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Three-dimensional structure of the human plasmin alpha2-macroglobulin complex

S J Kolodziej1, H U Klueppelberg, N Nolasco

  • 1Department of Pathology and Laboratory Medicine, The University of Texas Health Science Center, Houston, Texas, 77030, USA.

Journal of Structural Biology
|December 9, 1998
PubMed
Summary

Three-dimensional electron microscopy reveals how human plasmin binds alpha2-macroglobulin. This complex prevents plasmin from degrading proteins by trapping its active site within the alpha2-macroglobulin cavity.

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Area of Science:

  • Biochemistry
  • Structural Biology
  • Molecular Medicine

Background:

  • Alpha2-macroglobulin (α2M) is a large plasma proteinase inhibitor.
  • Plasmin is a key enzyme in fibrinolysis, involved in blood clot breakdown.

Purpose of the Study:

  • To determine the three-dimensional structure of the human plasmin-α2M complex.
  • To elucidate the mechanism of plasmin inhibition by α2M.

Main Methods:

  • Three-dimensional reconstructions using electron microscopy of stained and frozen-hydrated specimens.
  • Analysis of the structural arrangement of the binary complex.

Main Results:

  • The human plasmin-α2M complex has dimensions of approximately 170 x 140 x 140 Å.

Related Experiment Videos

  • Plasmin's C-terminal catalytic domain is entrapped within the α2M cavity, while N-terminal domains protrude.
  • The complex exhibits asymmetry due to the plasmin structure, unlike complexes with methylamine or chymotrypsin.

    • Conclusions:

    • The entrapment mechanism prevents plasmin's catalytic site from accessing plasma proteins.
    • Plasmin enters the α2M cavity after bait-domain cleavage, requiring α2M conformational changes.
    • This structural insight clarifies a crucial regulatory mechanism in protein degradation and fibrinolysis.