Microcalorimetric determination of the reaction enthalpy changes associated with the carboxylase and oxygenase reactions catalysed by ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBISCO)

Abstract

The total reaction enthalpy change Δ_rH associated with the carboxylation and oxygenation of D-ribulose 1,5-bisphosphate (RuBP) catalysed by RUBISCO from spinach and Rhodospirillum rubrum was measured for the first time with sensitive microcalorimetry. The enthalpy changes Δ_r,cH and Δ_r,oH associated with the carboxylase and oxygenase reactions were determined separately using the known ratio of these reactions. Δ_r,cH and Δ_r,oH corrected for the heat of neutralisation due to proton release during the overall carboxylation or oxygenation processes. For the corrected enthalpy change Δ_r,cH′ of the carboxylase reaction catalysed by RUBISCO of spinach a value of −21.34±0.4 kJ mol⁻¹ was determined. In the case of the oxygenase reaction for RUBISCO of spinach a reaction enthalpy change Δ_r,oH′ of −319.1±9.6 kJ mol⁻¹ which is typical for energy rich reactions, was found. This value is comparable with Δ_r,oH′=−291.4±5.8 kJ mol⁻¹ associated with the oxygenase reaction catalysed by Co²⁺-activated RUBISCO from R. rubrum.