Tomato pectin methylesterase: modeling, fluorescence, and inhibitor interaction studies-comparison with the bacterial (Erwinia chrysanthemi) enzyme.
The molecular model of Lycopersicon esculentum (tomato) pectin methylesterase (PME) was built by using the X-ray crystal structure of PME from the phytopathogenic bacterium Erwinia chrysanthemi as a template. The overall structure and the position of catalytically important residues (Asp132, Asp 153, and Arg 221, located at the bottom of the active site cleft) are conserved. Instead, loop regions forming the walls of the catalytic site are much shorter and form a less deep cleft, as already revealed by the carrot PME crystal structure. The protein inhibitor of pectin methylesterase (PMEI) isolated from kiwi fruit binds tomato PME with high affinity. Conversely, no complex formation between the inhibitor and PME from E. chrysanthemi is observed, and the activity of this enzyme is unaffected by the presence of the inhibitor. Fluorescence quenching experiments on tomato PME and on PME-PMEI complex suggest that tryptophanyl residues present in the active site region are involved in the interaction and that the inhibitor interacts with plant PME at the level of the active site. We also suggest that the more open active site cleft of tomato PME allows the interaction with the inhibitor. Conversely, the narrow and deep cleft of the active site of E. chrysanthemi PME hinders this interaction. The pH-dependent changes in fluorescence emission intensity observed in tomato PME could arise as the result of protonation of an Asp residue with unusually high pKa, thus supporting the hypothesis that Asp132 acts as acid/base in the catalytic cycle.
D'Avino
Rossana
R
Institute of Protein Biochemistry, CNR, Napoli, Italy.
Camardella
Laura
L
Christensen
Tove M I E
TM
Giovane
Alfonso
A
Servillo
Luigi
L
D000595:Amino Acid Sequence; D002265:Carboxylic Ester Hydrolases / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics*; D002586:Cesium / Q000737:chemistry; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D018552:Daucus carota / Q000201:enzymology; D004791:Enzyme Inhibitors / Q000737:chemistry / Q000494:pharmacology; D004885:Erwinia / Q000201:enzymology; D007454:Iodides / Q000737:chemistry; D007527:Isoenzymes / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics; D018551:Solanum lycopersicum / Q000201:enzymology*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013050:Spectrometry, Fluorescence
D004791:Enzyme Inhibitors; D007454:Iodides; D007527:Isoenzymes; D002586:Cesium; D002265:Carboxylic Ester Hydrolases; C022323:pectinesterase