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Function
Saporin is a ribosome-inactivating protein (RIP); alone, saporin does not selectively inactive ribosomes but rather conjugate with other molecules like peptides [3]. Saponaria officinalis is the plant from which saporin is extracted [4]. Type I and type II RIPS exist. Of these types, saporin is a type I. Ribosome inactivating proteins catalyze a cleavages N-glycosidic bond that is formed between the ribosome and adenine [5]. This adenine has the role of binding EF-1 and EF-2 to a ribosome [5]. EF stands for elongation factor. Since adenine no longer has a bond to the ribosome, the elongation step in translation cannot occur because the elongation factors cannot bind to just the ribosome. The specific elongation factor that is inhibited is elongation factor 2, which causes irreversible damage and disallows protein synthesis [4].
Disease
Relevance
Structural highlights
Type 1 RIPS are monomeric, meaning they have one part [6]. Saporin-S6 at maturity is 256 amino acids long [4]. While saporin consists of different residues and molecules, there is only one Chain A in it, and thus is monomeric. Chain A is a polypeptide weighing 30 KDa [7]. This chain consists of beta-sheets and alpha-helixes. The β-sheets make up the N-terminal domain, while the 𝛼-helix portion is the C-terminal domain [5]. In the figure of Chain A, the 𝛼-helices are spiral-shaped strands, while the β-sheets are more of a flat strand.
There is an active site within this chain that consists of five residues. These residues are Tyr⁷², Tyr¹²⁰, Glu¹⁷⁶, Arg¹⁷⁹, and Trp²⁰⁸ [4]. Other RIPs also have these same residues in their active sites. The saporin active has Glu¹⁷⁶, Arg¹⁷⁹, and Trp²⁰⁸ in the exact same position as the other ribosome-inactivating proteins. There is a difference in Tyr⁷², which has different side-chain conformations in RIPs and thus is not the same in saporin and other RIPs. This Tyr⁷² is the residue that interacts with the adenine in the cleavage of adenine and the ribosome [5].
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