Facilitation by Carbonic Anhydrase of Carbon Dioxide Transport

Abstract

The enzyme carbonic anhydrase greatly increases the rate of conversion of carbon dioxide to bicarbonate ions and the rate of the reverse reaction. In aqueous solution this rapid interchange of carbon dioxide between the form of dissolved gas and the form of bicarbon. ate ions may be utilized to enhance carbon dioxide transport, the limit of enhancement being the transport of bicarbonate ions. As many biological media contain more than 20 times as many bicarbonate ions as dissolved CO2 molecules, the enhanced transport can thus become the major CO2 transport system.It can also be demonstrated that carbonic anhydrase alone, in the absence of bicarbonate ions, enhances CO2 transport. While the function of this type of facilitated transport in biological systems has not been established, it may exist in cell membranes. 44 One can describe three types of transport systems in which carbonic anhydrase facilitates CO2 transport: (i) flow transport, such as blood flow in pulmonary capillaries, in which the enzyme makes dissolved bicarbonate available for rapid conversion to CO2 and effectively increases the amount of transported CO2 by the amount of bicarbonate in the blood, and increases the amount of eliminated CO2 by the bicarbonate A-V difference; (ii) diffusion transport, such as occurs in extravascular space, in which the enzyme mobilizes bicarbonate diffusion for CO2 transport; and (iii) transport of CO2 by the enzyme alone. The role of the enzyme in the first type of transport system was demonstrated by tracer experiments in the lungs of intact dogs (1). The second and third types of transport facilitation are demonstrated by data that I now present. Diffusion of CO, and Bicarbonate Ions in Aqueous SolutionsThe reversible reaction of C02 with water to form bicarbonate ions may be expressed in the following form:Carbonic anhydrase C02 + H20 < LHCO3-+ HeThe direct reaction of C02 with water to give bicarbonate ion in the presence of the enzyme has been suggested by Gibbons and Edsall (2) and others. The relation between dissolved C02 and bicarbonate concentrations at equilibrium is given by the simplified equation pH = 6.1 + log [HCO3-]/[C02] (1)It follows that in a solution of pH greater than 6.1 there is more C02 in the form of bicarbonate ions than in the form of dissolved C02 at equilibrium: for example, human blood plasma at pH 7.4 contains 20 times as many bicarbonate ions as C02 molecules. Diffusion of dissolved CO2 and bicarbonate ions may be described in terms of the following model. Consider a cylinder of unstirred solution of fixed area and thickness, subject to a difference in C02 tension between the flat end faces. Dissolved C02 diffuses The author is a research physiologist and lecturer in physiology at Scripps

Discovery of an algal mitochondrial carbonic anhydrase: molecular cloning and characterization of a low-CO2-induced polypeptide in Chlamydomonas reinhardtii.

“…A role for CA in pH regulation has been suggested in other systems (36). A mitochondrial CA could also be involved in the diffusion and transport of inorganic carbon species within the mitochondrial matrix as well as their efflux, in analogy to what has been shown in other systems (3,37).…”

Rate of disappearance of labeled carbon dioxide from the lungs of humans during breath holding: a method for studying the dynamics of pulmonary CO2 exchange

“…Fourth, the toxic effects during carbonic anhydrase inhibition were severe enough in one human to make us reluctant to administer large doses of the inhibitor to others, and, therefore, we have data on only one subject. Fifth, evidence has been recently presented in vitro that carbonic anhydrase, in addition to catalyzing the conversion of CO2 to bicarbonate, directly facilitates the transport of molecular CO2 (43,44). If this action of carbonic anhydrase is of significance in vivo, the CO2 exchange space in the lungs blocked by carbonic anhydrase inhibition might be greater than the bicarbonate space.…”

Internal carbonic anhydrase activity in the tissue of scleractinian corals is sufficient to support proposed roles in photosynthesis and calcification

“…The iCA activity per unit surface area required to maintain constant CO 2 deviations from equilibrium remained essentially constant, though there was a slight increase in iCA activity per unit surface area as tissue thickness increased, a result of its role in facilitated diffusion of C i through the tissue layers ( Fig. 7B; Enns, 1967). In contrast, the iCA activity per unit volume must increase dramatically as the metabolic rates are squeezed into a thinner tissue layer and can decrease as the tissue expands.…”