21 - Amino Acid Synthesis Flashcards
What is one of the essential amino acids that is basically nonessential?
Arginine is produced in the urea cycle but at levels insufficient to satisfy the requirements of protein synthesis. Need to get extra from diet.
What is one of the essential amino acids that is basically nonessential?
Exceptions to nonessential?
Arginine is produced in the urea cycle but at levels insufficient to satisfy the requirements of protein synthesis. Need to get extra from diet.
Cysteine is only non essential if we have enough methionine (both sulfur containing).
Tyrosine is only nonessential if we have enough essential tryptophan (aromatic)
Describe synthesis of nonessential (non diet acquired) amino acids
- Pathways of carbohydrate and lipid metbaolism are identical in almost all species
- Considerable variation in amino acid metabolism among different species, but there are some similarities in the pathways leading to all twenty amino acids
- Glutamic acid and glutamine are particularly important entry points to many of the biosynthetic pathways
- Transamination reactions (catalysed by transaminases) are important reactions in the incorporation of amino nitrogen
Describe transaminases
d
Describe transaminases and list the five amino acids that use it in biosynthesis.
Transaminases (aka aminotransferases) catalyze the transfer of the alpha amino group of the donor amino acid to the alpha carbon of an acceptor alpha ketoacid.
This is CENTRAL to the synthesis of
- Alanine
- Aspartate
- Glutamate
- Asparagine
- Glutamine
Redistribution of amino nitrogen to other amino acids using an alpha ketoacid is used to maintain sufficient quantities of each amino acid as it is required.
What do transaminases require to function? Why?
Transaminases use pyridoxal phosphate (a derivative of pyridoxine/vitamin B6 as cofactor).
Donor amino acid forms a Schiff base intermediate with pyridoxal phosphate to allow for a carbocation.
What do transaminases require to function? Why?
Transaminases use pyridoxal phosphate (a derivative of pyridoxine/vitamin B6 as cofactor).
Donor amino acid forms a Schiff base intermediate with pyridoxal phosphate to allow for an intermediate carbanion.
Describe the transaminase reaction mechanism
- Transaminase enzymes show specificity for the R group of the amino acid
- Enzyme forms a Schiff base with pyridoxal phosphate and then reacts with alpha amino acid to form geminal diamine intermediate
- The enzyme is released and the amino acid-PLP Schiff base (aldimine) remains
- Proton extraction by lysine amino group of enzyme forms carbanion resonance-stabilized intermediate
- Lysine and the enzyme leave with an amine leaving a ketimine
- Ketimine forms a carbinolamine intermediate from hydroxylation
- Carinolamine intermediate is hydrolyzed to form pyridoxamine phosphate (PMP) enzyme complex and alpha keto acid
Reverse reaction with the amine recipient alpha ketoacid regenerates pyridoxal phosphate
Transaminase reactions are used in biosynthesis of all amino acids except ___ and ___
threonine and lysine
What are the consequences of the reactants and products of transaminations being amino acid and alpha ketoacid?
The reactions are reversible.
Transaminases are important for both amino acid synthesis or alpha ketoacid replenishment via amino acid degradation (recall anaplerotic pathways in the TCA cycle).
What aminotransferases might you expect to see in the liver when there is liver disease?
- Alanine
- Aspartate
- Glutamate
These are abundant in the liver for protein synthesis
How can ATP dependent pathways produce asparagine or glutamine?
ATP dependent amidation by asparagine synthetase or glutamine synthetase produces asparagine or glutamine respectively.
How can ATP dependent pathways produce asparagine or glutamine?
ATP dependent amidation by asparagine synthetase or glutamine synthetase produces asparagine or glutamine respectively.
How can ATP dependent pathways produce asparagine or glutamine?
ATP dependent amidation by asparagine synthetase or glutamine synthetase produces asparagine or glutamine respectively.
Give the possible amino acid transamination products for each of these metabolites:
pyruvate (1)
Oxaloacetate (2)
alpha-ketoglutarate (2)
Pyruvate: alanine
Oxaloacetate: aspartate, asparagine
alpha-ketoglutarate: glutamate, glutamine
Give the possible amino acid transamination products for each of these metabolites:
pyruvate (1)
Oxaloacetate (2)
alpha-ketoglutarate (2)
Pyruvate: alanine
Oxaloacetate: aspartate, asparagine
alpha-ketoglutarate: glutamate, glutamine
Where do carbon skeletons used to synthesize amino acids come from?
Glucose from the TCA cycle
Oxaloacetate: aspartate: asparagine, methionine, threonine, lysine
Alpha ketoglutarate: glutamate: glutamine, proline, arginine
Which amino acids can be synthesized from 3-phosphoglycerate?
Transamination with 3-phospholglycerate yields serine.
Serine can be acetylated to O-acetylserine. Lyase can convert O-acetylserine to cysteine
Which amino acids can be synthesized from oxaloacetate?
- Aspartate
- Asparagine
- Methionine
- Threonine
- Lysine
- Isoleucine
Which amino acids can be synthesized from phosphoenolpyruvate?
- Tyrosine
- Phenylalanine
- Tryptophan
Which amino acid can be synthesized from ribose-5-phosphate?
Histamine
How is amino acid synthesis metabolically controlled?
There are way too many diverse products for simple feedback. So we have:
- Sequential feedback
- Polyvalent feedback (multiple enzymes effected)
- Cumulative feedback (based on level of each amino acid, best model).
Cumulative feedback involves partial inhibition and partial stimulation when different levels of different AAs are present (the effect of concentrations are additive)
How is bacterial glutamine synthetase regulated?
Cumulative allosteric inhibition and covalent modification
- Glutamine is the major amino group donor for the synthesis of multiple AAs, the productino of glutamine by glutamine synthetase is a vital control point for regulation of nitrogen metabolism.
- Adenylation site on each monomer (Try residues near active sites)
- Dodecamer (12 subunits), six hexameric rings stacked on each other
- Adenylation is inhibitory
If energy charge is high, GS is activated, if energy level is low or Gln levels are high, GS is less active.
List the nine inhibitory allosteric effectors of glutamine synthetase
N containing endproducts
- Histamine
- Tryptophan
- Carbamoyl phosphate
- Glucosamine-6-phosphate
- AMP
- CTP
Indicators of cellular nitrogen levels
- Alanine
- Serine
- Glycine
These bind more readily if the enzyme is adenylated
Describe covalent adenylation of glutamine synthetase
- Addition of AMP to a tyrosine residue adjacent to the active site on each subunit
- Converts enzyme to less active form and enhances sensitivity to allosteric inhibitors
- Adenylation and deadenylation catalyzed by adenylyltransferase complex
