3UCM: Coccomyxa Beta-carbonic Anhydrase in Complex With Thiocyanate

The beta-class carbonic anhydrases (beta-CAs) are widely distributed among lower eukaryotes, prokaryotes, archaea, and plants. Like all CAs, the beta-enzymes catalyze an important physiological reaction, namely the interconversion between carbon dioxide and bicarbonate. In plants the enzyme plays an important role in carbon fixation and metabolism. To further explore the structure-function relationship of beta-CA, we have determined the crystal structures of the photoautotroph unicellular green alga Coccomyxa beta-CA in complex with five different inhibitors: acetazolamide, thiocyanate, azide, iodide, and phosphate ions. The tetrameric Coccomyxa beta-CA structure is similar to other beta-CAs but it has a 15 amino acid extension in the C-terminal end, which stabilizes the tetramer by strengthening the interface. Four of the five inhibitors bind in a manner similar to what is found in complexes with alpha-type CAs. Iodide ions, however, make contact to the zinc ion via a zinc-bound water molecule or hydroxide ion--a type of binding mode not previously observed in any CA. Binding of inhibitors to Coccomyxa beta-CA is mediated by side-chain movements of the conserved residue Tyr-88, extending the width of the active site cavity with 1.5-1.8 A. Structural analysis and comparisons with other alpha- and beta-class members suggest a catalytic mechanism in which the movements of Tyr-88 are important for the CO(2)-HCO(3)(-) interconversion, whereas a structurally conserved water molecule that bridges residues Tyr-88 and Gln-38, seems important for proton transfer, linking water molecules from the zinc-bound water to His-92 and buffer molecules.
PDB ID: 3UCMDownload
MMDB ID: 95296
PDB Deposition Date: 2011/10/27
Updated in MMDB: 2011/11 
Experimental Method:
x-ray diffraction
Resolution: 2.51  Å
Source Organism:
Similar Structures:
Biological Unit for 3UCM: tetrameric; determined by author and by software (PISA)
Molecular Components in 3UCM
Label Count Molecule
Proteins (4 molecules)
Carbonic Anhydrase
Molecule annotation
Chemicals (10 molecules)
* Click molecule labels to explore molecular sequence information.

Citing MMDB