Amino acid catabolism Flashcards

1
Q

3 situations in which amino acids are catabolised

A

-normal synthesis and degredation of proteins -when protein is in excess in the diet -during starvation

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2
Q

Ultimate purpose of amino acid catabolism

A

remove the amino group and use the carbon skeleton for TCA or to make gluconeogenic precursors to feed to the rest of the body

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3
Q

Where are amino acids processed ?

A

Liver`

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4
Q

What happens to the amino groups in amino acids when catabolised ?

A

They are either recycled, or converted and excreted

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5
Q

How are nitrogen groups from amino acid catabolism ‘passed around’ in the liver ?

A

They are added to α-ketogluterate to form glutamate (or glutamine). Glutamate is then sent to the liver mitochondria where it is given up and becomes NH4+

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6
Q

In the muscle, what happens to the extra amino groups ?

A

They are added to pyruvate in order to become alanine, which is transported to the liver

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7
Q

What converts pepsin to pepsinogen

A

The acid of the stomach

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8
Q

What happens to excess amino acids ?

A

they are NOT stored, they are either used for protein synthesis or used for energy The rest are excreted, which is why high protein diets put strain on the kidneys

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9
Q

Where does pepsin cleave ?

A

F,W,Y

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10
Q

What are the pancreatic proteases:

A

trypsin, chymotrypsin, carboxypeptidases A and B

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11
Q

Where do the pancreatic proteases cleave ?

A

they cleave short peptides into free amino acids

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12
Q

difference between ketogenic and glucogenic amino acids

A

Glucogenic amino acids make oxaloacetate to form glucose Ketogenic amino acids make acetyl CoA or acetoacetyl CoA to form ketones

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13
Q

amino groups are transformed into one of these three groups in this organ

A

they are turned into a non-toxic, water soluble product (urea, uriv acid, ammonium) in the liver

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14
Q

Transamino reaction

A

α ketogluterate + L-amino acid –> L-glutamate +α-keto group (catalysed py pyridoxal phosphate (PLP))

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15
Q

How is the amino group removed from glutamate in the liver ?

A

It uses glutamate dehydrogenase, which catalyses the oxydative deamanation of glutamate

NAD or NADP can be used under different conditions

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16
Q

How are pyruvate and alanine interconverted ?

A

In the muscle, pyruvate gets an amino group from glutamate and becomes alanine
This reaction is catalysed by alanine aminotransferase

The alanine is transferred through the blood to the liver, where the reaction occurs in reverse

(glucose-alanine cycle)

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17
Q

What happens to glutamine when it enters the hepatic mitochondria ?

A

The amino groups are turned into NH4+and passed off to carbomoyl phosphate (or aspartate).

The carbomyl phosphate is then put into the urea cycle

18
Q

enzymes of the urea cycle (4)

A
  1. Ornithine Transcarbamoylase
  2. Arginosuccinate Synthetase
    3 Arginosuccinase
  3. Arginase
19
Q

What is put into the urea cycle, and what comes out ?

A

An -NH2 group is added by carbomyl phosphate, and aspartate is added in step 2b

Fumerate and urea are the products

20
Q

Regulation of the urea cycle

A

activated (allosterically) by acetyl CoA, glutamate, and arginine

21
Q

How is the fumerate from the urea cycle added to the TCA cycle ?

A

It is converted into malate in the cytosol, which enters the mitochondria and is added to the TCA cycle

22
Q

You are a bodybuilder and you boost your protein intake to maximize your bodybuilding capacity. What is the fate of excessive dietary protein or amino acid intake?

A

Building muscle requires some increase in amino acid intake, but not to the extent that many bodybuilders consume

As amino acids are not stored, they are sent to the liver to be catabolised into energy and may be used to make acetyl CoA (ketogenic) or oxaloacetate (glucogenic). Alternately, the carbon skeletons may enter directly into the TCA cycle.

As a result, more nitrogen must be excreted, which may put a strain on the kidneys over time.

23
Q

Some genetic diseases are caused by loss-of-function mutations of Urea Cycle enzymes. What would be the symptoms in humans lacking functional versions of these enzymes?

A

As the purpose of the urea cycle is to excrete nitrogen waste in a safe way, loss of function mutations in these enzymes would lead to a buildup of nitrogen products, detectable in the blood.

Accumulation may reach toxic levels in the brain, which lead to several neurological dysfunction, and eventually death.

