Heme structure of hemoglobin M Iwate [alpha 87(F8)His -> Tyr]: A UV and visible resonance Raman study

Heme structures of a natural mutant hemoglobin (Hb), Hb M Iwate [alpha 87(F 8)Hiss-Tyr], and protonation of its F8-Tyr were examined with the 244-nm ex cited UV resonance Raman (UVRR) and the 406.7- and 441.6-nm excited visible resonance Raman (RR) spectroscopy. It was clarified from the UVRR bands at 1605 and 1166 cm(-1) characteristic of tyrosinate that the tyrosine (F8) o f the abnormal subunit in Hb M Iwate adopts a deprotonated form. UV Raman b ands of other Tyr residues indicated that the protein takes the T-quaternar y structure even in the met form. Although both hemes of alpha and beta sub units in metHb A take a six-coordinate (6c) high-spin structure, the 406.7- nm excited RR spectrum of metHb M Iwate indicated that the abnormal ct subu nit adopts a 5c high-spin structure, The present results and our previous o bservation of the nu (Fe-o(tyrosine)) Raman band [Nagai et al. (1989) Bioch emistry 28, 2418-2422] have proved that F8-tyrosinate is covalently bound t o Fe(III) heme in the alpha subunit of Hb M Iwate. As a result, peripheral groups of porphyrin ring, especially the vinyl and the propionate side chai ns, were so strongly influenced that the RR spectrum in the low-frequency r egion excited at 406.7 nm is distinctly changed from the normal pattern, Wh en Hb M Iwate was fully reduced, the characteristic UVRR bands of tyrosinat e disappeared and the Raman bands of tyrosine at 1620 (Y8a), 1207 (Y7a), an d 1177 cm(-1) (Y9a) increased in intensity. Coordination of distal His(E7) to the Fe(II) heme in the reduced alpha subunit of Hb M Iwate was proved by the observation of the nu (Fe-His) RR band in the 441.6-nm excited RR spec trum at the same frequency as that of its isolated alpha chain, The effects of the distal-His coordination on the heme appeared as a distortion of the peripheral groups of heme. A possible mechanism for the formation of a Fe( III)-tyrosinate bond in Hb M Iwate is discussed.