Amino acid catabolism Flashcards
3 situations in which amino acids are catabolised
-normal synthesis and degredation of proteins -when protein is in excess in the diet -during starvation
Ultimate purpose of amino acid catabolism
remove the amino group and use the carbon skeleton for TCA or to make gluconeogenic precursors to feed to the rest of the body
Where are amino acids processed ?
Liver`
What happens to the amino groups in amino acids when catabolised ?
They are either recycled, or converted and excreted
How are nitrogen groups from amino acid catabolism ‘passed around’ in the liver ?
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+
In the muscle, what happens to the extra amino groups ?
They are added to pyruvate in order to become alanine, which is transported to the liver
What converts pepsin to pepsinogen
The acid of the stomach
What happens to excess amino acids ?
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
Where does pepsin cleave ?
F,W,Y
What are the pancreatic proteases:
trypsin, chymotrypsin, carboxypeptidases A and B
Where do the pancreatic proteases cleave ?
they cleave short peptides into free amino acids
difference between ketogenic and glucogenic amino acids
Glucogenic amino acids make oxaloacetate to form glucose Ketogenic amino acids make acetyl CoA or acetoacetyl CoA to form ketones
amino groups are transformed into one of these three groups in this organ
they are turned into a non-toxic, water soluble product (urea, uriv acid, ammonium) in the liver
Transamino reaction
α ketogluterate + L-amino acid –> L-glutamate +α-keto group (catalysed py pyridoxal phosphate (PLP))
How is the amino group removed from glutamate in the liver ?
It uses glutamate dehydrogenase, which catalyses the oxydative deamanation of glutamate
NAD or NADP can be used under different conditions
How are pyruvate and alanine interconverted ?
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)
What happens to glutamine when it enters the hepatic mitochondria ?
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
enzymes of the urea cycle (4)
- Ornithine Transcarbamoylase
- Arginosuccinate Synthetase
3 Arginosuccinase - Arginase
What is put into the urea cycle, and what comes out ?
An -NH2 group is added by carbomyl phosphate, and aspartate is added in step 2b
Fumerate and urea are the products
Regulation of the urea cycle
activated (allosterically) by acetyl CoA, glutamate, and arginine
How is the fumerate from the urea cycle added to the TCA cycle ?
It is converted into malate in the cytosol, which enters the mitochondria and is added to the TCA cycle
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?
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.
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?
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.
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?
Fatty acid -> fatty acyl CoA
UDP glucose production
What is the function of SAM ?
S-Adenosyl methionine converts phosphatidyl ethanolamine to phosphadityl choline.
(swaps out an X group)
What is Pterin ?
It is converted into vitamin B12, a coenzyme in many steps
It cannot be synthesized de novo
What can be made from tryptophan catalysis (3)
Nicotinate (niacin) (precursor of NAD an NADP
Serotonin (neurotransmitter)
indoleacetate (plant growth factor)
Is tryptophan ketogenic or glucogenic ?
Both
the ring can be used to make ketones
the alanine part can make oxaloacetate
Glutamate -> glutamine regulation
End product regulation (see below)
Adenylation (addition of AMP)
Regulated enzyme:
Glutamine synthase
What amino acid ca be synthesized by pyruvate ?
Alanine
What are the main pecursors to make heme ? (2)
Succinyl CoA
Glycine
They make δ-aminolevulinate (main unit of Hb)
Can also make from glutamate, but rarely
Describe the general synthesis of heme
Succynyl CoA and glycine make δ-aminolevulinate
δ-aminolevulinate units come together to make a porphyrin ring
Fe is added to the centre
What happens to heme in a bruise ?
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)
What is glutathione, how is it made, and what does it do ?
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
How many precursors are involved in the synthesis of a purine ring ?
6
Write a short note on glutamate metabolism and its role in general amino acid metabolism.
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.
Explain why phenylalanine is both ketogenic and glucogenic
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)
Outline the pathway for the conversion of ammonia to urea
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
Explain the alanine-glucose cycle
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
Which of the following statements are CORRECT:
- The two nitrogens of urea are derived directly in the biosynthetic pathway from ammonia.
- The correct sequence of intermediates in the urea cycle is: carbamoyl phosphate, citrulline, arginosuccinate, arginine, ornithine, fumarate
- Glutamate dehydrogenase can use either NADH or tetrahydrobiopterin as reducing cofactors.
- The decarboxylation of tryptophan produces histamine.
- The precursors for heme biosynthesis are glycine and succinyl CoA
- Incorrect; the nitrogens come from glutamate (via carbomyl phopshate) and alanine
- Incorrect. Carbomyl phosphate adds a nitrogen gtoup to ornitine to become citrulline;
Fumerate is a product (arginosuccinate -> fumerate + arginine) - Incorrect. It can use NADP, or NAD, but not tetrahydrobiopterin
- Incorrect. Histamine comes from histidine
- True
