Zinc metalloenzyme carbonic anhydrase catalyzes the hydration of carbon dioxide as part of mammalian respiration. Carbonic anhydrase interacts with several porphyrins and metalloporphyrins resulting in specific changes in the porphyrin absorbance spectra. Two copper metalloporphyrins, copper-monosulfonate tetraphenyl porphyrin and copper-monocarboxy tetraphenyl porphyrin, reversibly and competitively inhibit carbonic anhydrase binding at the active site of the enzyme. Non-metallated porphyrins do not inhibit enzymatic activity. The absorbance spectrum of immobilized porphyrin-enzyme complex is measured by planar waveguide evanescent wave absorbance spectroscopy with a blue LED as a light source and an Ocean Optics USB2000 spectrophotometer. The characteristics of the absorbance spectra of the porphyrins are specific and different when bound to the active site of the enzyme or bound non-specifically to the surface of the slide. The reversible competitive inhibition of immobilized carbonic anhydrase can be used to indicate the presence of the competitive inhibitors saccharin, 1,10-phenanthroline, and cysteine in solution as well as to quantify carbon dioxide in gas samples based on the porphyrin absorbance spectrum. When the porphyrin-enzyme complex is exposed to a competitive inhibitor or to carbon dioxide, the porphyrin is displaced from the active site and its the absorbance spectrum changes. Carbon dioxide levels between 1,000 (0.1%) and 10,000 parts per million (1.0%) can be measured.
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