Structure Molecular Function And Toxicology Of Ricin Chemistry Essay

Ricin is a protein which is isolated from Castor bean works normally known as Ricinus communis. Ricin is a Lectin which belongs to ribosome-inactivating proteins ( Type2 ) that block protein synthesis in normal eucaryotic cells.

It can be extracted easy from Castor beans or from left over waste mash. Ricin is a heterodimer, where one concatenation contributes in attaching to the cell membrane, while other concatenation plays a cardinal function in doing toxicity. The particular characteristic of Ricin toxin is retrograde conveyance to the cytosol ( wholly change by reversal to that of normal protein conveyance to cell membrane, i.e from cell membrane to Golgi composite, so to endoplasmic Reticulum and so to cytosol ) . Since ricin falls in ribosome inactivating proteins group it portions common mechanisms with other group members like Abrin, Shiga toxin. etc.

Toxicity of ricin has been known from long, but it came into light during decease of celebrated Bulgarian novelist Georgi Ivanov Markov. He was poisoned to decease by ricin in the signifier of pellet, which was shot with the aid of Umbrella ( umbrella gun ) .Ricin was used as bunch bombs or as a toxic dust during universe wars, and now it has been listed as possible bio panic agent.

Since Structure of ricin is studied extensively, chief current research work is traveling on to happen specific vaccine/antidotes against ricin and to potentially utilize ricin as anti-cancer agent.

Ricin Structure

It consists of 2 glycoprotein ironss, RTA ( Ricin Toxin A ; 32kDa ) and RTB ( Ricin Toxin B ; 34KDa ) consisting of 267 and 262 amino acid residues severally. The two ironss are held together by disulphide bond links between 259 residue of RTA and 4 residue of RTB.

Fig 1-Ricin Heterodimer with RTA on upper right and RTB on Bottom Left

RTA is more like a ball-shaped protein and is about 55A tens 45A ten 35A midst. RTA consists of 3 structural spheres being 30 % coiling with 7 alpha spirals ; It besides has 15 % beta construction which is made up of 5 strands of beta. First sphere is made of beta sheet, 2nd sphere is composed of residues 118-210 and is dominated by 5 alpha spirals, 3rd sphere is made of residues 211-267, is like a compact phonograph record like sphere which interacts with first two spheres and RTB. RTA has a outstanding cleft at the interface between all the three spheres, which is postulated to be the active site cleft. It is besides called as latent active cleft when it is in heterodimer signifier along with RTB, which partly blocks this active site, which is known to interact with sarcin-ricin cringle in ribosome causation depurination and thereby doing ribosome inactivation.

RTB is a merchandise of cistron duplicate. It consists of 2 spheres. Each sphere has similar topologies and contain Lactose adhering site ( shown as brace of circles in fig. 1 ) . By X-ray crystallographic surveies it has been seen that brain sugar of lactose prevarications in shallow pocket. This pocket/cleft is created by a three residue peptide kink on the underside, aromatic ring ( Try-37 sphere 1 & A ; Try 248 in sphere 2 ) at top. At back of cleft in sphere 1 ; C3 and C4 hydroxyl of galactose signifier H bond with Asparagine ( Asn-46 ) and Lysine ( Lys-40 ) the cardinal Asparagine residue is held in place by H bond to Aspartate ( Asp-22 ) , Glutamine ( Gln-35 ) forms hydrogen bond with the C4 intoxicant of brain sugar. In sphere 2 ; C3 and C4 signifier H bond with Asn-255 and a H2O molecule bond ( correspondent to lysine in sphere 1 ) to Asn-255. Asn-255 is stabilised by Asp-234, but there is no parallel of glutamine in sphere 2. The long arm of around 8-10 residues from aminic terminus of B concatenation interacts with carboxy terminus of A concatenation ( as shown in fig 1 ) .

[ Protein Databank Entry of Ricin 2AAI ( Structure at 2.5A ) ] .

