This research proposal involves the determination of the mechanism of an enzyme, tetrachlorohydroquinone (TCHQ) dehalogenase. The enzyme plays a critical role in the biodegradation of pentachlorophenol, by performing two successive reductive dehalogenation reaction replacing two chlorines with hydrogens. The reaction includes a glutathione conjugation step which is an essential step in the mechanism of glutathione S-transferases (GSTs). GSTs are very important enzymes present in many organisms for detoxification processes. TCHQ dehalogenase has some sequence identity and mechanistic similarities to GSTs. The specific aim of this work is to determine whether TCHQ dehalogenase accomplishes the removal of a chlorine from TCHQ via an initial tautomerization to follow for an S/N/2 reaction, or via a nucleophilic aromatic substitution reaction such as those commonly found in aromatic conjugation reactions of GSTs.The mechanism of TCHQ dehalogenase will be determined by experimental and theoretical techniques. The use of both experimental and theoretical techniques is important to the overall understanding of the mechanism of TCHQ dehalogenase and how similar its mechanism is to that of GSTs.