Ada (protein)
{{Short description|Microbial protein found in Escherichia coli str. K-12 substr. MG1655}}
{{Infobox nonhuman protein
|Organism=Escherichia coli
|UniProt=P06134
|Name=Bifunctional transcriptional activator/DNA repair enzyme Ada
|Symbol=Ada
}}
Ada, also called as O6 alkyl guanine transferase I (O6 AGT I), is an enzyme induced by treatment of E. coli cells with alkylating agents that mainly cause methylation damage. This phenomenon is called the adaptive response hence the name. Ada transfers the alkyl group from DNA bases and sugar-phosphate backbone to a cysteine residue, inactivating itself. Consequently, it reacts stoichiometrically with its substrate rather than catalytically and is referred to as a suicide enzyme. Methylation of Ada protein converts it into a self transcriptional activator, inducing its own gene expression and the expression of other genes which together with Ada help the cells repair alkylation damage.{{cite journal |vauthors=Lindahl T, Sedgwick B, Sekiguchi M, Nakabeppu Y |year=1988 |title=Regulation and expression of adaptive response to alkylating agents |journal=Annu Rev Biochem |volume=57 |issue=1 |pages=133–57 |pmid= 3052269 |doi=10.1146/annurev.bi.57.070188.001025 |doi-access=free }} Ada removes the alkyl group attached to DNA bases like guanine (O6-alkyl guanine) or thymine (O4-alkyl thymine) and to the oxygen of the phosphodiester backbone of the DNA.{{cite book |title=DNA Repair and Mutagenesis |edition=2 |last=Friedberg |first=Errol |author2=Graham C. Walker |author3=Wolfram Siede |author4-link=Richard D. Wood |author4=Richard D. Wood |author5=Roger A. Schultz |author6=Tom Ellenberger |year=2006 |publisher=ASM Press |location=Washington, DC |isbn=1-55581-319-4 |oclc=59360087 }}{{cite journal |doi=10.1093/nar/13.8.2683 |vauthors=McCarthy TV, Lindahl T |year=1985 |title=Methyl phosphotriesters in alkylated DNA are repaired by the Ada regulatory protein of E. coli |journal=Nucleic Acids Res |volume=13 |issue=8 |pages=2683–98 |pmid= 2987862 |pmc= 341187 }} However, Ada shows greater preference for O6- alkyl guanine compared to either O4-thymine and alkylated phosphotriesters. Ada enzyme has two active sites, one for the alkylated guanines and thymines and the other for alkylated phosphotriesters.