The crocodile, which can remain under water without breathing for up to 1 hour,

Question

The crocodile, which can remain under water without breathing for up to 1 hour, drowns its air-breathing prey and then dines at its leisure. An adaptation that aids the crocodile is that it can utilize virtually 100% of the O_2 in its blood, whereas humans can only extract about 65% of oxygen in the blood. Deoxygenated crocodile Hb cannot bind BPG. However, the deoxygenated form does preferentially bind bicarbonate. How does this adaptation help the crocodile obtain its dinner? As part of your answer, sketch several binding curves of crocodile Hb binding to O_2 at increasing [HCO_3].

Explanation / Answer

Hb(4O2) (4O2) + deoxyHB deoxyHB-CO3 -

At the tissues, there is an equilibrium between oxygenated and deoxygenated Hb, depending upon the oxygen concentration in the tissues. Utilization of oxygen by the crocodile tissues will remove the oxygen from the right of the first equilibrium expression, causing the dissociation of oxygen from oxyhemoglobin. At the same time, the crocodile tissues are generating carbon dioxide from metabolism. This binds to deoxyHb, generating the carboxylated form. This reaction depletes the deoxyHb from the center of the above equilibrium, causing further incentive for oxyHb to give away its oxygen

When the crocodile has its prey underwater and is using lots of energy to keep its jaws clamped on the prey, O2 is consumed and CO2 is produced. CO2 is converted to protons and bicarbonate ions as follows: CO2 + H2O H2CO3 H+ + HCO3 - H+ is an allosteric inhibitor that specifically binds the deoxy or T state of hemoglobin, shifting the equilibrium from the R state to the T state. This is the pH Bohr Effect. In addition, the problem states that HCO3 - also specifically binds the deoxy or T state of crocodile hemoglobin. The shift to the deoxy or T state by H+ and HCO3 - lowers Hb affinity for O2, allowing for nearly complete O2 unloading and utilization in the crocodile’s blood.

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