Fluorescence investigations on choline phospholipid binding and chemical unfolding of HSP-1/2, a major protein of horse seminal plasma.

The research studied the interaction of seminal fibronectin type-II proteins with phospholipids found in sperm plasma membrane, primarily focused on HSP-1/2, a primary protein in horse seminal plasma. The article contains the results of several tests measuring aspects relating to the microenvironment of tryptophan residues in HSP-1/2 and how it reacts to different conditions.

Investigating Tryptophan Residues in HSP-1/2

• Fluorescent spectroscopic studies were carried out to examine the heterogeneity and microenvironment of tryptophan residues in HSP-1/2. Tryptophan residues are crucial for the interaction of Fn-II proteins with choline phospholipids.
• The researchers tested different states: native, in presence of PrC and phosphatidylcholines with both short (valeryl, C-5) and long (myristoyl, C-14) chains, and upon denaturation using fluorescence quenching, time-resolved fluorescence, and red-edge excitation shift measurements.

Results of the Investigation

• The microenvironment of tryptophan residues in HSP-1/2 was found to be more heterogeneous than in PDC-109, its bovine counterpart.
• Binding of choline containing ligands provided protection to the tryptophan residues. The shielding strength was highest for dimyristoyl PC and least for PrC.
• The red-edge excitation shift value for HSP-1/2 was smaller than that for PDC-109. Binding to PrC and DVPC reduced the REES substantially.

Chemical Denaturation of HSP-1/2

• The researchers found that HSP-1/2 only partially unfolded when exposed to chemical denaturants and lacked cooperativity, which is the synchronization of folding or unfolding of proteins in a group.
• The protein exhibited complete unfolding in the presence of 10mM dithiothreitol, indicating that disulfide linkages prevented total unfolding of the protein under usual circumstances.
• The study also found that presence of PrC caused transition midpoints to shift to higher concentrations of denaturant. Therefore, ligand binding and polydispersity were found to affect the unfolding process.