic (enter the TCA directly)
26
Which are the BCAA?
leucine, valine, isoleucine
27
Describe the breakdown of a-keto acid carbon skeletons resulting from catabolism of BCAA.
1. in muscle, transamination takes place, creating alpha keto acids and glutamate. (gln forms from here) 2. oxidative decarboxylation of glutamate generates NADH and CO2, The enzyme responsible for this is branched chain a-keto acid dehydrogenase complex. 3. FAD-linked dehydrogenation generates FADH2 4. alanine produced from pyruvate from valine and isoleucine
28
What is unusual about the breakdown of a-keto acids from BCAA?
They are taken up by the muscle rather than the liver
29
Where are straight chain AAs metabolized?
in the liver
30
What amino acids are generated by the metabolism of BCAA?
glutamine and alanine carry the amino groups from BCAA.
31
What coenzymes are needed by branched chain a-keto acid dehydrogenase complex? How is it regulated?
1. E1-lipolic acid (TPP) 2. E2-FAD (regenerated by NAD) 3. E3-coA BCKAD is active when dephosphorylated. It is inhibited by the NADH generated via FA oxidation (activates its kinase)
32
Maple syrup urine disease?
Deficiency in BCKAD E2 or E3 subunits, causing accumulation of branched chain a-keto acids or BCAA. sweet body odor dietary limitation of BCAA
33
How is propionyl coA metabolized?
It is an odd-numbered fatty acyl coA. Biotin + carboxylase remove a carbon. vit B12 and mutase rearrange it to succinyl coA. This can be converted to pyruvate, or it can enter the TCA cycle. Pyruvate can also be converted to alanine, which carries toxic ammonia out of the muscle to the liver. In the liver, alanine transaminase can catabolize alanine to pyruvate and glutamate, using pyruvate for gluconeogenesis.
34
What is the effect of glucocorticoids on glutamine synthetase?
They upregulate glutamine synthetase
35
Glutamine is a carrier of ammonia. How is it then metabolzied?
-In the liver and kidneys, glutamine can be broken into glutamate and ammonia. Glutamate can then be broken into a-ketogluterate and ammonia. This is ok, because the liver and kidney can handle ammonia.
36
After glutamine metabolism in the liver and kidney, how is ammonia gotten rid of?
- in liver, NH3 is converted to urea - in kidney, ammonia picks up protons and excretes in the form of ammonium. this is needed when ketone bodies are being produced, because they are very acidic.
37
In what tissue type is the branched chain a keto acid transaminase expressed?
muscle, therefore BCAA can only be metabolized in the muscle
38
What are the final products of tryptophan catabolism?
It is glucoketogenic. Its products are alanine and acetoacetate. Quinalinate is a by-product and contributes to the amount of available NADP.
39
What is pellagra?
Insufficient uptake of tryptopan (remember, it is an essential AA). Prevelent where protein intake is low. Symptoms = 4 D's
40
To what amino acid is phenylalanine converted?
In the liver, phenylalanine is converted to tyrosine by phenylalanine hydroxylase. PAH requires THB/BH4 as a coenzyme.
41
What causes phenylketonuria?
Deficiency in phenylalanine dehydrogenase or enzyme which reduces coenzyme THB back to its original form. Phenylacetate and phenyllactate buildup in patients. They cannot consume phenylalanine, and must supplement their diet with tyrosine.
42
What is the final product of tyrosine?
acetoacetyl coA. It is glucoketogenic.
43
What is the result in a defect in homogentisate dioxegenase?
This enzyme breaks down tyrosine. Patients have black urine and early onset arthritis. Alkaptonuria.
44
What type of carrier is generated from methionine catabolism?
S-adenosyl methionine methyl donor
45
When S-adenosyl methionine, a product of methionine catabolism, loses its methyl, what pathway can be thrombogenic?
S-adenosyl homocysteine uses hydrolase to convert to homocysteine. If it accumulates, it activates the clotting cascade and can be thrombogenic.
46
How are homocysteine levels kept low in the blood?
1. homocysteine can be remethylated to form methionine. Tetrahydrofolate carries and donates the methyl group with the help of vit B12. 2. homocysteine reacts with serine to form cysteine
47
Which two reactions in the body require vit B12
remethylation of homocysteine to methionine by methionine synthase, and breakdown of propionyl coA to succinyl coA.
48
What is a folate trap?
tetrahydrofolate is a coenzyme needed by methionine synthase to remethylate homocysteine to methionine. If B12 is not sufficient, TFH cannot release methyl to homocysteine.
49
Which metabolites are oxidatively decarboxylated to propionyl coA?
VOMIT (valine, odd numbered fatty acids methionine, isoleucine, threonine)
50
Which one-carbon carriers are produced form amino acid metabolism?
1. SAM (methyl donor) 2. tetrahydrofolate 3. biotin (CO2)
51
What pathways do aromatic amino acids enter?
phenylalanine, tyrosine, and tryptophan are all glucoketogenic (as well as isoleucine)
52
What process supplies the NADH and ATP needed for gluconeogenesis?
fatty acid oxidation

CORE II Block 1 (5 decks)

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