Gelonin is a type 1 ribosome-inactivating protein and

toxin A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849– ...

of approximately 30 kDa found in the seeds of the Himalayan plant Gelonium multiflorum. In cell-free systems gelonin exerts powerful N-glycosidase activity on the 28S

rRNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosoma ...

unit of

eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

ribosomes Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to f ...

by cleaving out

adenine Adenine () ( symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its derivati ...

at the 4324 site. Gelonin lacks carbohydrate-binding domains so it is unable to cross the plasma membrane, making it highly effective only in cell free systems.

Structure

Gelonin is a 30 kDa

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

. Gelonin is a dimer, consisting of two identical monomers. Each monomer is composed of 251

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

, for a total of 502 residues. Gelonin is classified as an (α + β) protein, as its

secondary structure Protein secondary structure is the three dimensional conformational isomerism, form of ''local segments'' of proteins. The two most common Protein structure#Secondary structure, secondary structural elements are alpha helix, alpha helices and beta ...

consists of both

beta sheets The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a gen ...

and alpha helices. Each monomer’s first 100 amino acids form 10 beta sheets, while their last 151 amino acids form 10 alpha helices. Gelonin’s two dimers are stabilized by hydrophobic interactions and hydrogen bonds. Specifically, the Asn22, Arg178, Asn180, and Lys237 residues of each monomer

hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

with each other to stabilize the molecules. Likewise, the hydrophobic residues Tyr14, Ile15, Val16 and Pro38 from one monomer form hydrophobic interactions with the same residues in the adjacent monomer to further stabilize the dimer. Active Site of Gelonin with Adenine

Active Site

Gelonin’s

active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate (binding site) a ...

is a cleft formed by six key residues: Tyr74, Gly111, Tyr113, Glu166, Arg169, and Trp198. The shape of the active site is stabilized by hydrogen bonding between Gly111 and Tyr113. Tyr113, Glu166, and Arg169 residues in the activate site participate in the enzymatic removal of

at the 4324 site of eukaryotic

28S rRNA Crazy Eights is a shedding-type card game for two to seven players and the best known American member of the Eights Group which also includes Pig and Spoons. The object of the game is to be the first player to discard all of their cards. The ga ...

. Although the reaction mechanism of gelonin has yet to be characterized in detail, it is believed to take place in a manner that is conserved among other type 1 ribosome-inactivating proteins(RIP). According to research performed on other Type 1 RIPs, Tyr113 and Arg169 form hydrogen bonds with nitrogen atoms in the adenine nucleobase. This facilitates the cleavage of the glycoside bond connecting the nucleobase and ribose, creating a transition state with a positively charged

oxocarbenium An oxocarbenium ion (or oxacarbenium ion) is a chemical species characterized by a central sp2-hybridized carbon, an oxygen substituent, and an overall positive charge that is delocalized between the central carbon and oxygen atoms. An oxocarbenium ...

ion intermediate. The oxocarbenium ion is stabilized by the negative charge of Glu166. Gelonin’s active site also contains three water molecules, which act as nucleophiles and attack the oxocarbenium ion, completing the reaction.

Therapeutic applications

Because of its ability to inhibit translation by cleaving eukaryotic 28S rRNA, gelonin has the potential to be utilized as a cancer therapy. The anticancer activity of gelonin has been demonstrated in numerous ''in vitro'' models. However, because of its hydrophilicity, gelonin is unable to internalize in cells. This has made clinical applications of the macromolecule difficult. Multiple gelonin delivery systems have been engineered, including conjugation to a

cell-penetrating peptide Cell-penetrating peptides (CPPs) are short peptides that facilitate cellular intake and uptake of molecules ranging from nanosize particles to small chemical compounds to large fragments of DNA. The "cargo" is associated with the peptides either t ...

, liposome encapsulation using

listeriolysin O Listeriolysin O (LLO) is a hemolysin produced by the bacterium ''Listeria monocytogenes'', the pathogen responsible for causing listeriosis. The toxin may be considered a virulence factor, since it is crucial for the virulence of ''L. monocytogenes ...

, and attachment to

bispecific antibodies A bispecific monoclonal antibody (BsMAb, BsAb) is an artificial protein that can simultaneously bind to two different types of antigen or two different epitopes on the same antigen. Naturally occurring antibodies typically only target one antigen. ...

. All of these delivery systems have been shown to significantly decrease tumor size ''in vivo'' in a number of different cell lines. However, clinical trials for gelonin have yet to be authorized.

References

*Chemie, V.F (2012). The ribosome-inactivating protein gelonin and parts thereof to be employed for a potential treatment of cancer. (Doctoral dissertation). Retrieved from https://kluedo.ub.uni-kl.de/files/3215/Dissertation+M.+Badr.pdf *Stirpe, F., Olsnes, S. & Pihl, A. Gelonin, a new inhibitor of protein synthesis, nontoxic to intact cells. Isolation, characterization, and preparation of cytotoxic complexes with concanavalin A. J. Biol. Chem. 255, 6947-6953 (1980) Plant toxins Ribosome-inactivating proteins {{protein-stub

Structure

Active Site

Therapeutic applications

See also

References