Back to Index PageDescription of the reaction catalyzed by topoisomerase I
Topoismerase type I enzymes are monomeric and transiently break one strand of duplex DNA, allowing for single step changes in the linking number of circular DNAs (the number of times on strand of DNA crosses the other) (Redinbo, 1998). Prokaryotic enzymes require magnesium and a single-stranded segment of DNA; additionally they form a covalent intermediate with the 5' end of the broken DNA strand and relax only negatively supercoiled DNA (Redinbo, 1998). Eukaryotic topoisomerase I enzymes do not require any metal cofactors, work on double stranded DNA as well, form a covalent intermediate with the 3' end of the broken strand, and are able to relax both positive and negative supercoils (Redinbo, 1998).
Topoismerase I must also seal the break in the DNA. This reversible breakage is achieved because topoisomerases maintain the high energy status of the phosphodiester bond during unwinding (Nash, 1998). Topoismerases use an enzyme residue (typically a tyrosine) to break DNA. A new enzyme-DNA phosphodiester bond is formed in the process. The covalent DNA-enzyme intermediate can be readily attacked by the free end of the DNA because the enzyme-DNA phosphodiester bond is of comparable energy to the orginal bond (Nash, 1998).
Updated: December 5, 1998
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