An overview of the alpha-, beta- and gamma-carbonic anhydrases from Bacteria: can bacterial carbonic anhydrases shed new light on evolution of bacteria?

Carbonic anhydrases (CAs, EC 4.2.1.1) are metalloenzymes which catalyze a simple but physiologically crucial reaction in all life Domains, the carbon dioxide hydration to bicarbonate and protons: CO2+H2O double left right arrow HCO3-+H+. These enzymes are involved in many physiologic processes, such as photosynthesis, respiration, CO2 transport, as well as metabolism of xenobiotics. Five different, genetically distinct CA families are known to date: the alpha-, beta-, gamma-, delta- and zeta-CAs. alpha-, beta- and delta-CAs use Zn(II) ions at the active site, the gamma-CAs are probably Fe(II) enzymes (but they are active also with bound Zn(II) or Co(II) ions), whereas the zeta-class uses Cd(II) or Zn(II) to perform the physiologic reaction catalysis. Bacteria encode for enzymes belonging to the alpha-, beta-, and gamma-CA classes. They contain zinc ion (Zn2+) in their active site, coordinated by three histidine residues and a water molecule/ hydroxide ion (in the alpha and gamma) or by two cysteine and one histidine residues (in the beta class), with the fourth ligand being a water molecule/ hydroxide ion. Here we propose that bacterial CAs can be used as markers for understanding the evolution and genetic variability of the Gram-positive and Gram-negative bacteria. We addressed several questions such as: (1) why are alpha-CAs present only in the genome of Gram-negative bacteria; (2) why are alpha-CAs not present in all Gram-negative bacteria; (3) why do Bacteria show an intricate pattern of CA gene expression; (4) what are the physiologic roles of such diverse CAs in these prokaryotes. We proposed possible answers to the previous questions. Moreover, we speculated on the evolution of the CA classes (alpha, beta and gamma) identified in the Gram-negative and -positive bacteria. Our main hypothesis is that from the ancestral Ur-CA, the gamma-class arose first, followed by the beta-class; the alpha-class CAs came last it is found only in the Gram-negative bacteria.