DNA Photolyase is characteristic of a Michaelis-Menten enzyme with the exception that its catalysis is initiated by light (Sancar, 1994).  Initially the enzyme binds to the pyrimidine dimer in a light-independent step. The pyrimidine dimer is then flipped out of the DNA helix and into the cavity where the flavin is located. The light-harvesting cofactor, 8-HDF or MTHF, will absorb a blue light photon and by dipole-dipole interaction transfer the excitation energy to FADH.  An electron is then transferred to the pyrimidine dimer.  This causes a breakage in the 5-5 and 6-6 sigma bonds of the cis,syn-cyclobutane ring.  Upon breakage of the ring, the distance between the two pyrimidine increases from C-C bond length to Van der Waals distance (Park,etal.,1995).  The dimer then splits into monomers and the electron is transferred back to the flavin cofactor to restore its functional form.  It has been suggested that the increase in bond length forces the substrate out of the cavity and the enyzme then dissociates (Park, etal., 1995). 

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