Barnase (a

portmanteau In linguistics, a blend—also known as a blend word, lexical blend, or portmanteau—is a word formed by combining the meanings, and parts of the sounds, of two or more words together.

of "BActerial" "RiboNucleASE") is a bacterial

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

that consists of 110

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s and has

ribonuclease Ribonuclease (commonly abbreviated RNase) is a type of nuclease that catalyzes the degradation of RNA into smaller components. Ribonucleases can be divided into endoribonucleases and exoribonucleases, and comprise several sub-classes within th ...

activity. It is synthesized and secreted by the bacterium '' Bacillus amyloliquefaciens'', but is lethal to the cell when expressed without its inhibitor barstar. The inhibitor binds to and occludes the ribonuclease active site, preventing barnase from damaging the cell's

RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...

after it has been synthesized but before it has been secreted. The barnase/barstar complex is noted for its extraordinarily tight protein-protein binding, with an on-rate of 10⁸s⁻¹M⁻¹.

Protein folding studies

Barnase has no

disulfide bonds In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In in ...

, nor does it require divalent cations or non-peptide components to fold. This simplicity, in combination with its reversible folding transition, means that barnase has been extensively studied in order to understand how proteins fold. The folding of barnase has been extensively studied in the laboratory of Alan Fersht, who used it as the test case in developing a method of characterizing

protein folding Protein folding is the physical process by which a protein, after Protein biosynthesis, synthesis by a ribosome as a linear chain of Amino acid, amino acids, changes from an unstable random coil into a more ordered protein tertiary structure, t ...

transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked w ...

s known as phi value analysis.

Active site and catalytic mechanism

Barnase catalyzes

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

at diribonucleotide GpN sites. Cleavage occurs in two steps using a general acid-base mechanism: a cyclic intermediate is formed during the first

transesterification Transesterification is the process of exchanging the organic functional group R″ of an ester with the organic group R' of an alcohol. These reactions are often catalyzed by the addition of an acid or base catalyst. Strong acids catalyze the r ...

step, which is then hydrolysed to release the cleaved

. The two most important residues involved in catalysis are Glu73 and His102, which are both essential for enzymatic activity. Glu73 is the general base whilst His102 is the general acid. Although it is not directly involved in acid-base catalysis, Lys27 is also critical for activity; it has been implicated in transition-state substrate binding.

References

External links

InterPro entry
Ribonucleases Bacterial enzymes {{enzyme-stub

Protein folding studies

Active site and catalytic mechanism

See also

References

Further reading

External links