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Ionic-strength-dependent substrate inhibition of the lysis of Micrococcus luteus by hen egg-white lysozyme
I M Verhamme1, G W Van Dedem, A R Lauwers
1Laboratory of General Biochemistry and Physical Pharmacy, State University, Gent, Belgium.
Abstract:
The kinetics of lysis of Micrococcus luteus by hen egg-white lysozyme in dilute buffer media is characterized by pronounced substrate inhibition. This effect occurs within the complete pH range where lysozyme activity is detectable. The electrostatic potential of the negatively charged cell-wall proteoglycan increases with decreasing ionic strength, resulting in an enhanced affinity between proteoglycan and lysozyme and probably favouring multipoint substrate attachment. For the lysozyme-catalyzed hydrolysis of cell-wall proteoglycan three plausible mechanisms of substrate inhibition can be postulated. Two out of the three models fit our experimental data, the simplest of the two providing the most rigorous information on the kinetic parameters Km, V and Ki. Three graphical methods consistent with the chosen model were applied for preliminary parameter estimation and the constants obtained were compared to those from nonlinear least-squares analysis. If substrate inhibition is neglected it is shown that serious bias is imposed upon the parameters.
Insights
Substrate inhibition significantly impacts Micrococcus luteus lysis by hen egg-white lysozyme, particularly at low ionic strengths. Ignoring this inhibition leads to inaccurate kinetic parameter estimations.
Area of Science:
- Biochemistry
- Enzymology
- Microbiology
Background:
- Lysozyme is a key enzyme in bacterial cell wall hydrolysis.
- Substrate inhibition is a complex kinetic phenomenon observed in enzymatic reactions.
- The interaction between lysozyme and bacterial proteoglycans is influenced by ionic strength.
Purpose of the Study:
- To investigate the kinetics of Micrococcus luteus lysis by hen egg-white lysozyme.
- To characterize the phenomenon of substrate inhibition in this enzymatic reaction.
- To develop and validate models for understanding lysozyme-proteoglycan interactions and kinetic parameters.
Main Methods:
- Enzymatic assays measuring Micrococcus luteus lysis by hen egg-white lysozyme.
- Kinetic analysis including graphical methods and nonlinear least-squares analysis.
- Investigation of the effect of ionic strength and pH on enzyme activity.
Main Results:
- Pronounced substrate inhibition was observed across the detectable pH range of lysozyme activity.
- Decreasing ionic strength enhanced lysozyme-proteoglycan affinity, likely promoting multipoint attachment.
- Two out of three proposed mechanisms accurately described the experimental data, with one providing rigorous kinetic parameters (Km, V, Ki).
Conclusions:
- Substrate inhibition is a critical factor in lysozyme-mediated bacterial lysis.
- Accurate kinetic parameter determination requires accounting for substrate inhibition.
- Neglecting substrate inhibition can lead to significant bias in estimated kinetic parameters, impacting mechanistic understanding.