Nitrogen in Amino Acid Synthesis

Question 1

Describe the production of glutamine

Answer 1

• NH4+ reacts with glutamate to produce glutamine using glutamine synthetase
• Glutamine synthetase = 2 step mechanism

Question 2

Describe the 2 step mechanism of glutamine sythetase

Answer 2

• Step 1: Glutamate is phosphorylated to form γ-glutamyl phosphate
• step 2: Phosphorylation of glutamate creates a good leaving group that is easily displaced by ammonia

Question 3

Describe the two steps involved in nitrogen assimilation

Answer 3

• Step 1: NH4+ reacts with glutamate to produce glutamine using glutamine synthetase
• Step 2: Glutamine reacts with α-ketoglutarate to generate two molecules of glutamate
  This is catalyzed by glutamate synthase in bacteria and plants

Question 4

Are glutamine sythetase and glutamate synthase present in all animals?

Answer 4

• Glutamine sytnthetase = present in all organisms
• Glutamate synthase is NOT (only in plants and bacteria)

Question 5

Describe the regulation of glutamine synthesis

Answer 5

• Cumulative regulation
• Alanine, glycine, and at least 6 end products of glutamine metabolism = allosteric inhibitors of glutamine synthetase
  Alone each of these molecules produces only partial inhibition, all 8 together will shut down the enzyme

Question 6

What are all amino acids derived from?

Answer 6

All amino acids are derived from intermediates of glycolysis, the citric acid cycle, or the pentos phosphate pathway

Question 7

How does nitrogen enter the biosynthetic pathways of the TCA cycle, pentos phosphate pathway and glycolysis?

Answer 7

Glutamate and Glutamine

Question 8

What are the 6 precursors of amino acid biosynthetic families?

Answer 8

1. α-ketoglutarate
2. 3-phosphoglycerate
3. Oxaloacetate
4. Pyruvate
5. Ribose 5-phosphate
6. Phosphoenolpyruvate and erythrose 4-phosphate

Question 9

Describe the synthesis of proline

Answer 9

• Precursor: α-ketoglutarate
• Derived from glutamate
• Bacteria: glutamate —> proline
• Humans: glutamate —> glutamate γ-semialdehyde —> proline —> ornithine

Question 10

Describe the synthesis of arginine

Answer 10

• Precursor: α-ketoglutarate
• Derived from glutamate
• Generated by the urea cycle (ornithine is an intermediate)

Question 11

How is ornithine synthesized?

Answer 11

Ornithine is produced by the breakdown of arginine in the urea cycle
(It is also synthesized from glutamate γ-semialdehyde)

Question 12

Describe the synthesis of serine

Answer 12

• Precursor: 3-phosphoglycerate
• Same pathway in all organisms
• 3 steps

Question 13

What are the 3 steps involved in the synthesis of serine?

Answer 13

1. Oxidation of 3-phosphoglycerate to form 3-phosphohydroxypyruvate
2. Transamination of glutamate to form 3-phosphoserine
3. Dephosphorylation of 3-phosphoserine to form free serine

Question 14

What amino acid is serine the precursor for?

Answer 14

• Glycine
• Serine —> glycine by N5, N10-methylene-THF

Question 15

BRIEFLY Describe the synthesis of cysteine

Answer 15

• Almost identical to serine
• Serine and cysteine differ by one atom
• Serine = oxygen
• Cysteine = sulfer
• Serine to cysteine = oxygen to sulfer

Question 16

Describe the synthesis of cysteine from homocysteine

Answer 16

• Free homocysteine undergoes a reaction with serine catalyzed by cystathionine β-synthase to yeild cystathionine
• Cystathionine γ-lyase a PLP requiring enzyme catalyzes the removal of ammonia and cleavage of cystathionine to yield free cysteine

Question 17

Describe the synthesis for, Asparagine, Methionine, Lysine, and Threonine

Answer 17

• Precursor: oxaloacetate
• Derived from aspartate
  Asparagine = synthesized from the admiration of aspartate

Question 18

Describe the synthesis of alanine and aspartate

Answer 18

• Precursors: pyruvate and oxaloacetate
• Synthesized by the transamination from glutamate

Question 19

Describe the synthesis of valine, leucine, and isoleucine

Answer 19

• Precursor: pyruvate
• Pyruvate reacts with TPP to form a resonance stabilized carbanion
• The carbanion can go in two directions (either combine two pyruvate to form valine and leucine OR combine with
  α-ketobutyrate to form isoleucine)
• Addition of nitrogen is the last step in these reactions
• The amino group is donated from a molecule of glutamate

Question 20

Describe the synthesis of tyrosine, tryptophan, and phenylalanine

Answer 20

• Precursors: phosphoenolpyruvate and erythrose 4-phosphate
• Very complex chemistry (rings must be synthesized then closed then oxidized to create double bonds)
• Chorismate is a common intermediate