From Proteopediaproteopedia link
CONFORMATIONAL MOBILITY OF HIS-64 IN THE THR-200 (RIGHT ARROW) SER 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 three-dimensional structure of the Thr-200----Ser (T200S) mutant of human carbonic anhydrase II (CAII) has been determined by X-ray crystallographic methods at 2.1-A resolution. This particular mutant of CAII exhibits CO2 hydrase activity that is comparable to that of the wild-type enzyme with a 2-fold stabilization of the E.HCO3- complex and esterase activity that is 4-fold greater than that of the wild-type enzyme. The structure of the mutant enzyme reveals no significant local changes accompanying the conservative T200S substitution, but an important nonlocal structural change is evident: the side chain of catalytic residue His-64 rotates away from the active site by 105 degrees about chi 1 and apparently displaces a water molecule. The displaced water molecule is present in the wild-type enzyme; however, the electron density into which this water is built is interpretable as an alternate conformation of His-64 with 10-20% occupancy. The rate constants for proton transfer from the zinc-water ligand to His-64 and from His-64 to bulk solvent are maintained in the T200S variant; therefore, if His-64 is conformationally mobile about chi 1 and/or chi 2 during catalysis, compensatory changes in solvent configuration must sustain efficient proton transfer.
Conformational mobility of His-64 in the Thr-200----Ser mutant of human carbonic anhydrase II.,Krebs JF, Fierke CA, Alexander RS, Christianson DW Biochemistry. 1991 Sep 24;30(38):9153-60. PMID:1909891
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.