Isoalloxazine is the traditional name of the head group of a flavin cofactor: small molecules typically found bound to enzymes involved in electron transfer reactions in metabolism. It looks like this:

          H          R
          |          |
        H-C-H        N     N    O
           \  // \  / \  // \  //     
            \//   \/   \//   \//
             |    ||    |     |
             |    ||    |     |
            /\\   /\   //\    N
           /  \\ /  \ //  \  / \
        H-C-H        N     ||   H
          |                ||
          H                O

The 'R' on the top of the figure varies depending on the flavin in question (see flavin cofactors for a full description of the possible R-groups.) The extensive conjugation of the head group is what makes it amenable to the addition/subtraction of electrons or a hydride. This is a representation of the molecule without any added electrons (i.e. it is in it's oxidized state.) Electrons or a hydride typically are added to the nitrogen atom at the bottom of the center ring, forming reduced flavin species. This nitrogen is in a very unusual environment, as evinced by it's extreme NMR chemical shift of 350 ppm.