Oxygen-linked CO, Binding to Isolated p Subunits of Human Hemoglobin

It is known that most of the oxygen-linked carbamate which is formed in normal adult human hemoglobin (Hb A) is confined to the /3 subunits rather than to the LY subunits. In order to find out if similar differences exist in the isolated protomers of Hb A we have measured the effect of various pressures of carbon dioxide (pcoJ on the oxygen affinity in the following heme pigments: isolated (Y and p subunits with free -SH groups (rysti myoglobin (Mb) and $‘;“” mercurated p subunits (fi”“‘“), $L”p in which the terminal a-amino group of psi’ wa6 irreversibly blocked with pyridoxal phosphate (PLP). Similar measurements were done on Hb A and the fraction of oxygen-linked carbamate calculated from the effect ofp,,,, (at constant pH) on the oxygen half-saturation pressure (p5,,). A distinct influence of CO2 on par, was observed in psH which was absent in @I”p and thus indicates that the terminal a-amino group mediates the oxygen-linked binding of CO2 in p”” as it does in the /3 subunits of Hb A. However, the fraction of oxygen-linked carbamate was much less dependent on pH and pcoZ in /F” than in Hb A. Neither &“, p”‘“‘, or Mb, all of which are known to exist largely or wholly as monomers but have free terminal (Yamino groups, showed a shift ofp,,, upon addition of CO,. As both psH and pB,$ were shown to be tetrameric molecules, we conclude from this study that homotetramers composed of isolated p subunits do exhibit a reciprocal interaction between the binding of 0, and CO,.