L17 - Amino acid metabolism I Flashcards
Pathways of protein degradation
Lysosomal - long-lived cellular proteins
Ubiquitin-proteases - short-lived proteins
Intestinal - exogenous dietary proteins
Intestinal protein degradation
Dietary proteins are hydrolysed into amino acids and absorbed into the bloodstream
Provides amino acids for oxidative metabolism and gluconeogenesis
Why are amino acids broken down
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
Protein turnover
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)
Does turnover happen exclusively in humans
No, it occurs in all forms of life
How frequently does structural protein turnover occur?
Not that often, they are typically pretty stable
Collagen - half-life of months or years
Sources of amino acids
- Diet
- Body protein
- Non-essential amino acid synthesis
Fates of amino acids
- TCA cycle oxidation use
- N-compound synthesis
- Synthesis of tissue protein
- Synthesis of glucose, ketone bodies, fatty acids, etc
Amino acid metabolism: how the groups get metabolised
Amine group - excreted as urea
Carboxylic group and side chain - degraded to metabolic intermediates
Transamination: what is it, where does it occur, and why does it mainly occur here?
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
Alanine/aspartate + a-KG: what are the reactions?
Alanine + a-ketoglurtarate <-> pyruvate + glutamate
Aspartate + a-ketoglurtarate <-> oxaloacetate + glutamate
Reversible - may be used to generate AA if necessary
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?
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
Urea cycle: what is it used for, where does it occur, why is it required, and what is its energy usage?
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
Urea cycle: the process
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
Urea: what is its structure and where do its components come from?
H2N-C-NH2
⠀⠀⠀⠀\O
One nitrogen from ammonia, the other from aspartate and the oxygen from water
Links between urea cycle and citric acid cycle
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
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
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
Degradation of the C-skeleton: what is the general pathway?
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
Fates of the C-skeletons of amino acids
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
Ketogenic vs glucogenic amino acids
Glucogenic - used in the TCA cycle to create energy
Ketogenic - used to create fatty acids/ketones
Which amino acids have their carbon skeleton metabolised into acetyl-CoA?
- Isoleucine
- Leucine
- Tryptophan
Which amino acids have their carbon skeleton metabolised into acetoacetyl-CoA?
- Phenylalanine
- Leucine
- Lysine
- Tryptophan
- Tyrosine
Phe, Leu, Lys, Trp, Tyr
Which amino acids have their carbon skeleton metabolised into alpha-ketoglutarate?
- Arginine
- Glutamate
- Glutamine
- Histidine
- Proline
Arg, Glu, Gln, His, Pro
Which amino acids have their carbon skeleton metabolised into succinyl-CoA?
- Isoleucine
- Methionine
- Threonine
- Valine
Ile, Met, Thr, Val
Which amino acids have their carbon skeleton metabolised into fumarate?
- Aspartate
- Phenylalanine
- Tyrosine
Asp, Phe, Tyr
Which amino acids have their carbon skeleton metabolised into oxaloacetate?
- Aspartate
- Asparagine
Asp, Asn
Which amino acids have their carbon skeleton metabolised into pyruvate?
- Alanine
- Cysteine
- Glycine
- Serine
- Threonine
- Tryptophan
Ala, Cys, Gly, Ser, Thr, Trp
Gluconeogenesis: what is it, what are the main precursors for it, when does it occur, and what is the process?
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
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
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
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?
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
Maple syrup urine
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
Phenylketonuria
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)
