Analysis of pH-Induced Structural Changes of the Isolated Extrinsic 33 Kilodalton Protein of Photosystem II

• Published in: Biochemistry (Easton) 1997-05, Vol.36 (21), p.6350-6358
• Main Authors: Shutova, T, Irrgang, K.-D, Shubin, V, Klimov, V. V, Renger, G
• Format: Article
• Language: English
• Subjects: Anilino Naphthalenesulfonates; Circular Dichroism; Fluorescent Dyes; Hydrogen-Ion Concentration; Models, Molecular; Molecular Weight; Photosynthetic Reaction Center Complex Proteins - chemistry; Photosynthetic Reaction Center Complex Proteins - isolation & purification; Photosystem II Protein Complex; Potentiometry; Protein Structure, Secondary; Spectrometry, Fluorescence; Spinacia oleracea - chemistry
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi963115h

Structural properties of the isolated extrinsic regulatory 33 kDa protein of the water-oxidizing complex were analyzed at different pH values. It was found that (a) titrations of the buffer capacity reveal a characteristic hysteresis effect that is unique for the 33 kDa subunit and is not observed for the other extrinsic proteins, (b) changes of the emission from the fluorescence probe 1,8-ANS are indicative of an increased accessibility of the hydrophobic core of the 33 kDa protein to the dye at lower pH, (c) the near-UV circular dichroism spectrum of the polypeptide is altered owing to a pH decrease from 6.8 to 3.8 and becomes drastically changed at pH 2.8, and (d) the content of secondary structure elements remains virtually constant in the range 3.8 < pH < 6.8, with the following values gathered from far-UV CD spectra:  ∼8% α-helix, ∼33% β-strand, ∼15% turns, and ∼44% random coil. Further acidification down to pH 2.8 gives rise to a decreased α-helix and increased β-strand and random coil content. A theoretical model [Ptitsyn, O., & Finkelstein, A. (1983) Biopolymers 2, 15−22] was used to predict the probability and location of secondary structure elements within the protein sequence. On the basis of these calculations, an extended hydrophobic β-sheet domain could exist in the center of the protein and an α-helix in the C-terminal region. From these data, the 33 kDa protein is inferred to change its tertiary structure in vitro upon acidification of the aqueous environment. Possible implications of these features are discussed.