CA13 also known as carbonic anhydrase 13 belongs to the alpha-carbonic anhydrase family. The carbonic anhydrase from a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons, a reversible reaction that occurs relatively slowly in the absence of catalyst. The active site of most carbonic anhydrases contains a zinc ion; they are claasified as metalloenzymes. There are at least five distinct CA families (alpha, beta, gamma, delta, and epsilon). These families have no significant amino acid sequence similarity and in most cases are thought to be an example of convergent evolution. The alpha-CAs are found in humans. Recombinant human CA13, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography techniques.