Difference in enzymatic properties between "staphylothrombin" and free alpha-thrombin

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Chemistry

Background:

• Staphylocoagulase (STC) is a bacterial protein that binds human prothrombin (hPT).
• This interaction forms a coagulant complex, staphylothrombin (ST), with distinct enzymatic properties compared to alpha-thrombin.
• Understanding the STC region responsible for hPT interaction is key to elucidating its mechanism.

Purpose of the Study:

• To identify the specific region of staphylocoagulase responsible for binding and activating human prothrombin.
• To characterize the functional and binding properties of staphylocoagulase fragments.

Main Methods:

• Limited alpha-chymotryptic cleavage of staphylocoagulase to generate fragments.
• Affinity measurements (Kd) of fragments for human prothrombin.
• Assays for clotting and amidase activities of STC-hPT complexes.
• NH2-terminal sequencing of STC fragments.

Main Results:

• Three fragments (43, 30, 20 kDa) were generated from staphylocoagulase cleavage.
• The 43-kDa N-terminal fragment showed high affinity for hPT (Kd = 1.7 nM) and reconstituted clotting and amidase activities.
• The 30-kDa fragment had weaker affinity and only weak amidase activity, while the 20-kDa fragment only bound hPT.
• NH2-terminal sequencing confirmed the 43-kDa fragment contains the entire functional binding region.

Conclusions:

• The N-terminal region of staphylocoagulase, specifically within the 43-kDa fragment, harbors the functional site for human prothrombin binding and activation.
• This finding localizes the key interaction domain of staphylocoagulase, providing insights into coagulant complex formation.