From Proteopediaproteopedia link
Crystal structure of the Y7I mutant of human carbonic anhydrase II
[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 rate limiting step in catalysis of bicarbonate dehydration by human carbonic anhydrase II (HCA II) is an intramolecular proton transfer from His64 to the zinc-bound hydroxide. We have examined the role of Tyr7 using site-specific mutagenesis and measuring catalysis by the (1)O exchange method using membrane inlet mass spectrometry. The side chain of Tyr7 in HCA II extends into the active-site cavity about 7 A from the catalytic zinc atom. Replacement of Tyr7 with eight other amino acids had no effect on the interconversion of bicarbonate and CO, but in some cases caused enhancements in the rate constant of proton transfer by nearly 10-fold. The variant Y7I HCA II enhanced intramolecular proton transfer approximately twofold; its structure was determined by X-ray crystallography at 1.5 A resolution. No changes were observed in the ordered solvent structure in the active-site cavity or in the conformation of the side chain of the proton shuttle His64. However, the first 11 residues of the amino-terminal chain in Y7I HCA II assumed an alternate conformation compared with the wild type. Differential scanning calorimetry showed variants at position 7 had a melting temperature approximately 8 degrees C lower than that of the wild type.
Kinetic and crystallographic studies of the role of tyrosine 7 in the active site of human carbonic anhydrase II.,Mikulski R, Avvaru BS, Tu C, Case N, McKenna R, Silverman DN Arch Biochem Biophys. 2011 Feb 15;506(2):181-7. Epub 2010 Dec 9. PMID:21145876
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.