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Hemoglobin can bind to four molecules of carbon dioxide. The carbon dioxide molecules form a carbamate with the four terminal-amine groups of the four protein chains in the deoxy form of the molecule. Thus, one hemoglobin molecule can transport four carbon dioxide molecules back to the lungs, where they are released when the molecule changes ...
The average red blood cell contains 250 million hemoglobin molecules. [7] Hemoglobin contains a globin protein unit with four prosthetic heme groups (hence the name heme-o-globin); each heme is capable of reversibly binding with one gaseous molecule (oxygen, carbon monoxide, cyanide, etc.), [8] therefore a typical red blood cell may carry up to one billion gas molecules.
Hemoglobin's affinity for oxygen increases as successive molecules of oxygen bind. More molecules bind as the oxygen partial pressure increases until the maximum amount that can be bound is reached. As this limit is approached, very little additional binding occurs and the curve levels out as the hemoglobin becomes saturated with oxygen.
Hemoglobin has an oxygen-binding capacity of 1.34 mL of O 2 per gram, [6] which increases the total blood oxygen capacity seventy-fold compared to dissolved oxygen in blood plasma alone. [7] The mammalian hemoglobin molecule can bind and transport up to four oxygen molecules. [8] Hemoglobin also transports other gases.
This releases hydrogen ions from hemoglobin, increases free H + concentration within RBCs, and shifts the equilibrium towards CO 2 and water formation from bicarbonate. The subsequent decrease in intracellular bicarbonate concentration reverses chloride-bicarbonate exchange: bicarbonate moves into the cell in exchange for chloride moving out.
In hemoglobin, the iron is in one of four heme groups and has six possible coordination sites; four are occupied by nitrogen atoms in a porphyrin ring, the fifth by an imidazole nitrogen in a histidine residue of one of the protein chains attached to the heme group, and the sixth is reserved for the oxygen molecule it can reversibly bind to. [5 ...
Carbon monoxide, for example, is extremely dangerous when carried to the blood via the lungs by inhalation, because carbon monoxide irreversibly binds to hemoglobin to form carboxyhemoglobin, so that less hemoglobin is free to bind oxygen, and fewer oxygen molecules can be transported throughout the blood. This can cause suffocation insidiously.
Histidine residues in hemoglobin can accept protons and act as buffers.Deoxygenated hemoglobin is a better proton acceptor than the oxygenated form. [1]In red blood cells, the enzyme carbonic anhydrase catalyzes the conversion of dissolved carbon dioxide to carbonic acid, which rapidly dissociates to bicarbonate and a free proton: