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From Proteopedia
STRUCTURE OF RICIN A CHAIN MUTANT
Structural highlights
Function[RICI_RICCO] Ricin is highly toxic to animal cells and to a lesser extent to plant cells. The A chain acts as a glycosidase that removes a specific adenine residue from an exposed loop of the 28S rRNA (A4324 in mammals), leading to rRNA breakage. As this loop is involved in elongation factor binding, modified ribosomes are catalytically inactive and unable to support protein synthesis. The A chain can inactivate a few thousand ribosomes per minute, faster than the cell can make new ones. Therefore a single A chain molecule can kill an animal cell. The B chain binds to beta-D-galactopyranoside moieties on cell surface glycoproteins and glycolipids and facilitates the entry into the cell of the A chain; B chains are also responsible for cell agglutination (Lectin activity). Evolutionary Conservation Check, as determined by ConSurfDB. You may read the explanation of the method and the full data available from ConSurf. Publication Abstract from PubMedThe A chain of ricin (RTA) is an N-glycosidase which inactivates ribosomes by removing a single adenine base from a conserved region of rRNA. X-ray structures and site-directed mutagenesis revealed that Arg 180 interacts with the target adenine hydrogen bonding with N3. It may fully or partially protonate that atom as part of the hydrolysis mechanism. Arg 180 was previously converted to His (R180H) and shown to greatly reduce activity. Here R180H is shown to reduce overall activity 500-fold against Artemia salina ribosomes. A 2.2 A crystal structure reveals the mutation causes a rearrangement of the active site cleft, with Tyr 80 moving to block access to the adenine recognition site. His 180 forms a strong aromatic interaction with Trp 211, Tyr 80, and Tyr 123. A complex is formed with 250 mM AMP. The nucleotide binds in the active site region, but in an apparently nonproductive orientation. His 180 cannot bond to N3 and is screened from the substrate analog by the intervening Tyr 80. It may be that natural polynucleotide substrates, using additional interactions, can displace Tyr 80 and effect a productive binding. Structure and activity of an active site substitution of ricin A chain.,Day PJ, Ernst SR, Frankel AE, Monzingo AF, Pascal JM, Molina-Svinth MC, Robertus JD Biochemistry. 1996 Aug 27;35(34):11098-103. PMID:8780513[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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Categories: Castor bean | Large Structures | RRNA N-glycosylase | Day, P J | Ernst, S R | Frankel, A E | Monzingo, A F | Pascal, J M | Robertus, J D | Svinth, M | Duplication | Glycoprotein | Glycosidase | Hydrolase | Lectin | Toxin
