L17 - Amino acid metabolism I

Question 1

Pathways of protein degradation

Answer 1

Lysosomal - long-lived cellular proteins

Ubiquitin-proteases - short-lived proteins

Intestinal - exogenous dietary proteins

Question 2

Intestinal protein degradation

Answer 2

Dietary proteins are hydrolysed into amino acids and absorbed into the bloodstream

Provides amino acids for oxidative metabolism and gluconeogenesis

Question 3

Why are amino acids broken down

Answer 3

There is no storage form of amino acids so any in excess get broken down

Nitrous part is converted to ammonia and urea and excreted, the carbon skeleton is converted to either A-CoA acetoA-CoA, pyruvate, or a citric acid cycle intermediate

Question 4

Protein turnover

Answer 4

Daily protein intake 70-100g, amino acid oxidation provides 10-20% of total oxidative metabolism

Plus body protein is huge potential fuel reserve - 100,000KJ (24,000 kcal)

Question 5

Does turnover happen exclusively in humans

Answer 5

No, it occurs in all forms of life

Question 6

How frequently does structural protein turnover occur?

Answer 6

Not that often, they are typically pretty stable

Collagen - half-life of months or years

Question 7

Sources of amino acids

Answer 7

• Diet
• Body protein
• Non-essential amino acid synthesis

Question 8

Fates of amino acids

Answer 8

• TCA cycle oxidation use
• N-compound synthesis
• Synthesis of tissue protein
• Synthesis of glucose, ketone bodies, fatty acids, etc

Question 9

Amino acid metabolism: how the groups get metabolised

Answer 9

Amine group - excreted as urea

Carboxylic group and side chain - degraded to metabolic intermediates

Question 10

Transamination: what is it, where does it occur, and why does it mainly occur here?

Answer 10

Amino group transfer from a donor amino acid and a recipient keto acid (alpha-ketoglutarate), forming glutamate

Liver for almost all amino acids except valine, isoleucine, and leucine (they are branched amino acids and the liver doesn’t have the transaminases to break them down - they are instead broken down in other tissue: heart, muscle, brain etc)

This is where amino acids that have been broken down during digestion are first brought to so their breakdown (if the body has excess protein - no way to store it) begins here and the first step of protein breakdown is transamination

Question 11

Alanine/aspartate + a-KG: what are the reactions?

Answer 11

Alanine + a-ketoglurtarate <-> pyruvate + glutamate

Aspartate + a-ketoglurtarate <-> oxaloacetate + glutamate

Reversible - may be used to generate AA if necessary

Question 12

Deamination: what is it, what enzyme catalyses it, what is the reaction, where does it occur, and why is it so useful for amino acid metabolism?

Answer 12

Removal of amino group from glutamate, fomring NH4+

Glutamate dehydrogenase

Glutamate -> alpha-ketoglutarate

Inside the mitochondria

Essentially replenishes lost a-KG used in transamination reaction

Question 13

Urea cycle: what is it used for, where does it occur, why is it required, and what is its energy usage?

Answer 13

Nitrogen removal

In the liver in the cytosol/mitochondrial matrix:
* Ammonia generation occurs in the mitochondrial matrix
* Ornithine enters the mitochondria and gets turned into citrulline after reacting with ammonia before returning to the cytosol where the rest of the urea cycle occurs

Free ammonia - toxic

Uses 3 ATP equivalents

Question 14

Urea cycle: the process

Answer 14

1 - Carbomyl phosphate synthetase couples
a carbon dioxide with NH4+ using (2ATP), forming carbamoyl phosphate

2 - Ornithine transcarbamoylase produces citrulline by transferring the carbamoyl group from carbamoyl phosphate to ornithine

3 - Arginosuccinate synthase produces arginosuccinate by using an ATP molecule to compress aspartate with citrulline

4 - Arginosuccinate is cleaved by arginosuccinase, forming fumarate and arginine

5 - Arginine is hydrolysed by arginase using water to form urea and ornithine

Question 15

Urea: what is its structure and where do its components come from?

Answer 15

H2N-C-NH2
⠀⠀⠀⠀\O

One nitrogen from ammonia, the other from aspartate and the oxygen from water

Question 16

Links between urea cycle and citric acid cycle

Answer 16

Aspartate can be formed by a transamination reaction by using oxaloacetate

Aspartate leaves the mitochondria to the cytosol where it is either transaminated into oxaloacetate and then reduced by NADH to form malate, or used in the urea cycle to react with citrulline and form arginosuccinate

Fumarate leaves the urea cycle and can be converted to malate

The movement of malate and aspartate across the mitochondrial membrane is called the malate-aspartate shuttle

Question 17

Examination of a fetal blood sample from a young patient with symptoms including stunted growth, microcephaly and seizures, revealed elevated concentrations of arginine and ammonia.

Describe a possible mechanism by which this metabolic disorder might occur

Suggest a possible treatment

Answer 17

Nitogen removal system flawed somewhere

High ammonia/arginine - not an issue with steps 1-4 of AA cycle, may be with 5

Potential issue of arginase and the formation of urea for excretion

Treatment - limit protein intake, administer arginase/supportive techniques to return the intended effect of arginase

Question 18

Degradation of the C-skeleton: what is the general pathway?

