生物化学课件：2 Biosynthesis of amino acids

Rhizobia exist in nodules of
leguminous plants
➢Ammonia is incorporated into biomolecules through glutamate and
glutamine
• First, glutamine synthetase (Gln 合 成 酶 ) catalyzed the reaction of glutamate and NH4+ to yield glutamine.
The bacterial Gln synthetase has 12 subunits arranged as two rings of hexamers
Active sites
Tyr397 (adenylylation site)
Regulation by allosteric effectors
The glutamine synthetase is accumulatively inhibited by at least 8 allosteric effectors, mostly end products of glutamine metabolism
A specific Tyr residue (Tyr397) in bacterial glutamine synthetase can be reversible adenylylated
Biosynthesis of amino acids and related molecules
氨基酸及其衍生物 的生物合成
Contents
1. Overview of nitrogen metabolism
2. Reduction (fixation) of N2 into ammonia (NH3 or NH4+)
3. Biosythesis of amino acids
Several classes of reactions play special roles in the biosyntesis of amino acids and nucleotides
3. Synthesis of the 20 amino acids
4. Synthesi amino acids
1.The nitrogen cycle
2. Nitrogen is fixed by enzymes of the nitrogenase complex
• Activity of AT is modulated by binding to a regulatory protein, PII.
• The activity of PII, in turn, is regulated by the uridylylation (尿苷酰化) of a specific Tyr residue: PII-UMP stimulates the deadenylylation activity of AT, however, the unmodified PII stimulates the adenylylation activity of AT.
• Adenylylated enzyme is more sensitive to the allosteric inhibitor (变构抑制剂).
• The addition and removal of the AMP group to the glutamine synthetase are catalyzed by adenylyltransferase (AT, 腺苷酰基转移酶).
• Glutamate is produced from glutamine and αketoglutarate in a reaction catalyzed by glutamate synthase (Glu合酶).
• The amide group of Gln is the source of nitrogen in the synthesis of a variety of compounds, such as Trp, His, carbamoyl phosphate (氨基甲酰磷酸), glucosamine-6-P, CTP, and AMP.
• The amino groups of most other amino acids are derived from glutamate through transamination.
➢Glutamine synthetase is a primary regulatory point in nitrogen metabolism
Nitrogen fixation: Conversion of N2 to NH3 in diazotrophs (固氮生物) • Cyanobacteria (蓝绿藻, photosynthetic) • rhizobia (根瘤菌, symbiont 共生生物 )
Rhizobia
Cyanobacteria
The enzyme has 12 identical subunits (each having an independent active site forming two hexagonal rings ) and is regulated by both allosterically and by covalent modification.
• The addition and removal of UMP to PII are again catalyzed by a single protein, uridylyltransferase (尿苷酰基转移酶,UT): α-ketoglutarate and ATP stimulate the uridylylation, however, Gln and Pi stimulate the deuridylylation.