From Proteopediaproteopedia link
CRYSTALLOGRAPHIC ANALYSIS OF THR-200-> HIS HUMAN CARBONIC ANHYDRASE II AND ITS COMPLEX WITH THE SUBSTRATE, HCO3-
[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
A complex of carbonic anhydrase (CA) with one of its substrates, bicarbonate, has been studied crystallographically. Human isoenzyme II was mutated at position 200 from threonine to histidine, which results in higher affinity for bicarbonate. The HCO3- ion binds in the active site to the zinc ion as a pseudo-bidentate ligand which gives the metal a coordination geometry between tetrahedral and trigonal bipyramide. The water/hydroxide normally bound with tetrahedral coordination to the zinc is probably replaced by the OH group of the bicarbonate ion. The importance of residues Thr-199 and Glu-106 in controlling the binding orientation of HCO3- is discussed as well as the catalytic mechanism. Both the complex as well as the uncomplexed mutant were studied at 1.9 A resolution.
Crystallographic analysis of Thr-200-->His human carbonic anhydrase II and its complex with the substrate, HCO3-.,Xue Y, Vidgren J, Svensson LA, Liljas A, Jonsson BH, Lindskog S Proteins. 1993 Jan;15(1):80-7. PMID:8451242
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.