RESUMO
In C4 grasses of agronomical interest, malate shuttled into the bundle sheath cells is decarboxylated mainly by nicotinamide adenine dinucleotide phosphate (NADP)-malic enzyme (C4-NADP-ME). The activity of C4-NADP-ME was optimized by natural selection to efficiently deliver CO2 to Rubisco. During its evolution from a plastidic non-photosynthetic NADP-ME, C4-NADP-ME acquired increased catalytic efficiency, tetrameric structure and pH-dependent inhibition by its substrate malate. Here, we identified specific amino acids important for these C4 adaptions based on strict differential conservation of amino acids, combined with solving the crystal structures of maize and sorghum C4-NADP-ME. Site-directed mutagenesis and structural analyses show that Q503, L544 and E339 are involved in catalytic efficiency; E339 confers pH-dependent regulation by malate, F140 is critical for the stabilization of the oligomeric structure and the N-terminal region is involved in tetramerization. Together, the identified molecular adaptations form the basis for the efficient catalysis and regulation of one of the central biochemical steps in C4 metabolism.
Assuntos
Malato Desidrogenase/química , Malato Desidrogenase/metabolismo , Proteínas de Plantas/química , Proteínas de Plantas/metabolismo , Sorghum/enzimologia , Zea mays/enzimologia , Motivos de Aminoácidos , Biocatálise , Domínio Catalítico , Concentração de Íons de Hidrogênio , Malato Desidrogenase/genética , Malatos/metabolismo , Fotossíntese , Proteínas de Plantas/genética , Sorghum/química , Sorghum/genética , Zea mays/química , Zea mays/genéticaRESUMO
Pressure can shift the polymer-monomer equilibrium of Aß, increasing pressure first leads to a release of Aß-monomers, surprisingly at pressures higher than 180 MPa repolymerization is induced. By high pressure NMR spectroscopy, differences of partial molar volumes ΔV0 and compressibility factors Δß' of polymerization were determined at different temperatures. The d-enantiomeric peptides RD2 and RD2D3 bind to monomeric Aß with affinities substantially higher than those determined for fibril formation. By reducing the Aß concentration below the critical concentration for polymerization they inhibit the formation of toxic oligomers. Chemical shift perturbation allows the identification of the binding sites. The d-peptides are candidates for drugs preventing Alzheimer's disease. We show that RD2D3 has a positive effect on the cognitive behaviour of transgenic (APPSwDI) mice prone to Alzheimer's disease. The heterodimer complexes have a smaller Stokes radius than Aß alone indicating the recognition of a more compact conformation of Aß identified by high pressure NMR before.