Hemoglobin titration curves

The titration curve in the acid region is time-dependent and irreversible, as was first clearly demonstrated by Steinhardt and Zaiser (1951). This aspect of the titration of hemoglobin has also been reviewed previously (Steinhardt and Zaiser, 1955), but there was some ambiguity about the meaning of this phenomenon at the time of the review, which has been removed by more recent work, as will be briefly described here. 

A titration curve for sperm whale myoglobin has been reported by Bres-low and Gurd (1962). The most striking feature is that it exhibits a time-dependent acid denaturation, which resembles that observed for the similar protein hemoglobin. To elucidate the physical nature of this reaction, emphasis was placed on the back titration to neutral pH of denatured protein. As in the case of hemoglobin (mentioned earlier), there are two major differences between the titration curves of native and denatured myoglobin, as shown by the data of Table XV. 

B. German, and J. Wyman, The titration curves for oxygenated and reduced hemoglobin, J. Biol. Chem. 117, 533-550 (1937). 

Bohr and Haldane effects. Terminology. Henderson s interpretation of oxygen Bohr effect. Indirect estimates of titration curve of oxyhemoglobin. Direct differential titration of hemoglobin and oxyhemoglobin. Reversed Bohr effect at acid reactions. The existence of two oxygen-... 

The first and less specific body of evidence, which may be described as contributory rather than conclusive, comes from a study of the effect of temperature on the titration curve of the oxygenated hemoglobin (238). This evidence rests on the concept of the apparent heat of dissociation. The latter concept requires brief comment. This apparent heat is defined by the equation ... 

Pig. 11. Titration curves of hemoglobin and oxyhemoglobin of horse, from German and Wyman (56). Ordinates give equivalents per heme. Dashed line corresponds to buffering power of 3.0 equivalents heme per heme per pH unit. O " oxyhemoglobin. A = hemoglobin. 

Therefore the difference between the titration curves of oxyhemoglobin and hemoglobin respectively should be... 

Differential titration experiments were carried out on three systems (79). Ferroperoxidase and carbon monoxide ferroperoxidase gave identical curves. The peroxidase thus shows no Bohr effect. Since the two derivatives correspond as to bond types to the analogous hemoglobin and carbon monoxide hemoglobin, it can be stated with fair certainty that the heme-linked groups in horse-radish peroxidase are not imidazole. 

---