title: Folding pathway of the pyridoxal 5'-phosphate C-S lyase MalY from Escherichia coli.
abstract: MalY from Escherichia coli is a bifunctional dimeric PLP (pyridoxal 5'-phosphate) enzyme acting as a beta-cystathionase and as a repressor of the maltose system. The spectroscopic and molecular properties of the holoenzyme, in the untreated and NaBH4-treated forms, and of the apoenzyme have been elucidated. A systematic study of the urea-induced unfolding of MalY has been monitored by gel filtration, cross-linking, ANS (8-anilino-1-naphthalenesulphonic acid) binding and by visible, near- and far-UV CD, fluorescence and NMR spectroscopies under equilibrium conditions. Unfolding proceeds in at least three stages. The first transition, occurring between 0 and 1 M urea, gives rise to a partially active dimeric species that binds PLP. The second equilibrium transition involving dimer dissociation, release of PLP and loss of lyase activity leads to the formation of a monomeric equilibrium intermediate. It is a partially unfolded molecule that retains most of the native-state secondary structure, binds significant amounts of ANS (a probe for exposed hydrophobic surfaces) and tends to self-associate. The self-associated aggregates predominate at urea concentrations of 2-4 M for holoMalY. The third step represents the complete unfolding of the enzyme. These results when compared with the urea-induced unfolding profiles of apoMalY and NaBH4-reduced holoenzyme suggest that the coenzyme group attached to the active-site lysine residue increases the stability of the dimeric enzyme. Both holo- and apo-MalY could be successfully refolded into the active enzyme with an 85% yield. Further refolding studies suggest that large misfolded soluble aggregates that cannot be refolded could be responsible for the incomplete re-activation.
authors.affiliation: Dipartimento di Scienze Neurologiche e della Visione, Sezione di Chimica Biologica, Facoltà di Medicina e Chirurgia, Università degli Studi di Verona, Strada Le Grazie, 8, 37134 Verona, Italy.
authors.forename: Mariarita
authors.initials: M
authors.lastname: Bertoldi
authors.forename: Barbara
authors.initials: B
authors.lastname: Cellini
authors.forename: Douglas V
authors.initials: DV
authors.lastname: Laurents
authors.forename: Carla
authors.initials: C
authors.lastname: Borri Voltattorni
mesh_terms: D001665:Binding Sites; D003542:Cystathionine gamma-Lyase / Q000737:chemistry*; D004926:Escherichia coli / Q000201:enzymology*; D029968:Escherichia coli Proteins / Q000737:chemistry*; D007700:Kinetics; D011485:Protein Binding; D017510:Protein Folding; D011732:Pyridoxal Phosphate / Q000737:chemistry*; D012097:Repressor Proteins / Q000737:chemistry*; D014508:Urea / Q000737:chemistry
chemical_list: D029968:Escherichia coli Proteins; D012097:Repressor Proteins; C079405:MalY protein, E coli; D011732:Pyridoxal Phosphate; D014508:Urea; D003542:Cystathionine gamma-Lyase