X-ray absorption studies of halide binding to carbonic anhydrase.

Abstract
X-ray absorption measurements of bovine carbonic anhydrase B (EC 4.2.1.1) were made at the Stanford Synchrotron Radiation Project with a spectrometer operating in the fluorescence mode. Differences in absorption at and beyond the zinc K-edge near 9664 eV were observed upon the addition of Br- or I-. The additional absorption in k space, out to k .apprxeq. 7 .ANG.-1, obtained upon the addition of I- was compared with the absorption in this region of a ZnI2 sample. The similarities between these absorptions indicates that the Zn-I- distance in the protein is 2.65 .+-. 0.06 .ANG.; it is known to be 2.62 .ANG. in ZnI2. This shows that the I- binds directly to the Zn in the protein.