Answer 18

Amino acid - keto acid - metabolic intermediate - oxidation, FA, ketones, etc

Note - in the formation of the keto acid, the nitrous skeleton is removed via the urea pathway

Question 19

Fates of the C-skeletons of amino acids

Answer 19

All 20 amino acids can be oxidised to water and carbon dioxide by entering the TCA cycle through seven different molecules:

• Acetyl-CoA (Ile, Leu, Trp)
• Acetoacetyl-CoA (Phe, Leu, Lys, Trp, Tyr)
• alpha-ketoglutarate (Arg, Glu, Gln, His, Pro)
• Succinyl-CoA (Ile, Met, Thr, Val)
• Fumarate (Asp, Phe, Tyr)
• Oxaloacetate (Asp, Asn)
• Pyruvate (Ala, Cys, Gly, Ser, Thr, Trp)

Acetyl-CoA and acetoacetyl-CoA are both formed by ketogenic amino acids and the rest are glucogenic amino acids

Question 20

Ketogenic vs glucogenic amino acids

Answer 20

Glucogenic - used in the TCA cycle to create energy

Ketogenic - used to create fatty acids/ketones

Question 21

Which amino acids have their carbon skeleton metabolised into acetyl-CoA?

Answer 21

• Isoleucine
• Leucine
• Tryptophan

Question 22

Which amino acids have their carbon skeleton metabolised into acetoacetyl-CoA?

Answer 22

• Phenylalanine
• Leucine
• Lysine
• Tryptophan
• Tyrosine

Phe, Leu, Lys, Trp, Tyr

Question 23

Which amino acids have their carbon skeleton metabolised into alpha-ketoglutarate?

Answer 23

• Arginine
• Glutamate
• Glutamine
• Histidine
• Proline

Arg, Glu, Gln, His, Pro

Question 24

Which amino acids have their carbon skeleton metabolised into succinyl-CoA?

Answer 24

• Isoleucine
• Methionine
• Threonine
• Valine

Ile, Met, Thr, Val

Question 25

Which amino acids have their carbon skeleton metabolised into fumarate?

Answer 25

• Aspartate
• Phenylalanine
• Tyrosine

Asp, Phe, Tyr

Question 26

Which amino acids have their carbon skeleton metabolised into oxaloacetate?

Answer 26

• Aspartate
• Asparagine

Asp, Asn

Question 27

Which amino acids have their carbon skeleton metabolised into pyruvate?

Answer 27

• Alanine
• Cysteine
• Glycine
• Serine
• Threonine
• Tryptophan

Ala, Cys, Gly, Ser, Thr, Trp

Question 28

Gluconeogenesis: what is it, what are the main precursors for it, when does it occur, and what is the process?

Answer 28

Synthesis of glucose from non-carbohydrate precursors

AA and lactate are the major precursors for gluconeogenesis

During extended periods of carbohydrate shortage

Amino acid - pyruvate (OAA?) - glucose

Question 29

Fatty acid synthesis using ketogenic amino acids: what is the mechanism, what amino acids are exclusively in the ketogenic group, and why can this process be useful

Answer 29

Amino acids - A-CoA/acetoA-CoA - FA/ketone bodies

Leucine and lysine can ONLY give rise to FA / ketones

Body has ‘infinite’ capacity to store fatty acids as TAG - Excess dietary AA converted to fat

Question 30

Branched-chain amino acids: what are they, where are they degraded, how does degradation happen, what is used for degradation, and what is its regulation?

Answer 30

Valine, isoleucine, and leucine

Muscle, kidney, and brain - their aminotransferase are absent from the liver

Val, Ile, Leu - oxidized as fuels in muscle, kidney, brain

The amino acids undergo oxidative decarboxylation, producing CO2 and acetyl-CoA

Branched-chain α-keto acid dehydrogenase complex

Regulated by phosphorylation:
* Little Val, Ile, Leu in diet - phosphorylated & inactivated
* Addition of Val, Ile, Leu - dephosphorylation and activation

Question 31

Maple syrup urine

Answer 31

Disorder of the oxidative decarboxylation of α-ketoacids derived from valine, isoleucine, and leucine caused by the missing or defect of branched-chain dehydrogenase.

The levels of branched-chain amino acids and corresponding α-ketoacids are markedly elevated in both blood and urine.

The urine has the odor of maple syrup

The early symptoms:
lethargy
ketoacidosis
unrecognized disease leads to seizures, coma, and death
mental and physical retardation

Treatment = rigid control of the diet - limiting Val, Ile, Leu

Question 32

Phenylketonuria

Answer 32

Phenylketonuria is caused by an absence or deficiency of phenylalanine hydroxylase or of its tetrahydrobiopterin cofactor.

Phenylalanine accumulates in all body fluids and converts to phenylpyruvate.

Defect in myelination of nerves
The brain weight is below normal.
Mental and physical retardations.
The life expectancy is drastically shortened.

Diagnostic criteria:
phenylalanine level in the blood
FeCl3 test
DNA probes (prenatal)