22: Amino Acid Biosynthesis and Regulation of Synthetic Pathways

key to nitrogen metabolism in the biosphere

nitrogen-fixing bacteria

Nitrogenase complex components

• dinitrogenase reductase- provides electrons

• dinitrogenase- uses electrons to reduce atmospheric N₂ to NH₄⁺

N₂ to NH₄⁺ redox?

N₂ reduced to NH₄⁺

move amino groups between different carbon skeletons

aminotransferase

glutamine synthetase

glutamate —> glutamine

• 12 subunits regulated by allosteric inhibition AND covalent modification

glutamate synthase

glutamine —> glutamate

How is glutamine synthetase covalently modified?

on key Tyr residue via adenylation —> turns enzyme activity DOWN

glutamine synthetase regulation

• enzyme active = deadenylated

• enzyme inactive = adenylated

• inhibited by glutamine

• activated by alpha-ketoglutarate and ATP

enzymes that catalyze the removal of an ammonia group from glutamine and transfer it to a recipient substrate

glutamine amidotransferase

intermediates that can be used to synthesize ALL 20 aa

• 3-phosphoglycerate- glycolysis

• PEP- glycolysis

• Pyruvate- glycolysis

• OAA- TCA

• Alpha-ketoglutarate- TCA

• Ribose-5-phosphate- PPP

• Erythrose-4-phosphate- PPP

Goal of regulation of pathways that make amino acids

have about equal amounts of all 20 amino acids to synthesize proteins

If concentrations of aa not equal?

mistakes in translation, esp. charging tRNAs via amino acyl-tRNA synthetases

final product inhibits one of initial steps or enzymes in pathway

simple feedback inhibition

Step that is usually allosterically regulated

The first step

each product inhibits a little, all products together inhibit a lot

concerted inhibition

product of pathway may inhibit multiple steps/enzymes of pathway

sequential feedback inhibition

single step has multiple enzymes that regulate/catalyze that reaction

enzyme multiplicity

Why is enzyme multiplicity useful?

allows enzymes to be regulated differently in different organs

How are non-essential amino acids synthesized

• from essential amino acids

• from other metabolites

3 amino acids and their alpha keto acids

1. alanine —> pyruvate

2. glutamate —> alpha-ketoglutarate

3. aspartate —> OAA

Ketoacids

changes amount of enzymes NOT enzyme activity

transcriptional regulation

Regulation of pathways

• simple feedback inhibition

• concerted feedback inhibition

• sequential feedback inhibition

• enzyme multiplicity

• transcriptional regulation