Molecular mechanism of Ricin Toxin

Ricin is toxicant if inhaled or injected. In contrary, it is less toxic when taken orally. It leads to toxic consequence by the undermentioned paths of entry to eucaryotic cells

1 ) Endocytosis

2 ) Retrograde Transport

3 ) Ribosome Inactivation

1 ) Endocytosis

RTB concatenation of Ricin normally forms Hydrogen bonds with Carbohydrate medieties present on cell membrane. Ricin is known to adhere galactose with its two spheres present on RTB, but Ricin besides binds to mannose in absence of brain sugar on cell membrane in some cell types.

Once bound to membrane, entry into cell is driven by following procedures

Harmonizing to our survey we were able to happen 3 mechanisms which lead to ricin endocytosis

a ) Clathrin Dependent

Clathrin is a protein which enhances the formation of little cysts ( membrane edge ) in the cytol ; clathrin coated cysts aid in selective conveyance of molecules to trans-golgi web.

This procedure dependant on cholesterin.

B ) Caveolae

This are little introversions in plasma membrane ( specialised invaginated type of raft ) , besides known as lipid tonss. This flask shaped constructions are rich in cholesterin and have known to play major function in endocytosis.

degree Celsius ) Cholestrol independent

There have been posits that these tracts are dependent on protein kinase A, heterotrimeric G proteins and Cyclo-oxgenase tract.

Fig 2. Ricin conveyance path in a Cell.

2 ) Retrograde Transport of Ricin

This conveyance procedure is broken down into farther 2 procedures

Golgi setup to ER

Ricin is frequently transported from early endosomes to golgi complex, and it has been postulated that this tract is dependent on Rab9 and a protein called Dynamin.

This is assumed to be based on 2 tracts

a ) Based on KDEL sequence

It ‘s a C-terminal sequence ( Lys-Asp-Glu-Leu ) , signal for lasting keeping of proteins in ER.

The toxins bind to KDEL receptors that rhythm between Golgi and ER, therefore by normal manner toxins are retrieved back to the ER in the same mode as cellular KDEL tagged sequences.

2 ) Calreticulin

Ricin has been thought to adhere protein called calreticulin, a storage protein associated with ER, with the aid of RTB. ( RTB binds to terminal galactose residue on calreticulin ) . Catreticulin have been known to transport KDEL sequence.

From the ER Lumen to Cytosol.

This conveyance is based on Sec61p protein composite, Sec61p is common translocon nowadays on ER, it is known to be involved in retrograde conveyance of falsely folded proteins back in cytosol, where they are degraded by proteosomes ( ricin is stable in cytosol, due to less lysine residues, which decrease the activity of proteosomes ) .

For ricin to move as a potent toxin, RTA must be cleaved from RTB ; but exact site and procedure of this is non understood clearly. But it has been stated that RTA is recognised as a misfolded protein by some chaperones involved in ERAD ( Endoplasmic Reticulum associated protein debasement tract ) , which transport RTA out for debasement.

3 ) Ribosomes Inactivation

Subunit A besides called as Ribosomal RNA N-glycosidase removes a specific A from 28S-rRNA. RTA concatenation is known to split glycosidic bonds within rRNA. Ricin targets extremely conserved sequences of 12 bases normally present in about all eucaryotic ribosomes.

The sequence, 5′-AGUACGAGAGGA-3 ‘ , besides is known as Sarcin-ricin cringle plays a really of import function in protein synthesis, by supplying adhering sites to elongation factors. Depurination of Adenine in these sequences inactivates protein synthesis.

Depurination Reaction

Several amino acids are involved in this measure

Tyr-80, Tyr-123, Glu-177 and Arg-180.

1 ) RTA active site binds to Sarcin -Ricin cringle, stacking against Tyr80 and Tyr 123.

2 ) Arginine protonates N-3 A, taking to bond interruption between N-9 of Adenine and C-1 of ribose.

