Effect of Organic Anions on the Photoreaction of Photoactive Yellow Protein

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Abstract
In order to clarify changes in the structure and surface properties of photoactive yellow protein (PYP) upon light absorption, the spectroscopic properties and solution structure of its photo-intermediate (PYPM) were examined in the presence of various anions. At identical ionic strengths, citrate slowed the decay rate of PYPM more than acetate. Although the absorption spectrum in the dark was not affected by organic anions, citrate induced a 5-nm blue shift of the absorption maximum for PYPM Solution X-ray scattering experiments indicated that the radius of gyration (Jig) and apparent molecular weight in the dark were constant in all buffer systems. However, the Rg of PYPM in citrate buffer at high concentration was 16.2 (± 0·2) A, while the Rg of PYPM in acetate buffer was 15·6 (± 0.2) A- The apparent molecular weight increased 7% upon PYPM formation in citrate buffer at high concentration compared to other conditions. These results suggest that citrate molecules specifically bind to PYPM. A cluster of basic amino acid residues with a hydrogen bond donor would be exposed upon PYPM formation and responsible for the specific binding of citrate.

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