Carb_anhydrase

Eukaryotic-type carbonic anhydrase		 Carb_anhydrase
SMART accession number:SM01057
Description: Carbonic anhydrases are zinc metalloenzymes which catalyse the reversible hydration of carbon dioxide to bicarbonate (PUBMED:18336305), (PUBMED:10978542). CAs have essential roles in facilitating the transport of carbon dioxide and protons in the intracellular space, across biological membranes and in the layers of the extracellular space; they are also involved in many other processes, from respiration and photosynthesis in eukaryotes to cyanate degradation in prokaryotes. There are five known evolutionarily distinct CA families (alpha, beta, gamma, delta and epsilon) that have no significant sequence identity and have structurally distinct overall folds. Some CAs are membrane-bound, while others act in the cytosol; there are several related proteins that lack enzymatic activity. The active site of alpha-CAs is well described, consisting of a zinc ion coordinated through 3 histidine residues and a water molecule/hydroxide ion that acts as a potent nucleophile. The enzyme employs a two-step mechanism: in the first step, there is a nucleophilic attack of a zinc-bound hydroxide ion on carbon dioxide; in the second step, the active site is regenerated by the ionisation of the zinc-bound water molecule and the removal of a proton from the active site (PUBMED:9336012). Beta- and gamma-CAs also employ a zinc hydroxide mechanism, although at least some beta-class enzymes do not have water directly coordinated to the metal ion.
Interpro abstract (IPR001148):

Carbonic anhydrases (CA: EC 4.2.1.1) are zinc metalloenzymes which catalyse the reversible hydration of carbon dioxide to bicarbonate [(PUBMED:18336305), (PUBMED:10978542)]. The alpha-CAs are found predominantly in animals but also in bacteria and green algae. There are at least 15 isoforms found in mammals, which can be subdivided into cytosolic CAs (CA-I, CA-II, CA-III, CA-VII and CA XIII), mitochondrial CAs (CA-VA and CA-VB), secreted CAs (CA-VI), membrane-associated (CA-IV, CA-IX, CA-XII and CA-XIV) and those without CA activity, the CA-related proteins (CA-RP VIII, X and XI).

This entry represents a domain characteristic of alpha class carbonic anhydrases. The dominating secondary structure is a 10-stranded, twisted beta-sheet, which divides the molecules into two halves [(PUBMED:9336012)]. Alpha-CAs contain a single zinc atom bound to three conserved histidine residues. The catalytically active group is the zinc-bound water which ionizes to a hydroxide group. In the mechanism of catalysis, nucleophilic attack of CO2 by a zinc-bound hydroxide ion is followed by displacement of the resulting zinc-bound bicarbonate ion by water; subsequent deprotonation regenerates the nucleophilic zinc-bound hydroxide ion [(PUBMED:8673298), (PUBMED:11493685)].

Family alignment:
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There are 1578 Carb_anhydrase domains in 1570 proteins in SMART's nrdb database.

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