21 - Amino Acid Synthesis

Question 1

What is one of the essential amino acids that is basically nonessential?

Answer 1

Arginine is produced in the urea cycle but at levels insufficient to satisfy the requirements of protein synthesis. Need to get extra from diet.

Question 2

What is one of the essential amino acids that is basically nonessential?

Exceptions to nonessential?

Answer 2

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)

Question 3

Describe synthesis of nonessential (non diet acquired) amino acids

Answer 3

• 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

Question 4

Describe transaminases

Answer 4

d

Question 5

Describe transaminases and list the five amino acids that use it in biosynthesis.

Answer 5

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.

Question 6

What do transaminases require to function? Why?

Answer 6

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.

Question 7

What do transaminases require to function? Why?

Answer 7

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.

Question 8

Describe the transaminase reaction mechanism

Answer 8

• 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

Question 9

Transaminase reactions are used in biosynthesis of all amino acids except ___ and ___

Answer 9

threonine and lysine

Question 10

What are the consequences of the reactants and products of transaminations being amino acid and alpha ketoacid?

Answer 10

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).

Question 11

What aminotransferases might you expect to see in the liver when there is liver disease?

Answer 11

• Alanine
• Aspartate
• Glutamate

These are abundant in the liver for protein synthesis

Question 12

How can ATP dependent pathways produce asparagine or glutamine?

Answer 12

ATP dependent amidation by asparagine synthetase or glutamine synthetase produces asparagine or glutamine respectively.

Question 13

How can ATP dependent pathways produce asparagine or glutamine?

Answer 13

ATP dependent amidation by asparagine synthetase or glutamine synthetase produces asparagine or glutamine respectively.

Question 14

How can ATP dependent pathways produce asparagine or glutamine?

Answer 14

ATP dependent amidation by asparagine synthetase or glutamine synthetase produces asparagine or glutamine respectively.

Question 15

Give the possible amino acid transamination products for each of these metabolites:

pyruvate (1)
Oxaloacetate (2)
alpha-ketoglutarate (2)

Answer 15

Pyruvate: alanine

Oxaloacetate: aspartate, asparagine

alpha-ketoglutarate: glutamate, glutamine

Question 16

Give the possible amino acid transamination products for each of these metabolites:

pyruvate (1)
Oxaloacetate (2)
alpha-ketoglutarate (2)

Answer 16

Pyruvate: alanine

Oxaloacetate: aspartate, asparagine

alpha-ketoglutarate: glutamate, glutamine

Question 17

Where do carbon skeletons used to synthesize amino acids come from?

Answer 17

Glucose from the TCA cycle

Oxaloacetate: aspartate: asparagine, methionine, threonine, lysine

Alpha ketoglutarate: glutamate: glutamine, proline, arginine

Question 18

Which amino acids can be synthesized from 3-phosphoglycerate?

Answer 18

Transamination with 3-phospholglycerate yields serine.

Serine can be acetylated to O-acetylserine. Lyase can convert O-acetylserine to cysteine

Question 19

Which amino acids can be synthesized from oxaloacetate?

Answer 19

• Aspartate
• Asparagine
• Methionine
• Threonine
• Lysine
• Isoleucine

Question 20

Which amino acids can be synthesized from phosphoenolpyruvate?

Answer 20

• Tyrosine
• Phenylalanine
• Tryptophan

Question 21

Which amino acid can be synthesized from ribose-5-phosphate?

Answer 21

Histamine

Question 22

How is amino acid synthesis metabolically controlled?

Answer 22

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)

Question 23

How is bacterial glutamine synthetase regulated?

Answer 23

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.

Question 24

List the nine inhibitory allosteric effectors of glutamine synthetase

Answer 24

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

Question 25

Describe covalent adenylation of glutamine synthetase

Answer 25

• 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