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[Substrate specificity of E. coli glutamate decarboxylase]
Molekuliarnaia Biologiia
|March 1, 1977
Summary
E. coli glutamate decarboxylase catalyzes decarboxylation of various amino acid analogs, suggesting a transamination side reaction. The enzyme binds some analogs, altering spectral properties, but not others.
Area of Science:
- Biochemistry
- Enzymology
- Protein Structure and Function
Background:
- Glutamate decarboxylase (GAD) from E. coli is a key enzyme in GABA synthesis.
- Understanding GAD's substrate specificity is crucial for enzyme mechanism studies.
Purpose of the Study:
- To investigate the substrate specificity of purified E. coli glutamate decarboxylase.
- To elucidate the catalytic mechanism and binding interactions of GAD with various substrate analogs.
Main Methods:
- Enzyme kinetics studies using substrate analogs.
- Spectroscopic analysis (absorption and circular dichroism) to detect binding.
- Determination of kinetic parameters (Km, Ki, I50).
Main Results:
- Decarboxylation confirmed for gamma-methylene glutamate, threo-beta-hydroxyglutamate, allo-gamma-hydroxyglutamate, threo-beta-methylglutamate, homocysteate, aminoadipate, and cysteinesulfinate.
- Transamination to pyridoxal-phosphate (PLP) coenzyme suggested as a side reaction for these analogs.
- Binding of aliphatic dicarboxylic acids and valeric acid caused minor spectral shifts (420 to 423-425 nm).
- Several compounds, including gamma-aminobutyrate and D-glutamate, failed to bind or be decarboxylated.
Conclusions:
- E. coli glutamate decarboxylase exhibits broader substrate specificity than previously assumed.
- A transamination mechanism likely contributes to the enzyme's interaction with certain analogs.
- Specific structural features of substrate analogs dictate binding and catalytic activity by GAD.