We begin with a few important reactions that can either move backward or forward depending on conditions.174 Molecular oxygen (O2) is a vital component in a number of reactions in our bodies, such as aerobic respiration, the evolutionarily ancient process by which we capture energy from food.175 O2 must be transported to every cell so that it can participate in cellular reactions. O2 diffuses into the bloodstream in the lungs, but it is not very soluble in water (the main component of blood). If we relied on the solubility of oxygen in water to transport it around the body, we would be in trouble. Instead O2 reacts with (we usually say “binds to”, but this is definitely a chemical reaction) a protein called hemoglobin. The structure of hemoglobin is complex: it is composed of four polypeptide subunits and each polypeptide is associated with a heme group.176 The heme group contains an iron ion (Fe2+) complexed to four nitrogenous bases linked into a ring (called a porphyrin) to form a more or less planar arrangement, as shown in the figure. Heme is also the central active portion of one of the major components of our immune system, myeloperoxidase177. When you blow your nose, that familiar green color is actually caused by the light absorbing properties of the heme group in this enzyme, rather than the bacterial infection. Because the heme group is in a different molecular environment, its color appears green rather than red. Chlorophyll, a similar molecule, differs most dramatically from heme in that the iron ion is replaced by a magnesium ion (as shown in the figure). Its function is not to bind O2 (or CO2), but rather to absorb visible light and release an energetic electron as part of the photosynthetic process.
Iron is a transition metal. Recall that these elements have d orbitals, some of which are empty and available for bonding. Iron II (Fe2+) has plenty of energetically-available orbitals, and therefore can form Lewis acid–base complexes with compounds that have available electrons (such as nitrogenous bases). Within the porphyrin ring, four nitrogens interact with the Fe2+ ion. Typically, transition metals form complexes that are geometrically octahedral. In the case of the heme group, four of these interactions involve nitrogens from the four rings; a fifth involves a nitrogen of histidine residue of one of the protein’s polypeptides that approaches from below the ring plane. This leaves one site open for the binding of an O2 molecule, which has available lone electron pairs.178 When an O2 binds to one of these heme groups,
Hemoglobin + O2 ⇄ Hemoglobin-O2.
Note that this way of depicting the reaction is an oversimplification. As we said initially, each hemoglobin molecule contains four polypeptides, each of which is associated with a heme group (green in the figure), so there are four heme groups in a single hemoglobin molecule. Each heme group can bind one O2 molecule. When an O2 molecule binds to the heme iron, there are structural and electronic changes that take place within the protein as a whole. This leads to a process known as cooperativity, wherein the four heme groups do not act independently. Binding O2 to one of the four heme groups in hemoglobin causes structural changes to the protein, which increases the affinity for O2 in each of the remaining three heme groups. When a second O2 binds, affinity for O2 is once again increased in the remaining two heme groups.
As you might suspect, this process is reversible. Imagine a hemoglobin protein with four bound oxygen molecules. When an O2 is released from the hemoglobin molecule, the affinity between the remaining O2s and the heme groups is reduced, making it more likely that more of the bound O2s will be released. This is an equilibrium reaction, and we can apply Le Chatelier’s principle to it. Where O2 is in abundance (in the lungs), the reaction shifts to the right (binding and increasing affinity for O2). Where O2 is present at low levels, the reaction shifts to the left (releasing and reducing affinity for O2). The resulting hemoglobin molecule has a high capacity for binding O2 where O2 is present at high concentrations and readily releases O2 where O2 is present at low concentrations. In the blood [hemoglobin] ranges between 135–170 g/L, approximately 2 millimoles per liter (mM), and because there are four O2 binding sites per hemoglobin, this results in approximately ~ 250 mg/L or s8-mM concentration of O2.
By comparison, O2‘s solubility in water is ~ 8mg/L at 37oC, or 250 micromoles per liter (μM). The reaction can be written like this:
O2 in the air ⇄ O2 in the blood (liquid) + hemoglobin ⇄ hemoglobin-O2 + O2 in the blood ⇌ hemoglobin-2O2 + O2 in the blood ⇄ hemoglobin-3O2 + O2 in the blood ⇄ hemoglobin-4O2
When the hemoglobin reaches areas of the body where [O2] is low, the oxygen dissociates from the hemoglobin into the blood. The dissolved O2 is then removed from the blood by aerobic (oxygen-utilizing) respiration:
C6H12O6 + 6O2 ⇄ 6CO2 + 6H2O.
The combination of Le Chatelier’s principle and the cooperativity of the O2 + hemoglobin reaction now leads to the complete dissociation of the hemoglobin—4O2 complex, releasing O2. The products of aerobic respiration (essentially a combustion reaction) are carbon dioxide and water. Clearly the water can be carried away in cellular fluid, but the carbon dioxide must be removed in a variety of ways: a small part is removed by reacting with the hemoglobin (but not at the Fe center), some is dissolved in the blood, and some takes part in the buffering system present in the blood, and most is released in the lungs, into the air that you breath out.
Questions to Answer
- What complicates reaction systems in the real world (outside the lab)?
- Why is O2 not very soluble in water?
- By what factor does binding with hemoglobin increase solubility of O2 in water?
- Draw Lewis structures for O2 and CO. Why do you think they bind in similar ways to hemoglobin?
- Why does CO2 react differently with hemoglobin from the way O2 interacts with hemoglobin?
Questions to Ponder
- Why does it make physiological sense that O2 binds to oxygen-free hemoglobin (deoxyhemoglobin) relatively weakly and cooperatively?
174 Of course this designation is entirely arbitrary, as backward and forward depend on how the initial reaction is written.
175 We recognize such evolutionarily conserved processes because they used essentially (but not quite) the same reaction components and strategies. For example, aerobic respiration (whether in bacteria, potatoes, or humans) uses a structurally similar membrane system to transfer electrons from molecule to molecule (redox reaction). This generates an H+ gradient then used by a rotatory protein “generator” to synthesize ATP.
176 Amino acid chains are referred to as polypeptides; a protein is a functional unit, which can be composed of multiple polypeptides and non-polypeptide components such as heme groups.
178 This binding site can also be occupied by other types of molecules, in particular carbon monoxide (CO). Because the binding of O2 to hemoglobin is much weaker and less stable than the CO–hemoglobin interaction, exposure to CO blocks O2 transport through the body, leading to suffocation.