Insight into the conformational dynamics of specific regions of porcine pancreatic phospholipase A2 from a time-resolved fluorescence study of a genetically inserted single tryptophan residue.
The effects of Ca2+ and substrate analogue binding on the conformational dynamics of porcine pancreas phospholipase A2 (PLA2) in different regions was explored by combining site-directed mutagenesis and time-resolved fluorescence measurements. The single tryptophan residue (Trp-3) of the wild-type protein (W3), in the alpha-helix A, was replaced by a phenylalanine residue (W3F), whereafter Trp was substituted either for leucine-31 (W31), located in the calcium binding loop, or for phenylalanine-94 (W94), located at the "back side" of the enzyme. Furthermore, mutants lacking the 62-66 sequence were constructed with the Trp at position 3 (delta W3) or 31 (delta W31). The total fluorescence intensity decays of Trp in each protein, in the protein-calcium and the protein-calcium-substrate analogue complexes, analyzed by the maximum entropy method (MEM) can be interpreted as distributions of separated lifetime classes. In the case of the W94 mutant, a major short-lived excited-state population (tau approximately 50 ps) is observed, probably deactivated by the interaction with two proximate disulfide bridges via a radiationless process. For the four other mutants, the respective barycenters of the four lifetime classes display comparable values, but the amplitude distributions are different for Trp-3 and Trp-31. The rotational mobility of the Trp residue varies along the peptide chain. Trp-3 experiences only a fast hindered motion. Trp-31 is sensitive to an additional local flexibility that is absent in the N-terminal part of the protein. The largest wobbling angle is observed at position 94. No effect of calcium binding occurs on the lifetime distribution of the Trp-3 and Trp-94 residues. Their mobilities are not affected. In contrast, calcium binding displays a strong influence on the excited-state population distribution of Trp-31. A major population decaying with the longest lifetime is selected in the W31 protein and contributes to approximately 50% of the decay. The local flexibility and the amplitude of motion of Trp-31 is wider in the protein-calcium complex than in the unliganded protein. Binding of the monomeric substrate analogue n-dodecylphosphocholine (C12PN) in the presence of calcium slightly affects the Trp-3 excited-state population distribution and its mobility. Trp-31 is more sensitive to this binding. In particular, a more restricted rotation of the Trp-31 residue and a decrease of the peptide local flexibility as protein-calcium complexes are observed in both the W31 and delta W31 mutants.(ABSTRACT TRUNCATED AT 400 WORDS)
Kuipers
O P
OP
Laboratoire pour l'Utilisation du Rayonnement Electromagnétique, Centre National de la Recherche Scientifique, Orsay, France.
Vincent
M
M
Brochon
J C
JC
Verheij
H M
HM
de Haas
G H
GH
Gallay
J
J
D000818:Animals; D001483:Base Sequence; D002118:Calcium / Q000737:chemistry; D005454:Fluorescence Polarization; D007700:Kinetics; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010179:Pancreas / Q000201:enzymology*; D010455:Peptides / Q000737:chemistry / Q000502:physiology; D010741:Phospholipases A / Q000737:chemistry* / Q000235:genetics; D054467:Phospholipases A2; D011485:Protein Binding; D011487:Protein Conformation; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D013552:Swine; D014364:Tryptophan / Q000235:genetics* / Q000502:physiology
D010455:Peptides; D014364:Tryptophan; D010741:Phospholipases A; D054467:Phospholipases A2; D002118:Calcium