3 ) Above measure leads to formation of oxycarbonium on ribose, this is stabilised by Glu-177 nowadays on ricin

4 ) Protonation of Arg-180 of ricin and N3 of Adenine leads to deprotonation of nearby H2O molecule, from where hydroxyl group onslaughts on ribose carbonium ion.

5 ) This leads to depurination of Ribose, which is integral in phosphodiester anchor.

Ricin Antidotes

At present there are no counterpoisons for Ricin, but there are several possible inhibitors which can be used in future as counterpoisons against ricin.

We can sort Ricin Antidotes into three subclasses where suppression can be done by utilizing purines or pterin group or by a pyramidine.


Purines are natural inhibitors of Ricin, since ricin Acts of the Apostless on depurination in ribosome. So we are non concentrating much on this group

Pterin Group

Pterioc Acid

Tautomers of Pterin exist with Ricin, normally it can organize 3/6 and 4/2 strong and weak H bonds severally.

Normally Pterin is sandwiched between the rings of Tyr-80 and Tyr-123.

Hydrogen bond formation takes topographic point between

– Carbonyl O atoms of Gly-121 and 2-amino group of Pterin

– N3 of pterioc acerb signifiers two H bonds with Val-81 and Ser-176

– Arg-180 signifiers two H bonds at the 4-oxo and N5 places.

Due to Benzoate group PTA bends around side-chain of Tyr-80 by doing some non-polar interactions, benzoate group is held by H bonds from carboxylate group and Asn-78 on one side, and one from H2O molecule which is bound to Arg-258 of RTA.

Fig 3a Fig 3b

Figure 3b-Chemical Structure of Pterin,

3a-Pterin -Ricin composite ( Pterin edge to RTA concatenation of Ricin )

[ Protein informations bank entry for Ricin-pterin is 1BR6 ] .


The orientation of the pterin ring is similar to that seen for PTA. One difference is the

bonding of Arg180 to the inhibitor. It bonds with the 4-oxo group of neopterin, but does non bond to N5 as occurs in the PTA composite. Alternatively, a 2nd bond is formed with the proximal hydroxyl of the propane triol mediety ; associated with this rearrangement is a 7 rotary motion of the pterin ring. The other atoms of propane triol mediety of the neopterin does non do any interaction with RTA. Since the propane triol mediety of neopterin is much smaller than the corresponding substituents in PTA, it does non interact with Tyr80 in the same manner and appears to miss the new wave der Waals part to adhering which might be expected in PTA.

[ Protein informations bank entry for Ricin-neopterin is 1BR5 ] .


PBA { 4- [ 3- ( 2-Amino-1,4-dihydro-6-hydroxy-4-oxo-5-pyrimidinyl ) propyl ] -benzoic acid } binds to Ricin in a really similar manner to that of purines and pterins.

Aromatic ring of PBA is stacked between side ironss of Try-80.

Hydrogen bonds are formed between:

Exocyclic amino group of PBA with carbonyl O of Val-81 and Gly-121,

N1 ( one ring ) and Amido group of valine

N3 ( 2nd ring ) and Carbonyl O of Gly-121

The pyrimidine ring of PBA normally overlaps half pterin ring, ( when both constructions are superimposed ) but matching polar atoms play the same function in RTA binding.

The lone difference between pterin and PBA is the adhering between Arg-180and oxyanion at place 6 of PBA, which is considered stronger than the ketone at same place of of PTA.

Fig 5. PBA-Ricin composite

Comparison with Other Toxins.


It is a potent toxin besides belonging to ribosome demobilizing protein ( RIP 2 ) , which is obtained from seeds of Abrus precatorius ( or Rosary pea ) . Abrin is a hetrodimer consisting of ironss A and concatenation B with a disulfide bond between cys247 of A concatenation and Cys8 of B concatenation.

A concatenation is composed of 251 aminoacids and divided into 3 folding spheres, B concatenation is composed of 268 aminic acids. A concatenation depurinates adenine from place 4 and 324 of 28s rRNA and inhibits protein synthesis.

