unspoken devotion to my chosen hemoglobin portion has awoken this notion that my emotion is stolen
Thank you @DatPotato for providing me with words that rhyme with “hemoglobin.”
Hemoglobin (Hb) is the key protein that makes red blood cells (RBCs) red. Hb functions in oxygen (O2) transport. However, Hb can bind things other than oxygen. In this article, we will discuss the consequences of Hb binding to carbon dioxide (CO2), protons (H+), and nitric oxide (NO).
Carbon Dioxide
Hb bound to CO2 is called carbaminohemoglobin (HbCO2). In the red blood cell, HbCO2 can split into Hb and CO2. This allows CO2 unloading into the alveolus for exhalation.
You may be wondering - because Hb has 4 oxygen binding sites, why doesn’t HbCO2(O2)3 form? Or HbO2(CO2)3? Basically, why doesn’t Hb typically bind a combination of CO2s and O2s? Consider this: when CO2 is already bound to Hb, Hb loses its affinity for O2. Thus, O2 can no longer bind as well. Additionally, when O2 is already bound to Hb, cooperativity dictates that Hb will have a further affinity for O2. Thus, it’s highly unlikely that CO2 will bind.
However, note that most CO2 is not transported bound to Hb; more often, dissolved CO2 reacts with water with the help of carbonic anhydrase to form bicarbonate (HCO3-) which is the primary form of CO2 transport in the blood. HCO3- is mostly transported within RBCs, although about 25% is dissolved in plasma.
When it comes time for exhalation of the CO2 stored as bicarbonate, the HCO3-/Cl- antiporter in the RBC membrane exports the HCO3- out, where the aforementioned carbonic anhydrase-catalyzed reaction occurs in reverse and produces CO2 suitable for exhalation. Because the HCO3- is exported by an antiporter, the Cl- enters the cell. Because an anion (Cl-) is entering as another anion (HCO3-) exits the cell, electroneutrality is maintained. The resultant increase in the concentration of Cl- in the RBC is called the chloride shift.
Protons
Proton-bound Hb (HbH) is similar to HbCO2 in that the affinity of deoxyHb for H+ is higher than the affinity of oxyHb for H+. An important function of HbH in the body is that it serves to buffer the blood. Thus, even though venous blood might be very acidic after passing by a very metabolically active tissue, the pH only decreases slightly.
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Here is an image that summarizes the interaction of Hb with CO2 and H+:
Nitric Oxide
NO binds to the thiol groups on Hb, forming HbNO. This has two consequences; first, Hb is able to transport NO to various tissues, and NO is able to exert its local vasodilatory effects accordingly. This can help tissues take up more oxygen (as their vessel volume has increased due to the vasodilation). Second, NO makes Hb into methemoglobin (metHb). This form results from oxidation of heme iron to the ferric state, which has a +3 charge (the oxygen-binding version is ferrous, which has a +2 charge). Generally, NO can also alter Hb’s oxygen affinity.
That’s it! Thanks for reading. Hope you liked it. Please send me more article ideas :3
Here are some more resources to learn about this topic:
Vander’s Chapter 13 in my edition.
More advanced stuff about the types of Hb: https://mriquestions.com/types-of-hemoglobin.html.
More about the chloride shift: https://en.wikipedia.org/wiki/Chloride_shift.
More on Hb in general: https://en.wikipedia.org/wiki/Hemoglobin.
fun fact about CO: “The affinity of carbon monoxide for hemoglobin is 240 times that of oxygen. Once one molecule of carbon monoxide binds to hemoglobin, it shifts the hemoglobin-oxygen dissociation curve to the left, further increasing its affinity and severely impairing release of oxygen to the tissues.”
