Ada, also called as ''O''
6 alkyl guanine transferase I (''O''
6 AGT I), is an
enzyme
An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
induced by treatment of ''E. coli'' cells with
alkylating agents Alkylation is a chemical reaction that entails transfer of an alkyl group. 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 effectin ...
that mainly cause
methylation
Methylation, in the chemistry, chemical sciences, is the addition of a methyl group on a substrate (chemistry), substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replac ...
damage. This phenomenon is called the
adaptive response The adaptive response is a DNA damage response pathway prevalent across bacteria that protects DNA from damage by external agents or by errors during replication.Landini, P, Volkert MR. (2000Regulatory Responses of the Adaptive Response to Alkylatio ...
hence the name. Ada transfers the alkyl group from
DNA
Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
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.
Ada removes the alkyl group attached to DNA bases like
guanine
Guanine () (symbol G or Gua) is one of the four main nucleotide bases 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 ...
(O
6-alkyl guanine) or
thymine
Thymine () (symbol T or Thy) is one of the four nucleotide bases 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 ...
(O
4-alkyl thymine) and to the oxygen of the phosphodiester backbone of the DNA.
However, Ada shows greater preference for O
6- alkyl guanine compared to either O
4-thymine and alkylated phosphotriesters. Ada enzyme has two active sites, one for the alkylated guanines and thymines and the other for alkylated phosphotriesters.
See also
*
Ogt
*
O-6-methylguanine-DNA methyltransferase
Methylated-DNA--protein-cysteine methyltransferase (MGMT), also known as ''O''6-alkylguanine DNA alkyltransferase AGT, is a protein that in humans is encoded by the ''MGMT'' gene.
MGMT is crucial for genome stability. It repairs the naturally occu ...
References
Transferases
DNA repair
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