PREDICTING REDOX-SENSITIVE CYSTEINES IN PLANT ENZYMES BY HOMOLOGY MODELING

One of the unsolved problems in plant biochemistry has been the identi fication of the regulatory cysteines in the reductively light-activate d and -inactivated chloroplast enzymes. Homology modeling based on the sequences of these enzymes and the three-dimensional structures of ho mologous enzymes has now allowed tentative identification of the redox -sensitive Cys residues in four light-activated chloroplast enzymes. I n each case the regulatory disulfides are not positioned in the active site but instead appeal to be positioned so as to affect the flexibil ity or the conformation of the enzyme, and thereby to affect catalysis . In glyceraldehyde-3-P dehydrogenase and malate dehydrogenase inter-d omain movement would be restricted. In fructose bisphosphatase and sed oheptulose bisphosphatase the regulatory Cys residues are located on t he nucleotide binding domain iri a region Known to be sensitive to nit allosteric effector of other fructose bisphosphatases. Results of sit e-directed mutagenesis experiments to date are in general agreement wi th the domain-locking hypothesis. The redox sensitivity of a number of cytosolic enzymes suggests that reductive modulation might occur outs ide of the chloroplast in leaves, and in the roots, stems and germinat ing seeds of green plants. Our better understanding of the mechanism o f redox regulation may lead to new approaches for the regulation of en zyme activity with biotechnological applications.