Figures

1. MECHANISM FIGURE 18-6 Some amino acid transformations at the carbon that are facilitated by pyridoxal phosphate

Explanation and flow chart - PLP Mechanism Flow Chart

1. PLP Binds to Enzyme
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   Internal Aldimine (Schiff Base): PLP forms a Schiff base linkage with an enzyme lysine residue.

2. Amino Acid Substrate Binds
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   External Aldimine Formation: The amino acid’s α-amino group displaces the enzyme’s lysine, forming a Schiff base with PLP.

3. Three Possible Pathways from External Aldimine

   • Pathway 1: Transamination
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     Quinonoid Intermediate (PLP stabilizes negative charges)
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     New α-Keto Acid Formation: Amino group is transferred, producing a new α-keto acid and a new amino acid.

   • Pathway 2: Racemization
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     Quinonoid Intermediate (allows rotation around the α-carbon)
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     Stereochemistry Change: Converts an L-amino acid to a D-amino acid (or vice versa).

   • Pathway 3: Decarboxylation
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     Quinonoid Intermediate (stabilizes carbanion after CO₂ removal)
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     Amine Formation: Removal of CO₂ produces an amine from the amino acid.

4. Role of PLP as an Electron Sink
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   Stabilization of Intermediates: PLP’s structure enables electron delocalization, preventing unstable intermediates.