Influence of globin structures on the state of the heme. IV. Ferrous low spin derivatives

Abstract

Studies of high spin ferrous and ferric derivatives led us to conclude that in the quaternary R structure the state of the hemes is similar to that in the free alpha and beta subunits, but in the T structure a tension acts on the hemes which tries to pull the iron and the proximal histidine further from the plane of the porphyrin. We have now studied the effect of inositol hexaphosphate (IHP) on the three low spin ferrous compounds of hemoglobin with O2, CO, and NO. IHP failed to switch the quaternary structure of carbonmonoxy- and oxyhemoglobin A to the T state, but merely caused a transition to an as yet undefined modification of the R structure. IHP is known to cause a switch to the T structure in hemoglobin Kansas. We have found that this switch induces red shifts of the visible alpha and beta absorption bands and the appearance of a shoulder on the red side of the alpha band; these changes are very weak in carbonmonoxy- and slightly stronger in oxyhemoglobin Kansas. As already noted by previous authors, addition of IHP to nitrosylhemoglobin A induces all the changes in uv absorption and CD spectra, sulfhydryl reactivities, and exchangeable proton resonances normally associated with the R leads to T transition, and is accompanied by large changes in the Soret and visible absorption bands. Experiments with nitrosyl hybrids show that these changes in absorption are caused predominantly by the hemes in the alpha subunits. In the accompanying paper Maxwell and Caughey (J. C. Maxwell and W. S. Caughey (1976), Biochemistry, following paper in this issue) report that the NO in nitrosylhemoglobin without IHP gives a single ir stretching frequency characteristic for six-coordinated nitrosyl hemes; addition of IHP causes the appearance of a second ir band, of intensity equal to that of the first, which is characteristic for five-coordinated nitrosyl hemes. Taken together, these results show that the R leads to T transition causes either a rupture or at least a very dramatic stretching of the bond from the iron to the heme-linked histidine, such that an equilibrium is set up between five- and six-coordinated hemes, biased toward five-coordinated hemes in the alpha and six-coordinated ones in the beta subunits. The reason why IHP can switch nitrosyl-, but not carbonmonoxy- or oxyhemoglobin A, from the R to the T structure is to be found in the weakening of the iron-histidine bond by the unpaired NO electron and by the very short Fe-NO bond length.

Oxygen diffusion through hemoglobin and HbdesFe

“…in-' of nitrogen or oxygen. Since the absorption spectra of these proteins are sensitive to the conformation [8] the absorption studies indicate that elevated gas pressures do not disrupt the protein structure. When oxygen pressure was slowly removed the porphyrin lifetime returned to the unquenched value.…”

Turtle hemoglobin: evidence of a T‐state oxyhemoglobin

“…Nitric oxide derivatives were studied. Nitric oxide binds to hemes and is known to be a label sensitive to the quaternary structure of hemoglobin [5][6][7].…”

“…The T state is characterized by a well-resolved three-line hyperfme structure in the EPR spectrum, which has been assigned to the or-heroes [7][8][9].…”

Magnetic Studies on the Ferri‐haem Undecapeptide of Cytochrome c

“…The influence of globin on haem has been studied using the properties of intact haemoglobin [2], while iron-sulphur proteins have been approached in a complimentary manner by studying the effects of binding small peptides to their chemically synthesized iron-sulphur cores [3]. An intermediate approach, which we are beginning to investigate, is to make use of metal-containing peptide fragments resulting from the enzymic digestion of native enzymes.…”