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Q.
The enzyme essential for the transport of $CO_2$ as bicarbonate in blood is
MP PMTMP PMT 2012Breathing and Exchange of Gases
Solution:
The large fraction of carbon dioxide (about 70%) is converted to bicarbonate ions ($HCO_3^-$) and transported in plasma. When carbon dioxide diffuses into the RBCs,
it combines with water, forming carbonic acid ($H_2CO_3$). $H_2CO_3$ is unstable and quickly dissociates into hydrogen ions and bicarbonate ions.
$\underset{\overset{\text{Carbon}}{\text{dioxide}}}{CO_2} + \underset{\text{Water}}{H_2O} \xrightleftharpoons{\text{Carbonic anhydrase }} \underset{\overset{\text{Carbonic}}{\text{acid}}}{H_2CO_3}$
$\rightleftharpoons \underset{\overset{\text{Hydrogen}}{\text{ion}}}{H^+} + \underset{\overset{\text{Bicarbonate}}{\text{ion}}}{HCO_3^-}$
This reaction is thousands times faster in erythrocytes because they contain carbonic anhydrase, an enzyme that reversibly catalyses the conversion of carbon dioxide and water to carbonic acid.