THE PH DEPENDENT SPECTRAL PROPERTIES OF CLOSTRIDIUM-PASTEURIANUM 2[FE4S4] FERREDOXIN

The recently assigned H-1 NMR hyperfine signals of Clostridium pasteur ianum ferredoxin were investigated over the pH range 8-12 to monitor p ossible pH-dependent conformational changes of the protein. For very h igh pH Values minor perturbations were detected in the chemical shifts of three signals assigned to beta-CH2 cysteine protons of cluster II, while cluster I was not affected at all. These chemical shift variati ons, which can be fitted to a single pK(a) approximate to 10.9, are in terpreted as an effect of deprotonation of the phenolic group of Tyr-2 , located reasonably close to cluster II. This hypothesis has been sup ported by means of other techniques such as CD and absorption spectros copy that, on turn, are able to reveal minor pH-dependent spectral var iations at high pH. Finally a UV difference experiment has provided fu rther evidence for deprotonation of the phenolic group of Tyr-2. The p ossible influence of deprotonation of Tyr-2 on the redox properties of cluster II is discussed.