CONTROL OF CARBON-MONOXIDE BINDING STATES AND DYNAMICS IN HEMOGLOBIN-I OF LUCINA-PECTINATA BY NEARBY AROMATIC RESIDUES

The IR spectra of the CO-bound forms of monomeric hemoglobin I (HbI) o f Lucina pectinata and of a His64(E7) --> Gln64(E7), Leu29(B10) --> Ph e29(B10), Val68(E11) --> Phe68(E11) triple mutant of sperm-whale myogl obin which mimics the heme pocket of HbI, have been measured. HbI show s a main CO stretching component at nu(C-O) = 1936 cm(-1), while the t riple mutant shows it at nu(C-O) = 1935 cm(-1). Both proteins show a l arger population of this main A(3) conformer than the A(0) (HbI nu(C-O ) = 1960 cm(-1), triple mutant nu(C-O) = 1961 cm(-1)) or A(1,2) (nu(C- O) = 1950 cm(-1)) conformers when compared to wild-type sperm-whale my oglobin. The preference for the Ag conformer in these proteins is due to the existence of aromatic phenylalanine residues next to the heme. The positive electric field increase due to nearby phenyl multipoles o f the phenylalanines increase backbonding and reduce nu(C-O). The syne rgistic effect of the three unique residues in Lucina pectinata Hb I ( His64, Phe29, Phe68) is proposed to control the population of conforma tional states in this protein and the CO association rate.