In Vivo Control Mechanism of the Carboxylation Reaction
It is hardly surprising that both the synthesis and the activity of the enzyme ribulose 1,5-bisphosphate (RuBP) carboxylase are highly regulated. This most abundant enzyme on earth catalyzes the entry of CO2 into the reductive pentose phosphate pathway (Calvin cycle) (1), the pathway leading to the reduction of CO2 to sugar phosphates in all green plants (2), including those with a prelimin ary C4 cycle (3) for CO2 accumulation. Such first reactions are often the sites of important metabolic regulation. The carboxylation reaction is one of four steps in the Calvin cycle unique to that cycle and not found in the oxidative pentose phosphate cycle (the other such reactions are the ones converting fructose and sedo heptulose bisphosphates to their respective monophosphates and the reaction converting ribulose 5-phosphate to RuBP) (Figure 1). All four of these reactions are inactivated or are less active in the dark, when the oxidative pentose phosphate cycle and the glycolytic pathway operate. The inactivation in the dark of these four reactions unique to the reductive cycle is required to prevent the oper ation of futile cycles.
KeywordsCalvin Cycle Spinach Chloroplast Reconstituted System Carboxylation Reaction RuBP Carboxylase
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