From Proteopediaproteopedia link
Crystal structure of human carbonic anhydrase II complexed with an acetate ion
[CAH2_HUMAN] Defects in CA2 are the cause of osteopetrosis autosomal recessive type 3 (OPTB3) [MIM:259730]; also known as osteopetrosis with renal tubular acidosis, carbonic anhydrase II deficiency syndrome, Guibaud-Vainsel syndrome or marble brain disease. Osteopetrosis is a rare genetic disease characterized by abnormally dense bone, due to defective resorption of immature bone. The disorder occurs in two forms: a severe autosomal recessive form occurring in utero, infancy, or childhood, and a benign autosomal dominant form occurring in adolescence or adulthood. Autosomal recessive osteopetrosis is usually associated with normal or elevated amount of non-functional osteoclasts. OPTB3 is associated with renal tubular acidosis, cerebral calcification (marble brain disease) and in some cases with mental retardation.    
[CAH2_HUMAN] Essential for bone resorption and osteoclast differentiation (By similarity). Reversible hydration of carbon dioxide. Can hydrate cyanamide to urea. Involved in the regulation of fluid secretion into the anterior chamber of the eye. 
Publication Abstract from PubMed
The enzyme human carbonic anhydrase II (hCAII) crystallized in an acetate-bound complex belonging to space group P2(1)2(1)2(1), with unit-cell parameters a = 42.3, b = 71.8, c = 74.0 A. The structure was solved by the molecular-replacement method and refined to an R value of 0.18 and an R(free) of 0.21. The acetate molecule replaced the zinc-bound water molecule in the structure, differing from previous reports regarding the site of acetate binding. This mode of binding disrupts the hydrogen-bonded solvent network required for activity of the enzyme. This mode of inhibitor binding is a novel one that has not been observed previously.
A novel acetate-bound complex of human carbonic anhydrase II.,Mazumdar PA, Kumaran D, Swaminathan S, Das AK Acta Crystallogr Sect F Struct Biol Cryst Commun. 2008 Mar 1;64(Pt, 3):163-6. Epub 2008 Feb 23. PMID:18323598
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.