Sequence/structure relationships have been explored by site-directed mutagenesis using a structurally known adenylate kinase. In particular the effects of helix capping and nonpolar core expansion on thermodynamic stability have been analyzed. Six point mutations were produced and characterized by SDS/PAGE, native PAGE, isoelectric focussing, electrophoretic titration, enzyme kinetics, and X-ray structure analysis ...
Sequence/structure relationships have been explored by site-directed mutagenesis using a structurally known adenylate kinase. In particular the effects of helix capping and nonpolar core expansion on thermodynamic stability have been analyzed. Six point mutations were produced and characterized by SDS/PAGE, native PAGE, isoelectric focussing, electrophoretic titration, enzyme kinetics, and X-ray structure analysis. Heat-denaturation experiments yielded melting temperatures Tm and melting enthalpy changes delta Hm. The heat capacity change delta Cp of the wild-type enzyme was determined by guanidine hydrochloride denaturation in conjunction with Tm and delta Hm. Using the wild-type delta Cp value, Gibbs free energy changes delta G at room temperature were calculated for all mutants. Four mutants were designed for helix capping stabilization, but only one of them showed such an effect. Because of electrostatic interference with the induced-fit motion, one mutant decreased the catalytic activity strongly. Two mutants expanded nonpolar cores causing destabilization. The mutant with the lower stability could be crystallized and subjected to an X-ray analysis at 223-pm resolution which showed the structural changes. The enzyme was stabilized by adding a -Pro-His-His tail to the C-terminal alpha-helix for nickel-chelate chromatography. This addition constitutes a helix cap. Taken together, the results demonstrate that stabilization by helix capping is difficult to achieve because the small positive effect is drowned by adverse mutational disruption. Further addition of atoms to nonpolar cores destabilized the protein, although the involved geometry changes were very small, demonstrating the importance of efficient packing.
Related Citations:
High-Resolution Structures of Adenylate Kinase from Yeast Ligated with Inhibitor Ap5A, Showing the Pathway of Phosphoryl Transfer Abele, U., Schulz, G.E. (1995) Protein Sci 4: 1262
The C-DNA Sequence Encoding Cytosolic Adenylate Kinase from Baker'S Yeast (Saccharomyces Cerevisiae) Proba, K., Tomasselli, A.G., Nielsen, P., Schulz, G.E. (1987) Nucleic Acids Res 15: 7187
Structure of the Complex of Yeast Adenylate Kinase with the Inhibitor Ap5A at 2.6 Angstroms Resolution Egner, U., Tomasselli, A.G., Schulz, G.E. (1987) J Mol Biol 195: 649
The Complete Amino Acid Sequence of Adenylate Kinase from Baker'S Yeast Tomasselli, A.G., Mast, E., Janes, W., Schiltz, E. (1986) Eur J Biochem 155: 111
Organizational Affiliation:
Institut für Organische Chemie und Biochemie, Albert-Ludwigs-Universität, Freiburg im Breisgau, Germany.
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