DNA damage and repair Flashcards
Why are the nitrogenous bases so reactive
Carbon ring structures- allowing for potential of delocalisation of electrons
Double bonds- allowing for activation of these
Reactive side chains- just remove methyl group from thymine to make uracil
Generally, planar, cyclical-carbon structures are very reactive, and thus the bases are very reactive too.
What are the potential outcomes of base modifications
Prevent replication
Cause mutations
Describe deamination reactions
The primary amino groups of nucleic acid bases are somewhat unstable. They can be converted to ketogroups
NH2- =O (water in, ammonia out)
Other deamination reactions include conversion of adenine to hypoxanthine, guanine to xanthine, and 5-methyl cytosine to thymine.
Thousands of these occur daily in cells!
Describe chemical modification
The nucleic acid bases are susceptible to numerous modifications by a wide variety of chemical agents. For example, several types of hyper-reactive oxygen (singlet oxygen, peroxide radicals, hydrogen peroxide and hydroxyl radicals) are generated as byproducts during normal oxidative metabolism.
All of these can modify DNA bases. A common product of thymine oxidation is thymine glycol:
Describe the consequences of hyper-reactive oxygen species
Hyper-reactive oxygen species are also generated by ionizing radiation (X-rays, gamma rays).
Many environmental chemicals, including “natural” ones (frequently in the food we eat) can modify DNA bases, frequently by addition of a methyl or other alkyl group (alkylation). In addition, normal metabolism frequently leads to alkylation. Addition of larger molecules defines “adducts”.
These thymine glycols can take up these methyl groups and form adducts,
Why is DNA the target for many carcinogens
Chemical carcinogens are usually metabolically activated and converted into electrophiles (they want electrons)
DNA is very electron rich
What are the consequences of bulky DNA adducts
The electrophiles bind and form a covalent bond- bind covalently to DNA
The binding of these adducts causes problems, particularly during replication because it interferes with the ability of DNA polymerase to recognise the base (because of the bulky adduct)
Describe photo damage to DNA
Ultraviolet light is absorbed by the nucleic acid bases, and the resulting influx of energy can induce chemical changes. The most frequent photoproducts are the consequences of bond formation between adjacent pyrimidines within one strand.
Intra-strand effect
Thymine dimers can form
These actually help protect the body from UV damage
Describe the different types of DNA damage that can occur
Nick- break in phosphodiester bond as a result of ionising or radioactive damage
Gap- many nicks
Thymine dimers- disrupts topology of DNA double helix- which can act as a signal for DNA damage
Base pair mismatch- bulge in DNA double helix- no longer have complementary base pairing- can act as a signal for DNA damage.
Describe some of the chemical carcinogens that can damage DNA
dietary lifestyle- smoking environmental occupational medical endogenous
Describe how radiation can damage DNA
Radiation
ionizing
solar
cosmic
Describe the importance of carcinogens
DNA damage can lead to mutation
Mutation may lead to cancer
Damaging DNA is an important strategy in cancer therapy - introduce them into the cells to make them unstable and impossible for them to survive.
State some different types of DNA damage caused by carcinogens
DNA adducts & alkylation Base dimers & chemical cross-links Double & single strand breaks Base hydroxylations & abasic sites formed- hydroxylation damages the base that much it's destroyed.
Which group of chemicals can chemically modify and damage DNA
Polycyclic Aromatic Hydrocarbons which are produced by the combustion of organic material, aflatoxins which are microbial bi-products that contaminate grain crops in hot and humid climates and aromatic amines that are commonly used industrial intermediates. Common environmental pollutants Formed from combustion of fossil fuels Formed from combustion of tobacco
Summarise the mammalian metabolism of carcinogens
§ Phase 1 – addition of functional groups – oxidations, reductions, hydrolysis- takes place in liver- convert hydrophilic carbon ring structures to something that can become more polar and thus excreted.
o Mediated mainly by cytochrome p450 enzymes.
§ Phase 2 – conjugation of phase 1 functional groups – glucuronidation, sulphation, glutathione conjugation, methylation, acetylation & amino-acid conjugation.
o Generates polar (water soluble) metabolites to excrete. Most carcinogens are insidious and only become carcinogenic after phase 1 metabolism.
Where is benzo(a)pyrene BaP found
Coal tar
Cigarette smoke
Grilled meats
Involved in the carcinogenesis of lung and colorectal cancers.
Type 1 carcinogen- very potent
Describe the two step epoxidation of Benzo(a)pyrene
B[a]P is a substrate for CYP450, which converts it to B[a]P-7,8-oxide (this is an electrophile and toxic)
The body has a defence mechanism – epoxide hydrolase converts the oxide to a dihydrodiol (B[a]P-7,8-dihydrodiol)
This is inactive
However, this dihydrodiol is also a substrate for CYP450, which converts it to another oxide (B[a]P-7,8-dihydrodiol-9,10-oxide)
This even more reactive than the previous oxide – it goes on to form DNA adducts- positively charged
The adducts can joint to chemically modified bases- and thus form covalent bonds with DNA clumping the DNA up
Summarise aflatoxin b1
Formed by Aspergillus flavus mould
Common on poorly stored grains and peanuts
Aflatoxin B1 is a potent human liver carcinogen, especially in Africa and Far-East
Describe the epoxidation of aflatoxin B1
§ Aflatoxin B1 epoxidation process:
o P450 oxidises the aflatoxin B1.
o Aflatoxin B1 2,3-epoxide then adducts to DNA directly using its adjacent N7 positively charged carbon atom.
Adducts on to the N7 position of the guanine-forming a mutation.
Summarise 2-napthylamine
Past components of dye-stuffs
Include 2-naphthylamine and benzidine
Potent human bladder carcinogens
German dye industry epidemiology (1895 Rehn)
Explain the mechanism by which 2-napthylamine is a bladder carcinogen
2-naphthylamine is converted by CYP1A2 to a hydroxylamine derivative, which is reactive (N-hydroxy-2-napthylamine)
In the liver, this is glucuronidated (thus inactivating it)- by gluconoryl transferase
The inactive metabolite is excreted by the liver and then it goes to the bladder where it mixes with the urine
The ACIDITY of the urine causes hydrolysis of the glucuronides – this releases the hydroxylamine derivative, which forms a nitrenium ion
This is electrophilic so it leads to the formation of DNA adducts