24
Q

The enzyme: arginosuccinate synthetase uses both high-energy phosphate bonds of ATP, in the process generating pyrophosphate, PPi which is broken down by pyrophosphatase. Where have we encountered pyrophosphatase before in metabolic pathways?

A

Fatty acid -> fatty acyl CoA
UDP glucose production

25
Q

What is the function of SAM ?

A

S-Adenosyl methionine converts phosphatidyl ethanolamine to phosphadityl choline.

(swaps out an X group)

26
Q

What is Pterin ?

A

It is converted into vitamin B12, a coenzyme in many steps

It cannot be synthesized de novo

27
Q

What can be made from tryptophan catalysis (3)

A

Nicotinate (niacin) (precursor of NAD an NADP

Serotonin (neurotransmitter)

indoleacetate (plant growth factor)

28
Q

Is tryptophan ketogenic or glucogenic ?

A

Both

the ring can be used to make ketones
the alanine part can make oxaloacetate

29
Q

Glutamate -> glutamine regulation

A

End product regulation (see below)
Adenylation (addition of AMP)

Regulated enzyme:

Glutamine synthase

30
Q

What amino acid ca be synthesized by pyruvate ?

A

Alanine

31
Q

What are the main pecursors to make heme ? (2)

A

Succinyl CoA
Glycine

They make δ-aminolevulinate (main unit of Hb)

Can also make from glutamate, but rarely

32
Q

Describe the general synthesis of heme

A

Succynyl CoA and glycine make δ-aminolevulinate

δ-aminolevulinate units come together to make a porphyrin ring

Fe is added to the centre

33
Q

What happens to heme in a bruise ?

A

Heme has its iron removed and is synthesized into biliverdin by heme oxygnease

Biliverdin is then reduced to bilirubin by biliverdin reductase (uses NADPH)

The bilirubin is then excreted

(iron is the last step of synthesis, first step of breakdown)

34
Q

What is glutathione, how is it made, and what does it do ?

A

It is a tripeptide (γ-glu - cys - gly) that protecs the cell from oxidative damage

2 of them dimerize via disulphide bridge, and the hydrogen atoms are used to satisfy the free radicals

35
Q

How many precursors are involved in the synthesis of a purine ring ?

A

6

36
Q

Write a short note on glutamate metabolism and its role in general amino acid metabolism.

A

Glutamate is synthesized by adding an amino group to α-ketogluterate through a transaminase reaction. A second amino group can be added to for glutamine.

These amino groups come from other amino acids that are being metabolised. The glutamate and glutamine provides a safe way to transport nitrogen to the kidneys for disposal.

37
Q

Explain why phenylalanine is both ketogenic and glucogenic

A

The ring structure can be used to make acetyl CoA (ketogenic), while the carbon skeleton of the ‘alanine’ part can be used to make oxaloacetate (glucogenic)

38
Q

Outline the pathway for the conversion of ammonia to urea

A

The nitrogen transported via glutamine / glutamate is added to carbamoyl phosphate

It is then transported out of the mitochondria, and enters the urea cycle

A second amino group is added by aspartate in step 2 of the urea cycle

The end products of the cycle are urea and fumerate

39
Q

Explain the alanine-glucose cycle

A

In the muscle, pyruvate gets an amino group from glutamate and becomes alanine
This reaction is catalysed by alanine aminotransferase

The alanine is transferred through the blood to the liver, where the reaction occurs in reverse

The resulting pyruvate is synthesized into glucose by the liver for distribution, and the glutamate passes off the amino group into the urea cycle

40
Q

Which of the following statements are CORRECT:

  1. The two nitrogens of urea are derived directly in the biosynthetic pathway from ammonia.
  2. The correct sequence of intermediates in the urea cycle is: carbamoyl phosphate, citrulline, arginosuccinate, arginine, ornithine, fumarate
  3. Glutamate dehydrogenase can use either NADH or tetrahydrobiopterin as reducing cofactors.
  4. The decarboxylation of tryptophan produces histamine.
  5. The precursors for heme biosynthesis are glycine and succinyl CoA
A
  1. Incorrect; the nitrogens come from glutamate (via carbomyl phopshate) and alanine
  2. Incorrect. Carbomyl phosphate adds a nitrogen gtoup to ornitine to become citrulline;
    Fumerate is a product (arginosuccinate -> fumerate + arginine)
  3. Incorrect. It can use NADP, or NAD, but not tetrahydrobiopterin
  4. Incorrect. Histamine comes from histidine
  5. True