A search for the function of human carbonic anhydrase B

Interpreting the behavior of enzymes: purpose or pedigree?

To interpret the growing body of data describing the structural, physical, and chemical behaviors of biological macromolecules, some understanding must be developed to relate these behaviors to the evolutionary processes that created them. Behaviors that are the products of natural selection reflect biological function and offer clues to the underlying chemical principles. Nonselected behaviors reflect historical accident and random drift. This review considers experimental data relevant to distinguishing between nonfunctional and functional behaviors in biological macromolecules. In the first segment, tools are developed for building functional and historical models to explain macromolecular behavior. These tools are then used with recent experimental data to develop a general outline of the relationship between structure, behavior, and natural selection in proteins and nucleic acids. In segments published elsewhere, specific functional and historical models for three properties of enzymes--kinetics, stereospecificity, and specificity for cofactor structures--are examined. Functional models appear most suitable for explaining the kinetic behavior of proteins. A mixture of functional and historical models appears necessary to understand the stereospecificity of enzyme reactions. Specificity for cofactor structures appears best understood in light of purely historical models based on a hypothesis of an early form of life exclusively using RNA catalysis.

The importance of carbonic anhydrase B and C for the unloading of CO2 by the human erythrocyte

Carbonic anhydrase (carbonate dehydratase, EC 4.2.1.1) isoenzymes HCA B and HCA C from human erythrocytes were purified by affinity chromatography and characterized kinetically at 37 degrees C in 25 mM N-methylimidazole buffer, I = 0.15, pH 7.1, using a pH-indicator stopped-flow technique. The rate constants for the uncatalyzed hydration of CO2 and dehydration of H2CO3 were 0.12 . s-1 and 58 . s-1, respectively, Km and turnover numbers in the hydration reaction were 14.0 mM and 19.1 . 10(5) . s-1 for HCAC and 3.3 mM and 0.56 . 10(5) . s-1 for HCA B. Km and turnover numbers in the dehydration reaction were 70 mM and 5 . 10(5) . s-1 for HCA C and 16.8 mM and 0.27 . 10(5) . s-1 for HCA B. Ki for chloride was 14 mM for HCA B and greater than 200 mM for HCA C. Ki for acetazolamide was 0.9 microM against HCA B and 16 nM against HCA C. The rates of hydration of CO2 in hemolysates with known concentrations of HCA B and HCA C, and in mixtures of purified HCA B and HCA C with concentrations near those in the erythrocyte, were similar to the rates calculated from the kinetic parameters of each isoenzyme. HCA B is 86% inhibited by chloride in vivo. This strong inhibition, together with the low specific activity, explains why HCA B only accounts for 10% of the total carbonic anhydrase activity of the erythrocyte, en spite the cellular concentration HCA B is 8 times higher than that of HCA C. HCA B and HCA C can together accelerate the intracellular CO2-reaction 23 000-fold, which gives a margin of 25-fold over physiological needs in hard work and 50-fold at rest. Perceptible physiological effects on respiration should therefore be seen when total carbonic anhydrase activity of erythrocytes is 96 to 98% inhibited. These degrees of inhibition are achieved when plasma concentrations of acetazolamide reach 2 and 5 microM, respectively.

Carbonic anhydrase isoenzyme C in the human retina. An immunohistochemical study

The occurrence of carbonic anhydrase isoenzymes C and B in the retina of the human eye was examined with specific antisera against these enzymes. Immunological analysis were carried out by the double diffusion method of Ouchterlony, and the localization of the enzymes was studied by an application of immunoperoxidase (PAP) technique. A large amount of isoenzyme C was detected in tissue sections from the human retina, whereas the isoenzyme B was totally absent. Isoenzyme C was considered to be located primarily in the glial elements of the retina, correlating with the earlier findings obtained by traditional metal salt methods.