Abstract
The refined high resolution crystal structure of the bovine phospholipase A2 was compared with its counterpart from the venom of Crotalus atrox, the western diamondbacked rattlesnake. The strong similarity in their backbone conformations forms the basis of a common numbering system for the amino acid sequence. The three common major helices and much of the extended chain form a nearly identical "homologous core" structure. The variations in conformation usually arise from deletions/insertions or en bloc shifts of structural units. The exception to this is part of the highly conserved calcium-binding loop; however, this is to be expected as 1) there is no calcium ion sequestered in the venom dimer as there is in the case of the bovine enzyme and 2) two side chains in that segment form dimer-stabilizing interactions between the subunits of the C. atrox enzyme. The absolutely conserved catalytic network of hydrogen-bonded side chains formed by His 48, Tyr 52, Tyr 73, and Asp 99, as well as the hydrophobic wall that shields it, are virtually superimposable in the two structures. However, the details of the structural relationship between the amino terminus and the catalytic network differ in the two species and the ordered water molecules thought to be either functionally or structurally important in the pancreatic enzymes are not found in the crystal structure of the phospholipase A2 from C. atrox. The most striking difference from a functional standpoint is the fact that the surface depression in the region of the catalytic network that has been commonly considered the active site is shielded substantially in forming the intersubunit contact surface of the dimeric venom enzyme.
Highlights
From the $Laboratorium voor Chemische Fysica,Rijksuniversiteit, Groningen, Nijenborgh 16, 9747 Groningen, The Netherlands, the §Department of Biochemistry and Molecular Biology, University of Chicago, Chicago, Illinois 60637, the
The variations cionnformation usually arise from deletions/insertionsor en bloc shifts of structural units. The exception to this is part of the highly conserved calcium-binding loop; this is to be expected as 1) there is no calcium ion sequestered in the venom dimer as there is in thecase of the bovine enzyme and 2) two side chains in that segment form dimer-stabilizing interactionsbetween the subunitsof the C. atrox enzyme
Most notable are thefollowingfunctional similarities. ( a )All ofthe enzymes require certain structural features of the phospholipid substrate; thatis, they will hydrolyze only esters and notamides at the Cz of the naturally occurring enantiomer of phosphatidic acid derivatives. ( b ) There is a marked enhancement of activity-sometimes by a factor as high as 105-when the phospholipid is condensed in a micellar aggregate
Summary
The variations cionnformation usually arise from deletions/insertionsor en bloc shifts of structural units The exception to this is part of the highly conserved calcium-binding loop; this is to be expected as 1) there is no calcium ion sequestered in the venom dimer as there is in thecase of the bovine enzyme and 2) two side chains in that segment form dimer-stabilizing interactionsbetween the subunitsof the C. atrox enzyme. ( b ) There is a marked enhancement of activity-sometimes by a factor as high as 105-when the phospholipid is condensed in a micellar aggregate Whereas both the proenzyme and enzyme hydrolyze monomeric soluble substrates at about the same slow rate, only the enzyme’s activity isenhanced by substrate aggregation.
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