Ada, also called as ''O''⁶ alkyl guanine transferase I (''O''⁶ AGT I), is an

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

induced by treatment of bacterial cells with

alkylating agents Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecti ...

that mainly cause

methylation In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These t ...

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 Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equals ...

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. Ada removes the alkyl group attached to DNA bases like

guanine Guanine () (symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine ( uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is c ...

(O⁶-alkyl guanine) or

thymine Thymine () (symbol T or Thy) is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine n ...

(O⁴-alkyl thymine) and to the oxygen of the phosphodiester backbone of the DNA. 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.

References

Transferases DNA repair {{enzyme-stub

See also

References