Interactions between nucleotide binding sites on chloroplast coupling factor during ATP hydrolysis.

Area of Science:

• Biochemistry
• Enzyme kinetics
• Chloroplast function

Background:

• Chloroplast coupling factor 1 (CF1) plays a crucial role in ATP synthesis and hydrolysis.
• Understanding the regulatory mechanisms of CF1 is vital for elucidating energy transduction in chloroplasts.

Purpose of the Study:

• To investigate the initial hydrolysis of CaATP by CF1.
• To elucidate the role of Ca2+ and CaATP in CF1 activation and catalysis.
• To explore the kinetics of nucleotide exchange during CF1 activity.

Main Methods:

• Utilized the quenched-flow method to study rapid enzyme kinetics.
• Measured the time course of CaATP hydrolysis and ADP exchange.
• Investigated the effect of varying Ca2+ and CaATP concentrations on reaction rates.

Main Results:

• Ca2+ triggers a first-order conversion of CF1 to an active form, with a rate constant dependent on Ca2+ concentration.
• Steady-state hydrolysis rate exhibits sigmoidal dependence on CaATP concentration.
• Nucleotide exchange is complex, with a rapid initial phase accelerated by CaATP binding.
• Catalytic and nucleotide exchange rates are similar during steady-state catalysis.

Conclusions:

• A Ca2+-triggered mechanism for CF1 activation is proposed.
• The binding of CaATP promotes nucleotide exchange.
• The results suggest a catalytic mechanism involving at least two enzyme sites.