The gross protein structure, the microenvironment of the @entity5985 cluster, and the effect of neutral salts on the molecular structure of @entity2772 ferredoxin were studied by @entity1050 and absorption spectroscopy in the alkaline pH range. In the pH range of 7-11, the existence of reversible isomerization which consisted of at least two proton dissociation processes was indicated by the statical @entity1050 and absorption spectra. The @entity1050 changes in the visible and far-UV regions were dramatic upon elevation of the pH from neutral to alkaline, indicating a significant alteration of the microenvironment of the cluster and a decrease in the ordered secondary structures. The absorption change in the visible region due to pH elevation was small but clearly observed with a high signal-to-noise ratio. The numbers of protons involved in the respective processes and the apparent pK values obtained from the pH-dependence of the @entity1050 changes were in good agreement with those obtained from the pH-dependence of the absorption changes in the visible region. In addition, the rate constants obtained from the time courses of the @entity1050 and absorption changes agreed with one another. By the addition of 1 M @entity1219 , the @entity1050 and absorption spectra at alkaline pH were reversed almost to those at neutral pH without significant pH change. On the other hand, above pH 11, ferredoxin was found to be irreversibly denatured. Based on analyses of the statical @entity1050 and absorption spectra and of the time courses of the @entity1050 changes, the probable mechanism of the isomerization was considered to be as follows: (Formula: see text) where H stands for a proton, N-form for native ferredoxin at neutral pH, N*-form for alkaline ferredoxin below pH 11 which still has the @entity5985 cluster but with disordered secondary structures of the polypeptide chain, and D-form for completely denatured ferredoxin above pH 11. These results lead to the conclusions that (1) the interaction between the protein moiety and the @entity5985 cluster is essential for maintaining the native ferredoxin structure, and (2) neutral salts protect the polypeptide chain from unfolding through electrostatic interaction with the ionized side chains, resulting in the stabilization of ferredoxin.

Spectrophotometric studies on alkaline isomerization of XXXX ferredoxin.

@entity1219 :: ('CHEBI:26710', 'Chemical') :: ['NaCl'] @entity5985 :: ('CHEBI:30408', 'Chemical') :: ['iron-sulfur'] @entity1050 :: ('MESH:D006223', 'Disease') :: ['CD'] @entity2772 :: ('3562', 'Species') :: ['spinach']
@entity2772 :: (3562,Species) :: ['spinach']
