Excision Repair

The excision repair system consists of several enzymes, each involved in several steps. First, the error must be recognized. For example, an 

Reconstruction of a functional DNA molecule from undamaged fragments (recombinational repair), and 

Photoreactivation is a light-induced (300-600 nm) enzymatic cleavage of a thymine dimer to yield two thymine monomers. It is accomplished by photolyase, an enzyme that acts on dimers contained in single- and double-stranded DNA. 

The enzyme-DNA complex absorbs light and uses the photon energy to cleave specific C-C bonds of the cy-clobutylthymidine dimer. Photolyase is also active against cytosine dimers and cytosinethymine dimers, which are also formed by UV irradiation but much less frequently. 

Two modes of excision repair, (a) Escherichia coli mechanism. Two incision steps are followed by gap-filling and displacement by polymerase 1. (b) Micrococcus luteus mechanism. A pyrimidine dimer glycosylase breaks an N-glycosidic bond and makes a single incision. Pol 1 displaces the strand, which is removed by an exonucleolytic event. In both mechanisms, the final step is ligation. 

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Chemistry of glycosylases and endonucleases involved in base-excision repair 98CRV1221. 

Crosslinks result from the reaction of a bifunctional electrophilic species with DNA bases and imply a covalent link between two adjacent DNA strands which inhibits DNA replication. Primary targets within bases are N7 and 06 in guanine and N3 in cytosine. The initial lesions are removed by the suicide enzyme alkyltrans-ferase, whereas nucleotide excision repair is needed for frilly established crosslinks. 

Base excision- repair Spontaneous, chemical, or radiation damage to a single base Base removal byN-glycosylase, abasic sugar removal, replacement... 

Figure 36-23. Base excision-repair of DNA. The enzyme uracil DNA glycosylase removes the uracil created by spontaneous deamination of cytosine in the DNA. An endonuclease cuts the backbone near the defect then, after an endonuclease removes a few bases, the defect is filled in by the action of a repair polymerase and the strand is rejoined by a ligase. (Courtesy of B Alberts.)...

Figure 36-24. Nucleotide excision-repair. This mechanism is employed to correct larger defects in DNA and generally involves more proteins than either mismatch or base excision-repair. After defect recognition (indicated by XXXX) and unwinding of the DNA encompassing the defect, an excision nuclease (exinucle-ase) cuts the DNA upstream and downstream of the defective region. This gap is then filled in by a polymerase (5/e in humans) and religated.

Wood RD Nucleotide excision repair in mammalian cells. J Biol Chem 1997 272 23465. 

SCHEME 9.3 DNA adducts may reform after excision repair and return to react with DNA again. 

Kasparkova, J. Zehnulova, J. Farrell, N. Brabec, V. DNA interstrand crosslinks of novel antitumor trinuclear platinum complex BBR. Conformation, recognition by HMG-domain proteins and nucleotide excision repair. J Biol Chem 2002, 277, 48076-48086. 

This work provides important evidence for elucidating the cytotoxic effect of the ruthenium-arene complexes and the influence of the arene thereon, for instance with respect to excision repair of DNA lesions and DNA destabilization. It also established two different classes of Ru(II) arene anticancer drugs, i.e. those bearing an arene that has the possibility to intercalate and those that do not. This distinction is important as we will see further differences in DNA binding interactions for these two classes (vide infra). 

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