Free radical reactions with proteins and enzymes

A comparison of the inactivation of bovine carbonic anhydrase B (carbonate hydro-lyase, EC 4.2.1.1) by ^.OH, (SCN)₂^$\text{•}$ and Br₂^$\text{•}$ shows that the enzyme contains one or more essential tryptophan residues. Direct oxidation of histidine and tyrosine residues by the radicals is less important in causing inactivation of the enzyme. The effectiveness of all these radicals in inactivating carbonic anhydrase decreases with increasing pH in the region where the activity-linked ionizable group dissociates. Differences between the rates of reaction of Br₂^$\text{•}$ and (SCN)₂^$\text{•}$ with the holo- and apo-enzyme and between the resulting transient product spectra indicate that access to the reactive tyrosine and tryptophan residues is diminished by the presence of Zn²⁺ in the active site region.