Abstract
Background: Protein S is a central regulator of coagulation as it critically participates in down-regulation of both extrinsic and intrinsic pathways of the coagulation cascade. In this review, we aim to provide an update on protein S and its anticoagulant functions as a central hemostatic regulator. Methods: Electronic databases including, Google, Google Scholar, PMC, PubMed, Science Direct, and Scopus were rigorously searched using the terms protein S, hemostasis, natural anticoagulants, regulators of coagulation, and coagulation inhibitors for the completion of this descriptive review. Results: Literature review shows that protein S is a potent cofactor for activated protein C (APC) in the regulation of the intrinsic pathway and a cofactor for tissue factor pathway inhibitor (TFPI) in the regulation of the extrinsic pathway. The strong association between protein S deficiency either hereditary or acquired and increased risk for venous thrombosis indicates the important and central role of protein S in controlling the initiation and propagation phase of coagulation cascade and that protein S is an important determinant for optimal activity of both APC and TFPI in coagulation regulation. Conclusions: Available evidence suggests that the role of protein S in the down-regulation of blood coagulation is mainly mediated through its high affinity binding to negatively charged phospholipid surfaces. This high affinity binding to negatively charged phospholipids helps bring the anticoagulant proteins to the membranes, resulting in efficient and targeted regulation of coagulation. In the shade of current COVID-19 pandemic, protein S deficiency has been found to be a leading cause of thrombotic complications associated with COVID-19.