The first ribozyme structure to be solved was that of the hammerhead. It reveals a simple tertiary structure consisting of a two-domain catalytic core at the junction of three duplex stems.
This structure presumably represents a "ground state" of the catalytic reaction; a substantial conformational change would be required to align the reactive groups in a catalytic conformation.
The hammerhead ribozyme, a small RNA enzyme can target and cleave specific phosphodiester bonds in oligonucleotides. Papers describing the three-dimensional structure of the ribozyme and the dilemma it poses regarding catalytic mechanism are cited below.