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Human Erythrocyte Carbonic Anhydrase B.

Carbonicanhydrase is a group of enzymes that catalyze the hydration and hydrolysis of many carbonyl and related systems, such as hydration of carbon dioxide and aldehydes, and hydrolysis of esters [1].  The most common and effective-catalyzed reaction is the inter-conversion of carbon dioxide and water into carbonic acid, protons and bicarbonate ions [1]:

CO2 + H2O <-----> HCO3- + H+

Human erythrocyte Carbonic anhydrase (CA) B is a type of α-CA that is present in the blood [2]. This paper discusses the structure and symmetry of this enzyme and its biological importance.

Structure of human erythrocyte Carbonic anhydrase B

Like all other forms of carbonic anhydrase, CA B. consists of protein and one zinc ion at the bottom of the active site near the center of the molecule [1]. It is approximately ellipsoidal, with a backbone dimension of 41 X 41 X 47 A. The secondary structure of this enzyme is predominantly parallel and anti-parallel beta-pleated sheets that comprises about 40% of all of its residues, as revealed through X-ray diffraction by Kannan and his colleagues in 1974 [2]. Its main chain consists of a large β-structure formed by 10 polypeptidesegments.  On the surface of the molecule, there are six pieces of right-handed helix located on either side of the large β-structure. There are also some smaller β-structures near the surface. In addition, three different clusters of aromatic residues of about same distance from the zinc ion have been observed in the molecule.

 

Symmetry of human erythrocyte Carbonic anhydrase B

1). Geometry of the metal: The metal is liganded to the protein by three histidyl ligands, and the fourth ligand site is open to surrounding medium and possibly filled by a water molecule or OH ion. Therefore, the zinc ion has a distorted tetrahedral coordination [1][2].

2). The point group of each metal and its immediate ligand environment is C3v. as it has a C3 principle axis and 3 σv plane containing the C3 axis, but no 3 C2 axes nor a σh plane that is perpendicular to the C3 axis.

3). The overall point group of the enzyme C1 because it has no symmetry, so no IR or Raman data is expected.  

Biological importance of human erythrocyte Carbonic anhydrase B

Carbonic anhydrases are abundant in mammalian tissues, plants and bacteria. The several distinct classes of this enzyme (α, β, γ etc.) share little similarity in terms of structure and sequence, but they all perform the same function of catalyzing the hydration of carbon dioxide and the dehydration of bicarbonate [3]. Carbonic anhydrase B is one of the two major forms of carbonic anhydrase in human red blood cell (the other form is CA C). Like other members of the CA family, its main function is assisting the important chemical reaction that occurs in the blood

CO2 + H2O  <----->  HCO3- + H+

The above reaction is very slow in the absence of the enzyme. Hence, by assisting the inter-conversion between carbon dioxide and bicarbonate, Carbonic anhydrase B helps to maintain acid-base balance in blood, and transport carbon dioxide out of blood. Note that the carbonic acid (H2CO3) and hydrogen carbonate ions (HCO3-) is an important buffer system in the body. They equilibrate in the blood plasma to buffer PH. The inter-conversion between the two is shown in the second step of the reaction below [4]

CO2 + H2O <------> H+(aq) + HCO3-(aq) <-----> H2CO3(aq)

Another thing to note is that carbonic anhydrase B has very high catalytic power. For its catalytic activity of the hydration of dioxide, it increases the rate of the reaction by a factor of 109 (with a turnover number for carbon dioxide is roughly 106 s-1 under saturated conditions). However, the apoenzyme was almost catalytically inactive, which means that the enzyme cannot function without the zinc ion. Experiments have shown that the activity of carbonic anhydrase B can be restored when zinc ions are added in 1:1 molar ratio. In fact, it is zinc-aquo complex (rather than zinc ion alone) that needs to be present in the active site for the catalytic functionality of the enzyme [2]. 

References

  1. Prince, R. H. & Woolley, P. R. (1972). Metal Ion Function in Carbonic Anhydrase.Angew. Chem. internal. Edit. J Vol. I 1 (1972) No. 5. Retrieved from http://www3.interscience.wiley.com.l...79182/PDFSTART
  2. Kannan, K. K., Notstrand, B., Fridborg, K., Lovrgen, S., Ohlsson, A., & Petef, M. (1974). Crystal Structure of Human Erythrocyte Carbonic Anhydrase B. Three-Dimensional Structure at a Nominal 2.2-A Resolution. Retrieved from http://www.pnas.org/content/72/1/51.full.pdf+html
  3. Carbonic Anhydrase. (n.d.). Retrieved May 22, 2010 from
  4. http://www.rcsb.org/pdb/static.do?p=...h/pdb49_1.html
  5. Biological Buffers. (n.d.). Retrieved May 22, 2010 from http://www.scifun.org/chemweek/BioBu...ioBuffers.html