Ribonuclease
Ribonuclease (RNase) is the name of a group of enzymes that change ribonucleic acid (RNA) by digesting (cutting) phosphorus-oxygen bonds. The RNases are the subject of wide investigation in the laboratory, though scientists are still learning the many ways they work in living cells.
The best-studied RNase is from the pancreas of cattle. Its main portion, called ribonuclease A, was the first enzyme whose entire sequence of amino acids was determined. It was also the first protein to be totally synthesized from amino acid.
Pancreatic ribonuclease was first described in 1920 by the American biochemist Walter Jones (1865-1935), who showed that it could digest yeast RNA. It was partially purified in 1938 by the American microbiologist René Jules Dubos (1901-1982) and isolated in crystalline form late two years later by M. Kunitz. RNase's sequence and three-dimensional structure were determined in 1962 by the American biochemists Christian Anfinsen (1916-1995), Stanford Moore (1913-1982), and William H. Stein (1911-1980), who received the 1972 Nobel Prize in chemistry for the accomplishment.
Anfinsen was born in Monessen, Pennsylvania, received his Ph.D. in biochemistry from Harvard University in 1943, and joined the staff of the National Institutes of Health. He wanted to learn how the peptide (protein) chain was instructed to fold into its three-dimensional shape. By discovering the amino acid sequence in parts of the molecule, he showed that the sequence itself was all the information needed for folding.
Stein and Moore performed their sequencing work at Rockefeller University. Stein, from New York City, received a Ph.D. from Columbia University in 1938. Moore was born in Chicago, grew up in Nashville, Tennessee, and earned a Ph.D. in organic chemistry from the University of Wisconsin in 1938. The two scientists wanted to learn how ribonuclease's structure was related to its activity. An active site is the portion of the enzyme that binds to the reacting substance (the substrate). First Stein and Moore discovered that the amino acids at the active site--were much more active in the molecule than in free form. They discovered how to chemically identify the active amino acids within the chain, and finally, determined the entire amino acid sequence.
In 1968, ribonuclease was synthesized by two different methods. Ralph Hirschmann (1922-) at Merck Sharp and Dohme Inc. Research Laboratories synthesized individual proteins and then chained them together. Bruce Merrifield (1921-), at Rockefeller University, automated the synthesis process by attaching the amino acids one by one to a solid plastic matrix, which eliminated intermediate steps. For developing this process, Merrifield received the 1984 Nobel Prize in chemistry.
In the living cell, RNases may break down RNA that has served its purpose, so that the components can be used again. RNases may also play a part in forming an RNA molecule for a specific purpose, such as messenger RNA and ribosomal RNA. The roles of other RNAses are still unknown.
Some RNAses act only on specific groups, such as pyrimidine bases. Some RNases work only on specific RNA structures. Exoribonucleases act only the free ends of RNA molecules; endoribonucleases work elsewhere in the molecule. Some RNases work on RNA from the 5' to 3' direction, others from 3' to 5' (3' and 5' refer to locations where nucleotide bases attach to phosphates and sugars; each DNA and RNA strand has a 5' and a 3' end).
Ribonuclease P requires an RNA component in order to be active. Its discovery in the late 1970s by the American biophysicist Sidney Altman (1939-) earned him part of the 1989 Nobel prize in chemistry. RNase H functions by breaking down a copy of the RNA molecule when it is no longer needed for viral reproduction. It is a component of reverse transcriptase, an enzyme made by retroviruses (viruses with RNA genetic material) which allows them to make a copy of DNA from RNA.
Mammalian cells also produce RNase inhibitors, which keep RNases from breaking down RNA molecules.
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