Chemistry Reference
In-Depth Information
Repair of isolated nucleobase lesions by BER glycosylases involves a base-
fl ipping mechanism. A damaged guanine residue will be located by a glycosylase
scanning the DNA. It will then be fl ipped out from the interior regions of the DNA
strand and into the active site pocket of the attached glycosylase.
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This base - fl ipping
mechanism allows the glycosylase to gain access to the glycosidic bond of the
nucleobase lesion. Once the glycosidic bond has been cleaved, the base is released
and the glycosylase forms a Schiff base at the abasic site.
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This Schiff base will
remain in place until it is displaced by other proteins needed to further process the
abasic site. These proteins include apurinic/apyrimidinic endonuclease 1 (APE1)
which cleaves the phosphate backbone just to the 5
side of the abasic site, DNA
polymerases, which then fi ll in the gap in the DNA polymer, and fi nally DNA ligase,
which repairs the nick in the phosphate backbone. Many BER glycosylases harbour
intrinsic AP lyase activity, which would allow them to further process the abasic site.
This activity is very weak in most glycosylases and they do not complete the b -
elimination, but instead this cleavage step is typically carried out by APE1. NEIL1
and NEIL2 however, have strong lyase activity and are thought to complete b / d -
elimination at the abasic site.
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Recently, NEIL1 and NEIL2 have shown to have
considerably different roles with regard to replication associated repair (RAR).
NEIL1, but not NEIL2, shows strong cell cycle dependence with the S-phase.
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NEIL1, but not NEIL2, has also shown enhanced activity in the presence of prolif-
erating cell nuclear antigen (PCNA), a processivity factor for eukaryotic polymerase
d. The NEIL1/PCNA-stimulated activity suggests that NEIL1 may play an active
role in oxidized base lesion removal during replication.
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Recent modelling and dynamic studies on NEIL1 have shed light on why this
BER enzyme is specifi c for the nonplanar, further oxidized lesions, such as Sp and
other modifi ed pyrimidines.
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These studies have shown that the binding pocket is
somewhat fl exible and is more accommodating of the pyrimidine-like further oxi-
dized products, such as Sp and has complementary hydrogen-bond donor-acceptor
properties.
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In contrast, the planar 8-oxoG lesion binds only shallowly into the
pocket, has few hydrogen-bond donor-acceptor pairs, and has a solvent-exposed
six - membered ring.
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Taken together, the recent data on the formation of further oxidized guanine
lesions in DNA and the identifi cation of specifi c repair mechanisms suggest that
these lesions may be important in the genotoxicity of a variety of carcinogens,
including Cr(VI).
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17.3 Sugar Oxidation
17.3.1 C r ( VI ) - Induced Damage at Deoxyribose
Highly reactive Cr(V) and Cr(IV) species are capable of directly abstracting elec-
trons from the conjugated nucleobases, as discussed above. Similarly, these chro-
mium species can also oxidize the deoxyribose sugar by direct abstraction of
hydrogen atoms. Studies have shown that chromium preferentially attacks the C4
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