The Activation of Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase
A long-standing and continuing problem concerning ribulose bisphosphate carboxylase/oxygenase is the discrepancy between its activity in vivo and in vitro (1, 2). The apparently low affinity of the enzyme for CO2 was one of the better reasons Warburg had for dismissing the C3 photosynthetic carbon reduction cycle (3). When assayed in vitro with the naturally occurring concentrations of CO2 (10 µ,M) and O2 (250 µM), the purified enzyme is incapable of fixing CO2 for sustained periods (>90 sec) at rates equal to or greater than those of photosynthesis. Similarly, sustained synthesis of phosphoglycolate under natural conditions for >2 to 3 min at rates equal to or greater than the in vivo rates has not yet been observed. Nevertheless, beginning with the observations of Bahr and Jensen (4, 5), some progress towards resolution of these discrepancies has been made, and, provided that both reactions are initiated with fully activated enzyme, adequate rates of carboxylation or oxygenation under natural conditions can be achieved or even exceeded, if only for a minute or two (6–12). Thereafter, progressive inactivation of the enzyme becomes apparent. At best this represent only a partial solution to the problem, for clearly there are no such restrictions upon the enzyme in vivo.
KeywordsSpinach Chloroplast Cyanuric Acid Envelope Membrane Triose Phosphate Hexose Monophosphates
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