Abrin portions carbohydrate adhering site with Ricin, about 60 % of residues are similar to that of RICIN. B concatenation a lectin which binds to galactose present on the cell membrane. Abrin is regarded as more toxic so ricin ( 70 times ) .

Fig 6. Crystallographic Structure of Abrin Fig7. 3-D construction of Shiga Toxin

Shiga Toxin

Shiga toxins are AB5-toxins consisting of a pentameric binding moeity ( stxB ) and enzymatically active A-moeity ( stxA ) . B-chain is a pentamer consisting of 5 little ironss of 7.7kDa each, which interact non-covalently. Each B concatenation contains a adhering site of glycosphingolipid gb3.

A concatenation is of 32.2kDa and is attached to B concatenation with aid of Disulfide bond between cysteines 242-261.

Shiga Toxin and Ricin portion really common construction, as in, both have Lectin as one of there fractional monetary units which specifically binds to carbohydrate mediety on cell membrane, during internationalization both the fractional monetary units are detached from themselves so as to let go of N-glycosidase enzyme which will specially move on Ribosome. Both toxins have been known to follow the same retrograde conveyance to make cytosol and depurinate ribosome.

Ricin as an Anti-Cancer Drug and other Medicinal utilizations.

Work is being carried out to utilize ricin as an anti-cancer drug. Ricin can bring on programmed cell death in Human Cervical Cancer cells ( HeLa ) but the exact mechanism is really ill understood and surveies are traveling on with the usage of RTA/RTB labeled antibodies.

The foreign molecule has been normally fused to the N-terminal sphere of RTB to avoid steric hinderance by the antigen with RTB galactose receptor binding sites. Research has demonstrated that ricin toxin B fractional monetary unit is an first-class campaigner to heighten immunogenicity at the i.p. and i.n. paths of HIV, and likely other, antigens, and potentially to hike cytotoxic T-lymphocyte responses in the context of mucosal protection, a major demand for a possible HIV vaccinum campaigner.


Our survey shows that Structure of Ricin has been studied extensively ; postulates back uping the manner of conveyance and toxicity have besides been established good.

During comparing of Ricin with other toxins ( Abrin and Shiga toxin ) from same household or RIP ( type2 ) , we found out that there is high resemblance in at that place structural topology, manner of entry into cell and mechanism of demobilizing ribsomes ( by depurination of A in sarcin-ricin cringle of ribosome ) .

At present, we did non happen any counterpoisons for ricin, but we have presented some possible counterpoisons like PBA and Pterins, among the two we did happen out that PBA binds more expeditiously to Ricin ( barricading wholly its active site )

Ricin protein came to light as toxin, but surveies have shown that it can bring on programmed cell death in extremely spliting cells ( Cancer cells ) but the exact mechanism and ground for this is yet to be explored but we suggest some manner where ricin can be used in countering malignant neoplastic disease cells:

There are some cases where RTB has been used successfully as a bearer fused to with other molecules ; the usage of affibody engineering and nano sized bearer system increases the selectivity of ricin. Affibody molecules are little in size, extremely specific, extremely stable, and easy engineered and have a high grade of turn uping and they are used in placing tumours due to their high selectivity. Therefore, toxin conjugated affibody can be used ( ricin conjugated affibody ) to increase the efficiency of drug bringing system in aiming malignant neoplastic disease cells. Besides, aiming ricin can be enhanced by nano sized bearer system through inactive targeting by adhering ricin on bearer composites where they are attracted to tumor receptors harmonizing to a certain belongings like pH taking advantage from the acidic medium of the micro-environment of malignant neoplastic disease unlike the pH normal cells, work outing the internalisation job and increasing selectivity.


In our survey we have found that Structure, manner of action of Ricin has been established, but many facets like ordinance of endocytic tract, retrograde conveyance and conveyance intermediates need to be researched with the particular focal point. Ricin counterpoisons and other possible medicative applications of ricin like in the malignant neoplastic disease and all should be concentrated.


Hi there, would you like to get such a paper? How about receiving a customized one? Check it out