Real-time enzyme kinetics monitored by dual-color fluorescence cross-correlation spectroscopy

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Abstract
A method for sensitively monitoring enzyme kinetics and activities by using dual-color fluorescence cross-correlation spectroscopy is described. This universal method enables the development of highly sensitive and precise assays for real-time kinetic analyses of any catalyzed cleavage or addition reaction, where a chemical linkage is formed or cleaved through an enzyme’s action between two fluorophores that can be discriminated spectrally. In this work, a homogeneous assay with restriction endonuclease Eco RI and a 66-bp double-stranded DNA containing the GAATTC recognition site and fluorophores at each 5′ end is described. The enzyme activity can be quantified down to the low picomolar range (>1.6 pM) where the rate constants are linearly dependent on the enzyme concentrations over two orders of magnitude. Furthermore, the reactions were monitored on-line at various initial substrate concentrations in the nanomolar range, and the reaction rates were clearly represented by the Michaelis–Menten equation with a K M of 14 ± 1 nM and a k cat of 4.6 ± 0.2 min −1 . In addition to kinetic studies and activity determinations, it is proposed that enzyme assays based on the dual-color fluorescence cross-correlation spectroscopy will be very useful for high-throughput screening and evolutionary biotechnology.