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Structure and function of microplasminogen: I. Methionine shuffling, chemical proteolysis, and proenzyme activation
1Department of Pharmacological Sciences, SUNY at Stony Brook 11794-8651, USA.
Abstract:
We have cloned and expressed microplasminogen (mPlg), consisting of the N-terminal undecapeptide of human glu-Plg spliced to its proenzyme domain. This truncated (approximately 28 kDa) proenzyme retained the distinctive catalytic activities of the larger parent. Replacement of M residues followed by M shuffling permitted subsequent scission by site-directed chemical proteolysis (in CNBr/formic acid) without impairing any of the protein's characteristic properties. Activation of chymotrypsinogen-related zymogens occurs by limited proteolysis; the newly liberated, highly conserved N-terminus (VVGG) forms a salt bridge with an aspartyl residue immediately upstream of the active site serine. The role of both of these elements in mPlg activation was probed using protein engineering and site-directed proteolysis to alter the length and amino acid composition of the N-terminus, and to replace the aspartate. All modifications affected both Km and Kcat. The results identify some structural parameters of the N-terminus required for proenzyme activation.
Insights
Researchers engineered microplasminogen (mPlg), a smaller plasminogen variant. Modifications to its N-terminus and an aspartate residue revealed key structural requirements for enzyme activation.
Area of Science:
- Biochemistry
- Molecular Biology
- Enzymology
Background:
- Plasminogen (Plg) activation is crucial for fibrinolysis.
- Zymogen activation typically involves limited proteolysis and specific N-terminal interactions.
- Understanding these mechanisms is vital for developing targeted therapies.
Purpose of the Study:
- To investigate the structural requirements for microplasminogen (mPlg) activation.
- To explore the role of the N-terminus and an aspartate residue in catalytic activity.
- To engineer a catalytically active, truncated plasminogen variant.
Main Methods:
- Cloning and expression of microplasminogen (mPlg).
- Site-directed chemical proteolysis using CNBr/formic acid.
- Protein engineering to modify the N-terminus and aspartate residue.
- Enzyme kinetic analysis (Km and Kcat measurements).
Main Results:
- Engineered mPlg (approx. 28 kDa) retained catalytic activity.
- Chemical proteolysis enabled efficient mPlg generation without loss of function.
- Alterations to the N-terminus and aspartate residue significantly impacted Km and Kcat.
- Identified specific structural parameters of the N-terminus essential for proenzyme activation.
Conclusions:
- The N-terminal undecapeptide and its interaction with an aspartate residue are critical for plasminogen activation.
- Engineered mPlg serves as a valuable tool for studying plasminogen activation mechanisms.
- Findings provide insights into the structural basis of zymogen activation.