A ribozyme is a polyribonucleotide (RNA molecule) with enzymatic activity. They are sometimes also referred to as "RNA enzymes" however, to avoid confusion with ribonucleoproteins in which the RNA lacks catalytic activity, "ribozyme" is generally preferred.
Ribozymes have been found in every known cell and are essential to cell functioning. The most well known example of an essential ribozyme is the 28S rRNA (called the 23S rRNA in prokaryotes), which carries out the aminoacyltransferase (peptide-lengthening) activity of ribosomes.
Structure and Function
Similar to proteins, RNAs derive their function from their structure. The primary structure of an RNA refers to its sequence, which allows the RNA to adopt local conformational structures (e.g. RNA hairpins), these secondary structures give the RNA molecule an overall tertiary structure. For RNAs that function in a complex, either with proteins or other RNAs or both, the structure of the complex is called quaternary structure.
The catalytic site (or sites) of a ribozyme function in a similar manner to the active sites of protein enzymes. They lower the ΔG‡ (Gibbs energy) of a chemical reaction by binding to the substrates in a manner that makes the reaction more energetically favorable. Also like protein enzymes, many ribozymes require cofactors (e.g. Mg2+) in order to function properly.
Notably, the antibiotic chloramphenicol works by inhibiting the aminoacyltransferase activity of the bacterial 23S rRNA.
Although self-splicing introns are relatively rare in eukaryotic genes, they are the only types of introns found in the few bacterial protein-coding genes that contain introns. Self-splicing introns appear to have originated in ancient retroviruses. Furthermore, the remarkable sequence homology between bacterial and eukaryotic self-splicing introns is considered a major piece of evidence for the common origin of introns in both domains.
RNase P is a ribozyme with ribonuclease (RNA cutting) activity found in all known forms of cellular life. It is responsible for processing precursor-tRNAs as well as precursors to some other functional RNAs within the cell.
The spliceosome (hypothesized)
In eukaryotes, the spliceosome is the ribonucleoprotein complex that removes the intronic sequences from most precursor-mRNAs. While it has not historically been considered a ribozyme, recent experimental evidence suggests that it might be. Researchers are currently investigating this possibility.