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PubMed1995

Inhibition of the glycine decarboxylase multienzyme complex by the host-selective toxin victorin.

Victoria blight of oats is caused by the fungus Cochliobolus victoriae. This fungus is pathogenic due to its ability to produce the host-selective toxin victorin. We previously identified a 100-kD protein that binds victorin in vivo only in susceptible genotypes and a 15-kD protein that binds victorin in vivo in both susceptible and resistant genotypes. Recently, we determined that the oat 100-kD victorin binding protein is the P protein of the glycine decarboxylase complex (GDC). In this study, we examined the effect of victorin on glycine decarboxylase activity (GDA). Victorin was a potent in vivo inhibitor of GDA. Leaf slices pretreated for 2 hr with victorin displayed an effective concentration for 50% inhibition (EC50) of 81 pM for GDA. Victorin inhibited the glycine-bicarbonate exchange reaction in vitro with an EC50 of 23 microM. We also identified a 15-kD mitochondrial protein that bound victorin in a ligand-specific manner. Based on amino acid sequence analysis, we concluded that the 15-kD mitochondrial protein is the H protein component of the GDC. Thus, victorin specifically binds to two components of the GDC. GDA in resistant tissue treated with 100 micrograms/mL victorin for 5 hr was inhibited 26%, presumably as a consequence of the interaction of victorin with the H protein. Victorin had no detectable effect on GDA in isolated mitochondria, apparently due to the inability of isolated mitochondria to import victorin. These results suggest that the interaction of victorin with the GDC is central to victorin's mode of action.

Amino Acid Oxidoreductases↗
PubMed1986

Immunological and biosynthetic studies on the mammalian 2-oxoglutarate dehydrogenase multienzyme complex.

High-titre, monospecific, polyclonal antisera have been raised against purified mitochondrial 2-oxoglutarate dehydrogenase complex (OGDC) from ox heart and two of its three constituent enzymes, 2-oxoglutarate dehydrogenase (E1) and lipoyl succinyltransferase (E2). These specific antisera have been employed to monitor molecular events in the biosynthesis, import and maturation of this multimeric assembly. Lipoamide dehydrogenase (E3) elicits a poor antibody response in comparison to the other polypeptides of the complex. In cultured pig kidney cells (PK-15), incubated with [35S]methionine in the presence of uncouplers of oxidative phosphorylation, appearance of stable higher-Mr forms of the individual enzymes can be detected by specific immunoprecipitation and fluorographic analysis. In the case of 2-oxoglutarate dehydrogenase, E1, the initial cytoplasmic translation product has a subunit Mr value of 1500-3000 greater than in the mature enzyme while the precursor of the lipoyl succinyltransferase, E2, contains an additional sequence of Mr 6000-8000. Competition studies have revealed the immunological similarity of the precursor molecules to the native subunits. On removal of uncouplers, processing of accumulated precursors is rapidly initiated and is complete within 40 min. Interestingly, antiserum to native 2-oxoglutarate dehydrogenase complex fails to recognise E2 precursor molecules (pre-E2), which can be immunoprecipitated, however, by antibodies raised against the denatured E2 subunit. It is concluded that pre-E2 is conformationally dissimilar to native E2, which exists normally as a highly ordered, multimolecular aggregate in the native complex.

Animals↗
PubMed1986

Glycine decarboxylase multienzyme complex. Purification and partial characterization from pea leaf mitochondria.

The P, H, and T proteins of the glycine cleavage system have been purified separately from pea leaf mitochondria and demonstrate molecular weights of 98,000, 15,500, and 45,000, respectively, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular weight of P protein by gel filtration was 210,000, indicating that this enzyme has a native homodimer conformation. Reconstitution assays containing purified P, H, and T proteins and yeast lipoamide dehydrogenase catalyze the oxidation of glycine and demonstrate a strict dependence on pyridoxal phosphate, tetrahydrofolate, NAD+, and dithiothreitol. The released CO2, methylamine-H protein intermediate, and methylenetetrahydrofolate are produced in stoichiometric amounts from glycine during the cleavage reaction. H protein acts as co-substrate with glycine during the decarboxylation reaction, demonstrating an apparent Km value of 2.2 microM. P and H protein alone jointly catalyze the glycine carboxyl-14 CO2 exchange reaction in the presence of pyridoxal phosphate and dithiothreitol. L protein of the glycine cleavage system was immunopurified using monoclonal antibodies. Antigenic and molecular weight similarities of the L protein with the lipoamide dehydrogenase component of the pyruvate dehydrogenase complex were shown suggesting the possibility of common isomers of lipoamide dehydrogenase for the two enzyme complexes.

Amino Acid Oxidoreductases↗
PubMed1993

The phenyl propanoid pathway enzymes in Solanum tuberosum exist as a multienzyme complex.

The elution profile of the core sequence enzymes of the phenyl propanoid pathway, namely phenyl alanine ammonia lyase, t-cinnamic acid 4-hydroxylase and p-coumaryl CoA ligase, on AcA 34 column suggested the existence of a high molecular form (P1) and a low molecular form (P2) for all the three enzymes. All the P1 forms eluted together in same fractions, while the P2 forms eluted out according to their respective molecular mass. Rechromatography of P1 form under identical conditions showed a similar elution profile (Q1 and Q2 forms). Further, the Q1 form did not show any significant increase in specific activity when compared to the P1 form. These results suggested the possibility of these enzymes existing as a protein cluster. Further confirmation was obtained on repeated column chromatography of the Q1 form in presence of 0.1 M KCl which did not result in complete dissociation of the complex to its individual enzyme components. The identification of the subunit polypeptide of the individual enzyme components in the multi enzyme complex and the in vitro demonstration of the phenyl propanoid core pathway reaction sequence using phenylalanine alone as a substrate supplementing the required cofactors for appropriate reactions substantiated that at least the core enzymes of the phenyl propanoid sequence existed as a multi enzyme complex.

Coenzyme A Ligases↗
PubMedDate not supplied

[Symmetry of multienzyme complexes].

A model for studying the symmetry of stable states arising from polyenzymic complex conformations is proposed. A formal scheme of submolecular structure self-assembly, on which the model is based, enables it not only to limit the class of conformations but in some cases to determine the structure of a complex in an unambigous manner. The model is shown in its application to polyenzymic complexes of dehydrogenases of alpha-keto acids.

Azotobacter↗