Hemoglobin carries oxygen from the lungs to the tissues. Four globin chains hold four hemes, and each heme binds one oxygen molecule. The four sites are not independent. The tetramer switches between a low-affinity T state and a high-affinity R state, so the binding curve is sigmoid. Acid, carbon dioxide and 2,3-BPG push the switch toward T. The molecule then releases oxygen where tissues need it.
4
Chains in the working unit
1Two alpha and two beta chains, arranged as a pair of alpha-beta dimers.
4
One heme per chain, one O2 per heme.
2.8
Unmodified, stroma-free human hemoglobin, the control in a crosslinking study. Phosphate buffer at 29 C gives values near 3.1.
26.6 mmHg
P50, standard conditions
18Human standard P50, 26.6 +/- 1.2 mmHg. Cell-free hemoglobin gives 12 to 15 torr.
-0.29 d log P50 / d pH
Measured over pH 7.1 to 7.7 on unmodified hemoglobin.
1.407 mL O2 per g Hb
14 degrees
Quaternary turn, T to R
24Measured by this site: superpose the alpha1-beta1 dimer of 2HHB on 2DN3, then fit the other dimer. 2HHB to 2DN3 gives 14.2 deg, 2HHB to 2DN1 gives 14.1 deg, 2HHB to 1HHO gives 13.0 deg. A published analysis gives 15.0 deg.
6.7 x 5.9 x 5.4 nm
Principal-axis extents of all atoms in 2HHB, measured in this work.