Rate-limiting steps in the DNA polymerase I reaction pathway

Area of Science:

• Biochemistry
• Molecular Biology
• Enzymology

Background:

• DNA polymerase I (Pol I) is crucial for DNA replication and repair.
• Phosphorothioate analogues are used to study enzyme mechanisms.
• Understanding Pol I's catalytic steps is key to DNA synthesis research.

Purpose of the Study:

• To investigate the kinetics of DNA synthesis using a phosphorothioate nucleotide analogue.
• To elucidate the rate-limiting step in DNA polymerase I-catalyzed reactions.
• To examine the mechanism of pyrophosphate exchange catalyzed by Pol I.

Main Methods:

• Rapid-quench kinetics to measure initial reaction rates.
• Positional isotope exchange experiments with labeled dATP.
• Kinetic analysis using the substrate analogue (Sp)-dATP alpha S.

Main Results:

• Thionucleotide incorporation rate decreased with increasing phosphorothioate residues due to helix instability.
• Negligible isotope exchange indicated rapid pyrophosphate release.
• A conformational change preceding the chemical step was identified as rate-limiting.
• Phosphorothioate substitution did not affect polymerization rate but attenuated pyrophosphate exchange.

Conclusions:

• Helix instability of phosphorothioate-containing DNA affects polymerase incorporation rates.
• DNA polymerase I likely undergoes a rate-limiting conformational change before catalysis.
• The pyrophosphate exchange mechanism involves two consecutive inversion reactions.
• Phosphorothioate modification provides insights into DNA polymerase I's catalytic mechanism.