MECHANISM OF LIGHT-MODULATION - IDENTIFICATION OF POTENTIAL REDOX-SENSITIVE CYSTEINES DISTAL TO CATALYTIC SITE IN LIGHT-ACTIVATED CHLOROPLAST ENZYMES

Light-dependent reduction of target disulfides on certain chloroplast enzymes results in a change in activity. We have modeled the tertiary structure of four of these enzymes, namely NADP-linked glyceraldehyde- 3-P dehydrogenase, NADP-linked malate dehydrogenase, sedoheptulose bis phosphatase, and fructose bisphosphatase. Models are based on x-ray cr ystal structures from non-plant species. Each of these enzymes consist s of two domains connected by a hinge. Modeling suggests that oxidatio n of two crucial cysteines to cystine would restrict motion around the hinge in the two dehydrogenases and influence the conformation of the active site. The cysteine residues in the two phosphatases are locate d in a region known to be sensitive to allosteric modifiers and to be involved in mediating structural changes in mammalian and microbial fr uctose bisphosphatases. Apparently, the same region is involved in cov alent modification of phosphatase activity in the chloroplast.