From Proteopediaproteopedia link
HUMAN CARBONIC ANHYDRASE II [HCAII] (E.C.220.127.116.11) MUTANT WITH ALA 65 REPLACED BY THR (A65T)
[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 three-dimensional structures of A65F, A65L, A65H, A65T, A65S, and A65G human carbonic anhydrase II (CAII) variants have been solved by X-ray crystallographic methods to probe the importance of residue 65 and the structural implications of its evolutionary drift in the greater family of carbonic anhydrase isozymes. Structure-activity relationships in this series of CAII variants are correlated with those established for other carbonic anhydrase isozymes. We conclude that a bulky side chain at position 65 hinders the formation of an effective solvent bridge between zinc-bound water and H64 and thereby hinders solvent-mediated proton transfer between these two groups [Jackman, J. E., Merz, K. M., Jr., & Fierke, C. A. (1996) Biochemistry 35, 16421-16428]. Despite the introduction of a polar hydroxyl group at this position, smaller side chains such as serine or threonine substituted for A65 do not perturb the formation of a solvent bridge between H64 and zinc-bound solvent. Thus, the evolution of residue 65 size is one factor affecting the trajectory of catalytic proton transfer.
X-ray crystallographic studies of alanine-65 variants of carbonic anhydrase II reveal the structural basis of compromised proton transfer in catalysis.,Scolnick LR, Christianson DW Biochemistry. 1996 Dec 24;35(51):16429-34. PMID:8987974
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.