Sugar binding effects on the enzymatic reaction and conformation near the active site of pokeweed antiviral protein revealed by fluorescence spectroscopy

Expanding role of ribosome-inactivating proteins: From toxins to therapeutics

Ribosome-inactivating proteins (RIPs) are toxic proteins with N-glycosidase activity. RIPs exert their action by removing a specific purine from 28S rRNA, thereby, irreversibly inhibiting the process of protein synthesis. RIPs can target both prokaryotic and eukaryotic cells. In bacteria, the production of RIPs aid in the process of pathogenesis whereas, in plants, the production of these toxins has been attributed to bolster defense against insects, viral, bacterial and fungal pathogens. In recent years, RIPs have been engineered to target a particular cell type, this has fueled various experiments testing the potential role of RIPs in many biomedical applications like anti-viral and anti-tumor therapies in animals as well as anti-pest agents in engineered plants. In this review, we present a comprehensive study of various RIPs, their mode of action, their significance in various fields involving plants and animals. Their potential as treatment options for plant infections and animal diseases is also discussed.

Ligand-Induced Conformational Changes near the Active Site Regulating Enzyme Activity of Momorcharins from Seeds of Bitter Gourd

It is reasonable to consider that Type I-ribosomal inactivation proteins (RIP) retain some specific affinity to harmful pathogens to complete the role as a bio-defense relating protein. In the present studies, it was shown that two Type I-RIPs, α- and β-momorcharins, maintained the abilities to bind with N-acetylglucosamine (NAG) to change the conformation around the active sites and to regulate their N-glycosidase activities. By the binding of NAG, the freedom of internal motion of Trp192 in α-momorcharin was increased 1.5 times near the active site and, on the other hand, the corresponding motion of Trp190 was limited 50% in β-momorcharin. The results in the fluorescence resonance excitation energy transfer experiments demonstrated that Trp-190 of β-momorcharin was kept away from Tyr-70 but Trp192 contrarily approached closer to the nearest neighboring Tyr residue consisting of the active center of α-momorcharin by the binding with NAG. These conformational changes near the active site close correlated with promotion and/or suppression of the N-glucosidase activities of β- and α-momorcharins.

Conformational Change Near the Redox Center of Dihydrolipoamide Dehydrogenase Induced by NAD(+) to Regulate the Enzyme Activity

Dihydrolipoamide dehydrogenase (LipDH) transfers two electrons from dihydrolipoamide (DHL) to NAD(+) mediated by FAD. Since this reaction is the final step of a series of catalytic reaction of pyruvate dehydrogenase multi-enzyme complex (PDC), LipDH is a key enzyme to maintain the fluent metabolic flow. We reported here the conformational change near the redox center of LipDH induced by NAD(+) promoting the access of the DHL to FAD. The increase in the affinity of DHL to redox center was evidenced by the decrease in K M responding to the increase in the concentration of NAD(+) in Lineweaver-Burk plots. The fluorescence intensity of FAD transiently reduced by the addition of DHL was not recovered but rather reduced by the binding of NAD(+) with LipDH. The fluorescence decay lifetimes of FAD and Trp were prolonged in the presence of NAD(+) to show that FAD would be free from the electron transfer from the neighboring Tyrs and the resonance energy transfer efficiency between Trp and FAD lowered. These results consistently reveal that the conformation near the FAD and the surroundings would be so rearranged by NAD(+) to allow the easier access of DHL to the redox center of LipDH.

Pokeweed antiviral protein: its cytotoxicity mechanism and applications in plant disease resistance

Pokeweed antiviral protein (PAP) is a 29 kDa type I ribosome inactivating protein (RIP) found in pokeweed plants. Pokeweed produces different forms of PAP. This review focuses on the spring form of PAP isolated from Phytolacca americana leaves. PAP exerts its cytotoxicity by removing a specific adenine from the α-sarcin/ricin loop of the large ribosomal RNA. Besides depurination of the rRNA, PAP has additional activities that contribute to its cytotoxicity. The mechanism of PAP cytotoxicity is summarized based on evidence from the analysis of transgenic plants and the yeast model system. PAP was initially found to be anti-viral when it was co-inoculated with plant viruses onto plants. Transgenic plants expressing PAP and non-toxic PAP mutants have displayed broad-spectrum resistance to both viral and fungal infection. The mechanism of PAP-induced disease resistance in transgenic plants is summarized.