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 ΔrH 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 Δr,cH and Δr,oH associated with the carboxylase and oxygenase reactions were determined separately using the known ratio of these reactions. Δr,cH and Δr,oH corrected for the heat of neutralisation due to proton release during the overall carboxylation or oxygenation processes. For the corrected enthalpy change Δr,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 Δr,oH′ of −319.1±9.6 kJ mol−1 which is typical for energy rich reactions, was found. This value is comparable with Δr,oH′=−291.4±5.8 kJ mol−1 associated with the oxygenase reaction catalysed by Co2+-activated RUBISCO from R. rubrum.