Hemoglobin electrostatic interactions

Figure 7.8 Equilibria among the R and T forms of hemoglobins ar-chains are represented by circles, /3-chains by squares. The clamps represent electrostatic interactions. >-< represents 2,3-diphosphoglycerate. a, b, c and d represent electrostatic linkages that are broken or formed in the processes of interconversion between the reversible tight to relaxed conformation.

The question has been essentially decided by more recent experiments (Beychok and Steinhardt, 1959 Steinhardt et al., 1962). One procedure was to reduce the temperature and increase the ionic strength, the increase in ionic strength being for the purpose of diminishing the importance of electrostatic interactions. A second procedure was to use hemoglobin derivatives which are especially stable in the native state, these being the... 

In subsequent work,Vasudevan et al. [23] conclusively showed that the hemoglobin was fully dissociated under the conditions presented in Ref. 18 and that only the heme (i.e., iron within porphyrin) was extracted into the microemulsion. Vasudevan and Wiencek [24] went on to prove that under very limiting circumstances, protein will partition into nonionic microemulsion liquid membranes. The underlying extraction mechanism is a weak electrostatic interaction between the trace impurities in the surfactant and the protein. Since the separation is based on an interaction between the surfactant (or some other interfacially active compound) and the solute, no stripping reaction is required, and the system is really an equilibrium microemulsion extraction system as described in Sec. II.B.2. 

As early as 1941 Bernal and Fankuchen [10] observed peaks in the scattering curve of concentrated virus solutions. Later, such peaks were also reported for concentrated solutions of bovine serum albumin (BSA), hemoglobin, and DNA [11]. It soon became clear that this phenomenon originates from intermolecul-arly oriented structures in the solution caused by the electrostatic interaction between the polyions. In recent years this general behaviour of polyelectrolytes has been extensively investigated. Nevertheless, many properties still remain unexplained and we are still far from accounting quantitatively for many experimental results. Even the qualitative interpretation of experiments is still a matter of considerable discussion and controversy. 

Proteins containing more than one polypeptide chain, such as hemoglobin (see Topic B4), exhibit a fourth level of protein structure called quaternary structure (Fig. 8). This level of structure refers to the spatial arrangement of the polypeptide subunits and the nature of the interactions between them. These interactions may be covalent links (e.g. disulfide bonds) or noncovalent interactions (electrostatic forces, hydrogen bonding, hydrophobic interactions). 

---