H-1-NMR HYPERFINE-SHIFTED RESONANCES FROM THE EXCHANGE-COUPLED FE4S4-SIROHEME OF THE ASSIMILATORY SULFITE REDUCTASE FROM DESULFOVIBRIO-VULGARIS (HILDENBOROUGH)

In this paper, we illustrate the use of high-field 'H NMR of paramagnetically shifted resonances to study the assimilatory sulfite reductase from Desulfovibrio vulgaris (Hildenborough). This enzyme is one of the smallest multielectron (>2-) metalloredox enzymes in nature; possessing a siroheme and an exchange-coupled Fe,S, cluster. We describe the first example of NMR experiments on a protein containing such an exchange-coupled prosthetic center, discuss preliminary assignments of resonances and the likely identity of the sixth ligand to the siroheme, and also identify a signal that displays an unusually large downfield shift that may be a reflection of the exchange coupling between the chromophores.