Abstract
The interaction of the major bovine seminal plasma protein, PDC-109 with lipid membranes was investigated by isothermal titration calorimetry. Binding of the protein to model membranes made up of diacyl phospholipids was found to be endothermic, with positive values of binding enthalpy and entropy, and could be analyzed in terms of a single type of binding sites on the protein. Enthalpies and entropies for binding to diacylphosphatidylcholine membranes increased with increase in temperature, although a clear-cut linear dependence was not observed. The entropically driven binding process indicates that hydrophobic interactions play a major role in the overall binding process. Binding of PDC-109 with dimyristoylphosphatidylcholine membranes containing 25 mol% cholesterol showed an initial increase in the association constant as well as enthalpy and entropy of binding with increase in temperature, whereas the values decreased with further increase in temperature. The affinity of PDC-109 for phosphatidylcholine increased at higher pH, which is physiologically relevant in view of the basic nature of the seminal plasma. Binding of PDC-109 to Lyso-PC could be best analysed in terms of two types of binding interactions, a high affinity interaction with Lyso-PC micelles and a low-affinity interaction with the monomeric lipid. Enthalpy-entropy compensation was observed for the interaction of PDC-109 with phospholipid membranes, suggesting that water structure plays an important role in the binding process.
Highlights
The seminal plasma in mammals serves as a carrier of freshly ejaculated spermatozoa through the female genital tract to their final destination, the uterus
The major protein fraction of bovine seminal plasma is composed of four acidic proteins designated as BSP-A1, BSP-A2, BSP-A3 and BSP30 kDa, which are collectively referred to as bovine seminal plasma proteins, or as BSP proteins [5,6]
It is clear from this figure that the turbidity of DMPC vesicles decreases abruptly at about 0.03 mM protein concentration and levels off
Summary
The seminal plasma in mammals serves as a carrier of freshly ejaculated spermatozoa through the female genital tract to their final destination, the uterus. During this passage spermatozoa undergo a series of biochemical and ultrastructural changes – collectively referred to as capacitation – a necessary event before they attain the ability to fertilize the egg [1,2]. Homologues of BSP protein are present in the seminal plasma of other mammalian species such as stallion, pig, goat etc [7,8,9,10,11]. These observations show that the BSP family of proteins are widely distributed in mammalian seminal plasma, exist in several forms in each species and may play a common biological role
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