Inequivalent Conformational Response of Chironomus Hemoglobins to Ligation with O2 and CO.

Abstract

For the monomeric Hb III and IV and the dimeric Hb of C. thummi thummi the tryptophan circular dichroism reports different conformational response to binding of different ligands. The fine structure bands at 292 nm are most strongly developed in the unliganded state and considerably reduced in the oxy form. The spectrum of the CO derivative is intermediate but clearly more deoxy-like. This correlates well with the corresponding Bohr effect magnitudes determined by measuring proton release upon ligation. IR difference spectroscopy on O2 vs. CO derivatives in aqueous solution shows a normal O2 stretching band position at 1107 cm-1 characteristic of asymmetric O2 binding. The CO stretching band is consistently blue-shifted by 11-13 cm-1 from the 1951 cm-1 position observed with mammalian Hb, indicating reduced CO binding strength. Structural factors relevant to an explanation of the observed phenomena are discussed.