The positions of radical intermediates in the active sites of adenosylcobalamin-dependent enzymes.

The radical intermediates generated during the catalytic cycles of adenosylcobalamin-dependent enzymes occur in pairs. The positions of radicals residing on the cofactor, substrate or protein, relative to the position of the low-spin Co(2+) from the cob(II)alamin intermediate, can be extracted from electron paramagnetic resonance (EPR) spectra of the spin-coupled pairs. Examples of radical-Co(2+) pairs that span a range of interspin distances from 3 to 13A have been presented. Interspin distances greater than 5A require motion of one or more of the participating species. EPR spectroscopy provides a convenient means to determine the structures of these transient intermediates.