The objective of the proposed studies is to compare the conformations of human erythrocyte carbonic anhydrase B in its crystalline and dissolved states. The problem arises because the only method for determining the detailed three dimensional structure of the enzyme involves x-ray diffraction from crystalline enzyme, but all the studies on its function have been carried out on dissolved enzyme. X- ray crystallography provides invaluable information, because the detailed mechanism of action of a protein cannot be developed without reference to its structure. The results of the proposed experiments will lend credence or caution to explanations of the properties of dissolved enzyme on the basis of the structure of crystalline enzyme. The experimental approach is to react crystalline and dissolved enzyme with iodoacetate. The only histidines which will react are those which are accessible to the solvent. Since conformational changes can easily change the reactivity of a histidyl residue, the method provides a basis for detecting such changes. The sites of reaction will be determined by preparation, purification, and characterization of peptides containing the modified histidines. The pattern of reaction of iodoacetate-C14 with crystalline enzyme and iodoacetate-H3 with dissolved enzymes will be determined. Iodoacetate is an affinity labeling reagent for dissolved carbonic anhydrase B and reacts with histidine 200 in the region of the active site. Therefore, the integrity of the active site of crystalline enzyme can be checked by determining the rate and specificity of the reaction for histidine 200.