Bauer C, Kurtz A
J Biol Chem. 1977 May 10;252(9):2952-5.
It is known that most of the oxygen-linked carbamate which is formed in normal adult human hemoglobin (Hb A) is confined to the beta subunits rather than to the alpha 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 (pCO2) on the oxygen affinity in the following heme pigments: isolated alpha and beta subunits with free --SH groups (alphaSH, betaSH), mercurated beta subunits (betaPMB), myoglobin (Mb), and betaSH/PLP in which the terminal alpha-amino group of betaSH was irreversibly blocked with pyridoxal phosphate (PLP). Similar measurements were done on Hb A and the fraction of oxygen-linked carbamate calculated from the effect of pCO2 (at constant pH) on the oxygen half-saturation pressure (p50). A distinct influence of CO2 on p50 was observed in betaSH which was absent in betaSH/PLP and thus indicates that the terminal alpha-amino group mediates the oxygen-linked binding of CO2 in betaSH as it does in the beta subunits of Hb A. However, the fraction of oxygen-linked carbamate was much less dependent on pH and pCO2 in betaSH than in Hb A. Neither alphaSH, betaPMB, or Mb, all of which are known to exist largely or wholly as monomers but have free terminal alpha-amino groups, showed a shift of p50 upon addition of CO2. As both betaSH and betaSH/PLP were shown to be tetrameric molecules, we conclude from this study that homotetramers composed of isolated beta subunits do exhibit a reciprocal interaction between the binding of O2 and CO2.
已知在正常成人血红蛋白（Hb A）中形成的大多数氧联氨基甲酸酯局限于β亚基而非α亚基。为了弄清楚在Hb A的分离原聚体中是否存在类似差异，我们测量了以下血红素色素中不同二氧化碳压力（pCO2）对氧亲和力的影响：具有游离-SH基团的分离α和β亚基（αSH、βSH）、汞化β亚基（βPMB）、肌红蛋白（Mb）以及βSH/PLP，其中βSH的末端α-氨基被磷酸吡哆醛（PLP）不可逆地阻断。对Hb A进行了类似测量，并根据pCO2（在恒定pH下）对氧半饱和压力（p50）的影响计算氧联氨基甲酸酯的比例。在βSH中观察到CO2对p50有明显影响，而在βSH/PLP中则不存在这种影响，因此表明末端α-氨基介导了βSH中氧联CO2的结合，就像在Hb A的β亚基中一样。然而，βSH中氧联氨基甲酸酯的比例对pH和pCO2的依赖性远小于Hb A。已知主要或完全以单体形式存在但具有游离末端α-氨基的αSH、βPMB或Mb，在添加CO2后均未显示p50的变化。由于βSH和βSH/PLP均显示为四聚体分子，我们从这项研究得出结论，由分离的β亚基组成的同型四聚体在O2和CO2的结合之间确实表现出相互作用。
