InterPro: IPR018338 Carbonic anhydrase, alpha-class, conserved site

Carbonic anhydrases (CA: EC:4.2.1.1) are zinc metalloenzymes which catalyse the reversible hydration of carbon dioxide to bicarbonate [1, 2]. 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 [3]. 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.

• 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).

• The beta-CAs are highly abundant in plants, diatoms, eubacteria and archaea [4, 5]. The beta-CAs are far more diverse in sequence than other classes, and can be divided into different clades based on sequence identity, with the plant enzymes forming two clades representing dicotyledonous and monocotyledonous plants. Characterisation of these enzymes reveals sharp differences between the beta class, which forms dimers, tetramers, hexamers and octomers, and the alpha and gamma classes, which form strictly monomers and trimers.

• The gamma-CAs may be the most ancient form of carbonic anhydrases, having evolved long before the alpha class, to which it is more closely related than to the beta-class [6, 7]. The reaction mechanism of the gamma-class is similar to that of the alpha-class, even though the overall folds are dissimilar and the active site residues differ.

• The delta-CAs are found in marine algae and dinoflagellates [8].

• The epsilon-CAs are found in prokaryotes such as Thiobacillus neapolitanus (Halothiobacillus neapolitanus) in which it is a component of the carboxysome shell, where it could supply the active sites of RuBisCO in the carboxysome with the high concentrations of carbon dioxide necessary for optimal RuBisCO activity and efficient carbon fixation [9].

This entry represents a conserved site based around one of the zinc-binding histidines in the alpha class of carbonic anhydrases.