From Proteopediaproteopedia link
Crystal structure of the W5F 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 tryptophan residue Trp5, highly conserved in the alpha class of carbonic anhydrases including human carbonic anhydrase II (HCA II), is positioned at the entrance of the active site cavity and forms a pi-stacking interaction with the imidazole ring of the proton shuttle His64 in its outward orientation. We have observed that replacement of Trp5 in HCA II caused significant structural changes, as determined by X-ray diffraction, in the conformation of 11 residues at the N-terminus and in the orientation of the proton shuttle residue His64. Most significantly, two variants W5H and W5E HCA II had His64 predominantly outward in orientation, while W5F and wild type showed the superposition of both outward and inward orientations in crystal structures. Although Trp5 influences the orientation of the proton shuttle His64, this orientation had no significant effect on the rate constant for proton transfer near 1mus(-1), determined by exchange of (18)O between CO(2) and water measured by mass spectrometry. The apparent values of the pK(a) of the zinc-bound water and the proton shuttle residue suggest that different active-site conformations influence the two stages of catalysis, the proton transfer stage and the interconversion of CO(2) and bicarbonate.
Structure and catalysis by carbonic anhydrase II: role of active-site tryptophan 5.,Mikulski R, Domsic JF, Ling G, Tu C, Robbins AH, Silverman DN, McKenna R Arch Biochem Biophys. 2011 Dec 15;516(2):97-102. doi: 10.1016/j.abb.2011.09.011. , Epub 2011 Oct 5. PMID:22001224
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.