Effect of dithiothreitol on the catalytic activity, quaternary structure and sulfonamide-binding properties of an extracellular carbonic anhydrase from Chlamydomonas reinhardtii

Carbonic anhydrase inhibition. Insight into the characteristics of zonisamide, topiramate, and the sulfamide cognate of topiramate

Some useful therapeutic agents inhibit certain carbonic anhydrase (CA) isozymes to varying degrees. We have conducted enzyme kinetics studies in a 4-nitrophenyl acetate (4-NPA) hydrolysis assay with the marketed antiepileptic drugs topiramate (1) and zonisamide (2) to determine if their full inhibition of human CA-II and CA-I requires extended preincubation conditions. We found that neither 1 nor 2 requires appreciable preincubation with either enzyme to manifest full inhibitory activity. We also examined the sulfamide cognate of topiramate (3) to characterize its CA inhibitory activity, and confirmed that it is a very weak inhibitor, unlike 1 or 2. In a CO(2) hydration assay, 3 behaved as a very weak, partial inhibitor of CA-II and CA-I. We conclude that topiramate (1), zonisamide (2), and sulfamide 3 do not require extended exposure to human CA-I or CA-II to manifest full inhibitory activity (4-NPA assay).

Structural analysis of periplasmic carbonic anhydrase 1 of Chlamydomonas reinhardtii

Periplasmic carbonic anhydrase 1 of the unicellular green alga Chlamydomonas reinhardtii is a heterotetrameric glycoprotein consisting of two large subunits and two small subunits [Kamo, K., Shimogawara, K., Fukuzawa, H., Muto, S. & Miyachi, S. (1990) Eur. J. Biochem. 192, 557-562]. The cDNA sequence showed that the two subunits are cotranslated as a 377-amino-acid precursor polypeptide (41.6 kDa) consisting of an N-terminal 20-amino-acid signal peptide, a large subunit (35.6 kDa) and a small subunit (4.1 kDa) [Fukuzawa, H., Fujiwara, S., Yamamoto, Y., DionisioSese, M. L. & Miyachi, S. (1990) Proc. Natl Acad. Sci. USA 87, 4383-4387]. In the present study, amino-acid-sequence analysis of the carbonic anhydrase 1 was carried out to determine the sites of disulfide bonds and N-glycosylation and the C-terminal amino acid of the large subunit. Disulfide bonds were detected between Cys21 and Cys21, Cys61 and Cys264, Cys194 and Cys198, and Cys296 and Cys351 in the amino acid sequence deduced from the cDNA. Cys21 and Cys21 link two large subunits and Cys296 and Cys351 link a large subunit to a small subunit. Thus, the holoenzyme is constructed of two disulfide-bound large subunits, each of which joins to a small subunit by a disulfide bond. The C-terminal amino acid residue of the large subunit was determined as Ala305. This indicates that the peptide consisting of 35 amino acid residues between the large and the small subunit is deleted from the precursor during maturation, since the N-terminus of the small subunit is Ala341. Three potential N-glycosylation sites in the large subunit were all glycosylated.