THE IRON-SULFUR CLUSTERS OF TETRANUCLEAR IRON-SULFUR PROTEINS AND ENZYMES OFTEN INTERCONVERT BETWEEN THE [4FE-4S] AND [3FE-4S] FORMS. THE DETAILS OF HOW THIS HAPPENS ARE IMPORTANT SINCE WE KNOW, FOR EXAMPLE, THAT THE KREB'S CYCLE [4FE-4S] ENZYME, ACONITASE, DEACTIVATES BY INTERCONVERSION OF ITS CLUSTER TO [3FE-4S] FORM. IN OUR WORK, WE KNOW THAT ONE OF THE [4FE-4S] CLUSTERS OF THE 2[4FE-4S] FERREDOXIN (FD) FROM CLOSTRIDIUM PASTEURIANUM INTERCONVERTS UPON OXIDATION BY FERRICYANIDE TO FORM [3FE-4S][4FE-4S] FD. WHILE WORK TO DATE INDICATES WHICH CLUSTER UNDERGOES INTERCONVERSION, IT DOES NOT INDICATE AT ALL WHICH OF THE FOUR CYSTEINES (8, 11, 14 OR 47) ACTUALLY RELEASES ITS SPECIFIC IRON DURING THE INTERCONVERSION. BY SELECTIVELY INSERTING 3,3-DIDEUTERO CYSTEINE SEQUENTIALLY AND SITE SPECIFICALLY INTO TOTALLY SYNTHETIC FD (WHICH WE THEN RECONSTITUTE TO THE HOLO-PROTEIN), WE WILL BE ABLE THROUGH NMR SPECTROSCOPY OF NATIVE AND FERRICYANIDE OXIDIZED FD TO CONCLUSIVELY DEMONSTRATE WHICH CYS LOSES ITS IRON AND THUS BETTER UNDERSTAND THE MECHANISM BY WHICH THIS AND OTHER CLUSTER INTERCONVERSIONS TAKE PLACE. IN NATIVE FD, THE ~-C 1H2 ARE CONTACTED SHIFTED; THE [3FE-4S] CLUSTER IN TURN HAS A SIGNIFICANTLY DIFFERENT AND MORE BROADENED ~-C 1H2 PATTERN OF SHIFTS. SITE SPECIFIC INSERTION OF THE DIDEUTERO CYS INTO THE POSITION THAT LOSES THE IRON WILL NOT CHANGE THE OBSERVED [3FE-4S] CLUSTER NMR SPECTRUM, WHILE ANY OF THE OTHER THREE WILL RESULT IN A DRAMATICALLY DIFFERENT NMR SPECTRUM.