pH-rate profiles support a general base mechanism for galactokinase (Lactococcus lactis)
Laurie A Reinhardt1, James B Thoden, Greg S Peters
1Institute For Enzyme Research and Department of Biochemistry, University of Wisconsin-Madison, Madison, WI 53726, USA. lareinha@wisc.edu
FEBS Letters
|July 23, 2013
Summary
This study reveals the kinetic mechanism of Lactococcus lactis galactokinase (GALK), identifying key amino acids Asp183 and Arg36 essential for catalyzing galactose metabolism. These findings clarify GALK
Area of Science:
- Biochemistry
- Enzymology
- Metabolic pathways
Background:
- Galactokinase (GALK) is crucial for galactose metabolism via the Leloir pathway.
- Understanding GALK's enzymatic mechanism is vital for metabolic studies.
Purpose of the Study:
- To elucidate the kinetic mechanism and pH profiles of Lactococcus lactis galactokinase (GALK).
- To determine the roles of specific amino acid residues (Asp183, Arg36) in GALK catalysis.
Main Methods:
- Enzyme kinetics assays
- Site-directed mutagenesis (D183A, D183N, R36A, R36N)
- Analysis of pH-dependent kinetic parameters (kcat, kcat/K(Gal))
Main Results:
- The reaction mechanism is sequential in both forward and reverse directions.
- Mutations D183A and D183N rendered GALK inactive, confirming Asp183's role as a catalytic base.
- A pKa of 6.9 ± 0.2 in the forward reaction pH-kcat profile points to Asp183.
- Mutations R36A and R36N significantly reduced enzyme activity, indicating Arg36 facilitates deprotonation by lowering the substrate's pKa.
Conclusions:
- Asp183 acts as a catalytic base, deprotonating the C-1 hydroxyl group of galactose.
- Arg36 assists catalysis by lowering the pKa of the galactose C-1 hydroxyl group.
- The findings provide detailed mechanistic insights into GALK function in galactose metabolism.
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