Alkaline bohr effect

To produce a strong alkaline Bohr effect and a Root effect at acid pH, Xenopus hemoglobin has conserved the essential Ser F9/3, Glu FGljS, and... 

His HC3j8, and it has Gly rather than Lys in position HCljS to prevent anchoring of the C terminus in R structure (Table I). On the other hand, it has a Lys at F6/3, one turn of the tt helix away from the Glu FG1)8 and capable of competing with His HC3/8 for the essential salt bridge with that Glu (Figs. 4 and 5). This competition may weaken its alkaline Bohr effect compared to that of carp. 

In another aquatic f rog, Amphiuma, the adult hemoglobin has an acid Bohr effect at physiological pH, converted in vivo to a weak alkaline one... 

In amphibia that do not need a Root effect, the alkaline Bohr effect is weakened by a variety of single substitutions Ser F9 — Ala in the tadpoles of Xenopus and R. catesbeiana Glu FGl/8 — Asn or Gly in adult frogs and in one of the tadpoles His HC3 — Phe in another tadpole. It looks as though once a function is no longer needed any mutation that inhibits the function becomes fixed (97). 

Very recently, Shibayama et al. (1991) reported that cross-linked (a(3)A (chP)aXL isolated by our procedure (i.e., gel filtration in the presence of 1 M MgCl2 followed by ion-exchange chromatography) was found to be contaminated with about 20% of an electrophoret-ically silent impurity that shows higher 02 affinity. They developed a purification technique to separate the desired cross-linked Hb A from the impurity. They found that their purified (aP)A(aP)AXL in the absence of organic phosphate has the same 02 affinity, cooper-ativity, and alkaline Bohr effect as native Hb A. 

The Bohr effect, i.e., the uptake of hydrogen ions above pH 6 (known as the alkaline Bohr effect), is physiologically important because the ions released on uptake of CO2 by the blood from the tissues are neutralized through this mechanism. Figure 4 illustrates the Bohr effect of normal hemoglobin the reverse effect, the acid Bohr effect, has no physiological significance because it is observed below pH 6. 

The alkaline Bohr effect is synchronous with the oxygenation reaction and the liberation of Bohr protons is exactly proportional to the amount of oxygen taken up. The effect also results from specific changes within the hemoglobin molecule and the latest data indicate that 50% of the effect is due to the salt bridge between the imidazole side chain of His... 

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