Difference in molecular structure of rod and cone visual pigments studied by Fourier transform infrared spectroscopy

To investigate the local structure that causes the differences in molecular properties between rod and cone visual pigments, we have measured the diff erence infrared spectra between chicken green and its photoproduct at 77 K and compared them with those from bovine and chicken rhodopsins. In contras t to the similarity of the vibrational bands of the chromophore, those of t he protein part were notably different between chicken green and the rhodop sins. Like the rhodopsins, chicken green has an aspartic acid at position 8 3 (D83) but exhibited no signals due to the protonated carboxyl of D83 in t he C=O stretching region, suggesting that the molecular contact between D83 and G120 through water molecule evidenced in bovine rhodopsin is absent in chicken green. A pair of positive and negative bands due to the peptide ba ckbone (amide I) was prominent in chicken green, while the rhodopsins exhib ited only small bands in this region. Furthermore, chicken green exhibited characteristic paired bands around 1480 cm(-1), which were identified as th e imide bands of P189 using site-directed mutagenesis. P189, situated in th e putative second extracellular loop, is conserved in all the known cone vi sual pigments but not in rhodopsins. Thus, some region of the second extrac ellular loop including P189 is situated near the chromophore and changes it s environment upon formation of the bathe-intermediate. The results noted a bove indicate that differences in the protein parts between chicken green a nd the rhodopsins alter the changes seen in the protein upon photoisomeriza tion of the chromophore. Some of these changes appear to be the pathway fro m the chromophore to cytoplasmic surface of the pigment and thus could affe ct the activation process of transducin.