[Study by fast kinetics of certain ligand combination and dissociation reactions with...

Bis(N-maleimidomethyl)ether-hemoglobin (HbBME) is a well known derivative of hemoglobin obtained by reaction with bis(N-maleimidomethyl)ether, which forms an intra-chain bond in each of the beta subunit of the protein. Previous cristallographic determinations suggest that this chemical modification exerts an effect on the structure of the COOH-terminal region in the beta chain. As a consequence, the quaternary structure of deoxygenated HbBME is strongly destabilized in favor of the oxy conformation. Kinetic studies including oxygen dissociation and carbon monoxide binding reactions have been performed on HbBME in order to correlate these changes in structure with the reactivity of hemes in the protein. The disappearance of heme-heme interaction in HbBME is expressed in the character of the oxygen dissociation kinetics. With respect of this reaction, we show that the reactivity of alpha chains remains unchanged inside the chemically modified tetramer, whilst that of beta chains is increased. Carbon monoxide binding kinetics show heterogeneity with two well separated phases. The fast phase contributes about 75 per cent of the total absorbance change and has a rate of binding similar to that of fastly reacting hemoglobins. The slow phase show the same rate as in native deoxyhemoglobin. The effects of inositolhexaphospate on the these kinetics have been studied. They consist mainly in the decrease of fast phase which can disappear completely according to the experimental conditions. These experiments suggest that deoxy-HbBME exists in solution as a mixture of two isomers in slow equilibrium. Taking account of previous structural studies, these results are discussed in comparison with an hemoglobin mutant with very similar properties, hemoglobin Bethesda (Tyr 145beta leads to His). The importance of the COOH-terminal region on the functional properties of hemoglobin is once again emphasized.