The pH dependent spectral properties of Clostridium pasteurianum 2[Fe4S4] ferredoxin

The recently assigned 1H NMR hyperfine signals of Clostridium pasteurianum ferredoxin were investigated over the pH range 8-12 to monitor possible pH-dependent conformational changes of the protein. For very high 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 variations, which can be fitted to a single pKa = 10.9, are interpreted as an effect of deprotonation of the phenolic group of Tyr-2, located reasonably close to cluster II. This hypothesis has been supported by means of other techniques such as CD and absorption spectroscopy that, on turn, are able to reveal minor pH-dependent spectral variations at high pH. Finally a UV difference experiment has provided further evidence for deprotonation of the phenolic group of Tyr-2. The possible influence of deprotonation of Tyr-2 on the redox properties of cluster II is discussed.