Pyrimidine dimers specificity

Pyrimidine dimers and other forms of DNA damage can be removed by a general excision repair mechanism. The first reaction in this form of repair involves forming nicks about the damaged region of the DNA. In (a) we see the mode of incision of UV-irradiated DNA by the pyrimidine-dimer-specific glycosylase and AP endonuclease... 

An example of direct repair is the photochemical cleavage of pyrimidine dimers. Nearly all cells contain a photoreactivating enzyme called DNA photolyase. The E. coli enzyme, a 35-kd protein that contains bound N lO-methenyltetrahydrofolate and flavin adenine dinucleotide cofactors, binds to the distorted region of DNA. The enzyme uses light energy—specifically, the absorption of a photon by the N, N lO-methenyltetrahydrofolate coenzyme—to form an excited state that cleaves the dimer into its original bases. 

One of the best-understood examples of nucleotide-excision repair is the excision of a pyrimidine dimer. Three enzymatic activities are essential for this repair process in E. coli (Figure 27.49). First, an enzyme complex consisting of the proteins encoded by the uvrABC genes detects the distortion produced by the pyrimidine dimer. A specific uvrABC... 

The DNA-PL of E. coli has been the most extensively studied of the photoreactivating enzymes. The only known substrates for the enzyme are cis-syn-cyclobutylpyrimidine dimers, and light is required for the dimer cleavage to occur but not for substrate binding (141). The enzyme is specific for UV-irradiated DNA, as untreated DNA does not bind to the lyase (142). The pyrimidine dimer... 

The enzyme SPL is the first identified nonphotoactivatable pyrimidine dimer lyase and it specifically targets SP and cleaves it into two thymines by a light-independent mechanism. Early publications " provided... 

Analysis of gene-specific removal of UV-C induced photolesions showed a lower NER activity in mES cells compared to MEF. In mES cells, UV-C induced cyclobutane pyrimidine dimers were not removed and (6-4) photoproducts were removed up to 30% compared to MEF that are able to remove 40-70% of (6-4) photoproducts and 80% of cyclobutane pyrimidine dimers (Van Sloun 1999). Furthermore a saturation of the NER activity was... 

In relation to DNA repair, van Noort et aL [62] used a home-made AFM, offering increased cantilever deflection sensitivity and a modified liquid cell, to investigate the action of the photolyase enzyme (obtained from Anacys-tis nidulans) on a restriction fragment of DNA. Photolysase repairs lesions in DNA and is specific to pyrimidine dimers resulting from UV damage [112]. AFM data suggested the photolyase enzyme showed mobUity on loose (immobilised) sections of the otherwise immobilised DNA molecules this enzyme mobility was limited to the sections of DNA that were not immobilised to mica. 

Morikawa K, Matsumoto O, Tsujimoto M, Katayanagi K, Ariyoshi M, Doi T, Ikehara M, Inaoka T, Ohtsuka E (1992) X-ray structure of T4 endonuclease V an excision repair enzyme specific for a pyrimidine dimer. Science 256 523 526... 

DNA Photolyase. - Ultraviolet light (> 300 nm) damages cellular DNA by the formation of thymine dimers from adjacent pyrimidine bases on the same DNA strand. The two main photoproducts are (cis,syn)-cyclobutane pyrimidine dimers (CPDs) and the (6-4) photoproduct. Such dimers may be restored to their monomeric form through the action of two photoactive (300 < X < 500 nm) damage-specific DNA repair enzymes, named CPD photolyase (also called DNA photolyase) and (6-4) photolyase. These enzymes are found in various... 

Mitchell, D.L., Jen, J., and Cleaver, J.E., Sequence specificity of cyclobutane pyrimidine dimers in DNA treated with solar (ultraviolet B) radiation, Nucl. Acids Res., 20, 225,1991. 

There is as yet insufficient evidence for proper evaluation of electron transfer reaction as the possible key process in the photohydration reactions (and perhaps in some of the dimerization processes). It has also been suggested that there is a distinct charge separation in the (overall) neutral excited pyrimidine molecule, and that the charge is sufficiently localized that reaction of the excited molecule with OH or H+ (or both successively) can become a competitive reaction pathway.116 Such a dipolar reactant species has also been specifically proposed by Wacker et al.60 (Chart 8). This is an electrophilic attack on water similar to that proposed above for uracil photohydration. 

We present here a brief account of the specific dimerization, and other related, reactions undergone by a variety of purine and pyrimidine derivatives, and a number of related compound s, during the course of their electrochemical reduction at the surface of a mercury electrode. A characteristic feature of these reactions is the transfer of an electron to the compound, accompanied, or preceded, by its protonation. The resultant free radicals, generated by a one-electron reduction process, rapidly dimerize to products in which each of the monomeric components possesses an additional electron and an additional proton, relative to the parent monomer1 7). (See Scheme 1)... 

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