Time Course Analysis of Enzyme-Catalyzed DNA Polymerization

Area of Science:

• Biochemistry
• Enzymology
• Molecular Biology

Background:

• Traditional initial-rate analysis is problematic for determining DNA polymerase kinetic parameters due to substrate heterogeneity and sequence-dependent incorporation rates.
• DNA template binding and deoxynucleoside triphosphate (dNTP) incorporation exhibit complex kinetics influenced by DNA sequence.
• Existing methods struggle to accurately capture the nuances of processive polymerization.

Purpose of the Study:

• To develop and validate a novel mathematical model for analyzing DNA polymerase kinetics during processive polymerization.
• To accurately estimate enzyme-template association and dissociation rates, and steady-state kinetic parameters (kcat, Km) for dNTP incorporation.
• To investigate the impact of temperature on enzyme activity and explore the mechanism of DNA polymerase action using kinetic isotope effects.

Main Methods:

• A mathematical model was developed, incorporating DNA template binding and Michaelis-Menten nucleotide incorporation on a finite-length template.
• The model was numerically integrated and globally fitted to experimental reaction time courses.
• Processive synthesis of oligonucleotides by DNA polymerase I (Klenow fragment exo-) was monitored using PicoGreen fluorescence.

Main Results:

• The model successfully estimated enzyme-template association rate (k1 = 7.4 μM-1 s-1), dissociation rate (k-1 = 0.07 s-1), and dissociation constant (Kd = 10 nM).
• Steady-state parameters for correct dNTP incorporation yielded kcat values of 2.5-3.3 s-1 and Km values of 0.51-0.86 μM.
• Analysis revealed an activation energy for kcat of 82 kJ mol-1, significant enzyme inactivation at lower temperatures (up to 73%), and a solvent deuterium kinetic isotope effect (KIE) of 3.0-3.2.

Conclusions:

• Numerical integration of rate equations offers a flexible and superior approach for studying DNA polymerase kinetics compared to initial-rate analysis.
• The findings challenge previous assumptions about the rate-limiting steps in DNA polymerization and suggest a need for re-evaluation of conformational changes.
• The model provides a robust framework for analyzing complex enzyme systems, enhancing our understanding of DNA replication fidelity and efficiency.