Hemoglobin carbamate

In addition, some COg is directly transported by hemoglobin in the form of carbamate (—NHCOO ). Free a-amino groups of Hb react with COg reversibly ... 

In addition to transporting Oj from the lungs to peripheral tissues, hemoglobin transports CO2, the byproduct of respiration, and protons from peripheral tissues to the lungs. Hemoglobin carries CO2 as carbamates formed with the amino terminal nitrogens of the polypeptide chains. 

Hemoglobin carbamates account for about 15% of the CO2 in venous blood. Much of the remaining COj is carried as bicarbonate, which is formed in erythrocytes by the hydration of COj to carbonic acid (H2CO3), a process catalyzed by carbonic anhydrase. At the pH of venous blood, HjCOj dissociates into bicarbonate and a proton. 

Binding of C02 Most of the carbon dioxide produced in metabolism is hydrated and transported as bicarbonate ion (see p. 9). However, some CO2 is carried as carbamate bound to the uncharged a-amino groups of hemoglobin (carbamino-hemo-globin see Figure 3.7), which can be represented schematically as follows ... 

A lesser amount is transported by hemoglobin in the form of an attached carbamate. 

The C02 is bound covalently to the free a-amino groups of hemoglobin to form a carbamate. This re-... 

Some of this bicarbonate is transported out of the erythrocytes and is carried dissolved in the blood serum. A portion reacts directly with hemoglobin, binding to the N-terminal amino groups of the chains to form carbamates, as follows ... 

Although most of the CO2 produced by metabolism in the tissues is carried to the lungs as bicarbonate, some of the CO2 is covalently bound to hemoglobin (Fig. 44.14). In the tissues, CO2 forms carbamate adducts with the N-terminal amino groups of deoxyhemoglobin and stabilizes the deoxy conformation. In the... 

Fig. 44.14. Binding of CO2 to hemoglobin. CO2 forms carbamates with the N-terminal amino gronps of Hb chains. Approximately 15% of the CO2 in blood is carried to the Inngs bound to Hb. The reaction releases protons, which contribnte to the Bohr effect. The overall effect is the stabilization of the deoxy form of hemoglobin.

Although mueh of the CO, in the blood is transported as HCOj", some also eombines with the terminal amino groups of the 3 ehains of hemoglobin and aets as an allosteric effector (Chap. 5). Speeifieally, CO, reaets with the unionized form of a-amino groups to form carbamates that in turn form salt bridges, thus stabilizing the tense (deoxyhemoglogin) stracture. The reaetion is as follows ... 

The curve for oxyhemoglobin may be obtained from the data used in constructing that for hemoglobin quite easily. Equation S6" shows that at constant pH the amount of carbamate is proportional to the pressure of carbon dioxide. From a knowledge of p — p" at any given pH, it is therefore possible to fix a corresponding point on the new curve, since p is already known as a function of pH. On the other hand, at constant pH, p — p" is given by... 

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