Subunit constitution of carbonic anhydrase from Chlamydomonas reinhardtii

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Abstract
Carbonic anhydrase purified from the cell surface of Chlamydomonas reinhardtii was inactivated by treatment with dithiothreitol. This treatment caused dissociation of the holoenzyme into 35-kDa (A) and 4-kDa (B) subunits as revealed by SDS/PAGE. The 35-kDa subunit was further separated into two components A1 (35 kDa) and A2 (36.5 kDa) by SDS/PAGE using a gradient gel. These two components have the same amino acid sequence up to at least the 10th amino acid from the N-terminus. The molecular masses were estimated at 76 kDa and 35 kDa for the holoenzyme and the large subunit, respectively, and the molar ratio of the former to the latter at 1:2, by using the techniques of low-angle laser light-scattering photometry and precision differential refractometry combined with gel-filtration HPLC. The molar ratio of the 35-kDa/4-kDa subunits was estimated at 1:1 the gel-filtration HPLC monitored with precision differential refractomety. Atomic-absorption spectrophotometry revealed that the holoenzyme contains two atoms of zinc. These results suggest that the holoenzyme is a heterotetramer composed of two large subunits (A1 and A2) and two small subunits (B).