An RNA enzyme is an RNA strand that is able to function as an enzyme by catalyzing particular biochemical reactions within an organism. Typically, RNA is involved in the process of protein synthesis; it contains genetic "instructions" that allow ribosomes to synthesize specific proteins. For RNA strands that are involved in protein synthesis, the sequence of components that make up the strand, called nucleotides, is the most important factor, as the genetic "instructions" are contained in that sequence. For these enzymes, on the other hand, the three-dimensional arrangement of the strand is the most important factor, as the particular structure, not the sequence of nucleotides, allows the strand to act as an enzyme. An RNA enzyme may also be referred to as a ribozyme or as catalytic RNA.
RNA enzymes are incredibly rare compared to normal RNA, but they are no less essential in several essential biological processes. They are, for example, essential in the process of RNA translation, by which the "instructions" contained on a normal RNA strand are "read" and a protein is synthesized. An organelle called a ribosome is responsible for the actual process of protein synthesis. While a ribosome is a large complex made up of many different proteins and RNA strands, one of its most important functional units is an RNA enzyme.
One example of a biologically important RNA enzyme is ribonuclease P, or RNAse P, which cleaves segments of RNA. RNAse P is made up of both an RNA strand and a complex of associated proteins, though researchers have shown that RNAse P actually retains its ability to cleave RNA even if the proteins are not present. RNAse P, then, acts as an enzyme; its role involves the catalysis of a chemical reaction, not the transportation of genetic information. It is, therefore, considered an RNA enzyme.
The discovery that RNA could function as an RNA enzyme or as a transportation unit for genetic information has led to some speculation about its role in some of the earliest life forms on Earth. One theory known as the RNA world hypothesis states that there was a time when RNA actually fulfilled all of the functions of RNA, DNA, and proteins. RNA could have both stored and expressed genetic information in the form of nucleotide sequences and these enzymes could have functioned through the three-dimensional arrangements of RNA strands. Those in favor of the RNA world hypothesis believe that RNA evolved to DNA and proteins and that DNA provides more stable data storage and proteins can serve a greater variety of roles in biochemical reactions.