Equilibrium binding of thioredoxin FB to chloroplastic fructose bisphosphatase

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Abstract
Thioredoxin fB, the protein activator of chloroplastic fructose 1,6‐bisphosphatase, strongly binds its target enzyme with a stoichiometry of one protein dimer per enzyme tetramer. The thioredoxin binding site is distinct from the active site and the dissociation constant of the protein‐enzyme complex has the extremely small value of 769 nM at pH 7.5. This interaction involves both ionic and hydrophobic contributions and is enhanced by a pH increase from 7 to 8. These results suggest that the above molecular properties may be involved in the light activation of chloroplastic fructose bisphosphatase.