O6-Methylguanine Repair by O6-Alkylguanine-DNA Alkyltransferase

Abstract

O6-alkylguanine-DNA alkyltransferase (AGT) repairs O6-methylguanine (O6mG) in DNA that is known to cause mutation and cancer. On the basis of calculations performed using density functional theory involving the active site of AGT, a mechanism for catalytic demethylation of O6mG to guanine has been proposed. In this mechanism, roles of six amino acids, i.e., Cys145, His146, Glu172, Tyr114, Lys165, and Ser159 in catalytic demethylation of O6mG are involved. This mechanism has three steps as follows. At the first step, Cys145 in the Cys145-water-His146-Glu172 tetrad is converted to cysteine thiolate anion while at the second step, abstraction of the Tyr114 proton by the N3 site of O6mG occurs in a barrierless manner. In the third step, abstraction of Lys165 proton by deprotonated Tyr114 and transfer of the methyl group of O6mG to the thiolate group of Cys145 anion occur simultaneously. As AGT is a major target in cancer therapy, identification of the roles of the different amino acids in demethylation of O6mG is expected to be useful in designing efficient AGT inhibitors.

Mutagenicity associated with O6-methylguanine-DNA damage and mechanism of nucleotide flipping by AGT during repair

“…It is thus interesting to note that before and after nucleotide flipping, Tyr114 is situated at the same position (figure S1(b), available at stacks.iop.org/PhysBio/8/046007/mmedia). This raises a question whether Tyr114 has any role in nucleotide flipping, although, its role in catalytic O6MG repair has been recently proposed by a theoretical study [69]. Owing to above facts, we have studied nucleotide flipping by constructing a model system ( figure 2) where O6MG is intrahelical and base paired with C and AGT is interacting with the O6MG:C base pair in a configuration similar to the superposed structure as shown in figure S1(b) (available at stacks.iop.org/PhysBio/8/046007/mmedia) (AGT shown in red).…”

Sugar Radical Formation by a Proton Coupled Hole Transfer in 2′-Deoxyguanosine Radical Cation (2′-dG•+): A Theoretical Treatment

“…In addition to experiments, theoretical studies were also done to show the involvement of PCET reactions, e.g. , in DNA-acrylamide complexes,19,20 and the mechanism of repair of guanyl radical by aromatic amino acids 21…”

Direct Reversal Repair in Mammalian Cells

“…The proposed reaction mechanism between the alkyl group and the active site of MGMT ( Figure 6) requires that His146 acts as a water-mediated general base to deprotonate Cys145, which mediates the attack at the O6-alkyl carbon of guanine and, results in generation of a cysteine thiolate anion and an imidazolium ion stabilized by Glu172. Residue Tyr114 donates a proton to N3 of O6-meG and abstracts a proton from Lys165, facilitating simultaneous transfer of the methyl group on O6-meG to the thiolate anion of the Cys145 residue [33,35].…”