Circulation and Coagulation
Why Vitamin K Cannot ‘Over-Clot’ Your Blood
- Vitamin K cannot over-clot your blood.
- Vitamin K is required both for circulation and for coagulation.
- Low levels of vitamin K can disrupt the circulation system, and lead to unnecessary clotting risk.
- High levels of vitamin K ensure that the circulation system can function effectively, as K activates anti-coagulation proteins.
- High levels of K are needed to activate the coagulation system so it functions effectively.
- Vitamin K does not trigger clotting, it only ensures that the clotting and anti-clotting systems work effectively.
This page will describe the circulatory system, which keeps blood circulating through the body and the coagulation system, which ensures that blood will properly coagulate and form a clot, if an injury has occurred. This review will cover the primary elements of these very complex systems, as they would take place in a healthy person. Both of these systems depend on sufficient amounts of vitamin K being available. Vitamin K does not initiate the formation of a blood clot, nor does it resolve or dissolve a clot. However, vitamin K does improve the functioning of both these systems. The key role of vitamin K in making sure these systems work effectively, and the research behind it will be reviewed and explained.
The following is the peer reviewed research behind vitamin K and its role in these systems.
CIRCULATORY SYSTEM
The circulatory system is enormous, containing approximately 60,000 miles of blood vessels, arteries and veins. It’s charged with delivering nutrients and oxygen to every cell and on the return trip, it removes waste products from the lungs, kidneys, spleen, and liver. Blood is the medium that makes all this possible with help from the heart.
Blood is a liquid that circulates throughout the body in every artery, vein and capillary. Blood flows freely, yet at the same time, it contains the capability of engaging instantaneously, in the formation of a lifesaving clot, within a matter of seconds, localized to the area of need, while the rest of the blood continues to flow through the body, undisturbed.
This delicate balance is achieved because the body has chemical elements that are anti-coagulant most of the time, maintaining the circulating blood in a fluid state, and chemical elements that are pro-coagulant, and prepared to clot when needed. This ‘dance’ is accomplished by an interactive and highly regulated network of proteins contained in the blood that maintain the flow, yet which can coagulate in seconds.
Our blood has two main parts, blood cells and plasma. The plasma is the liquid part of the blood. It is mainly water. It contains proteins, fats, salt, and other substances. Those proteins are sent into the bloodstream, typically from the liver, to circulate around the body, in an inactive form, ready for action at any time, poised to participate in blood coagulation upon tissue insult or injury. Some of the proteins work to form a clot and are called clotting factors.
Red blood cells carry oxygen to all parts of the body, and carry back carbon dioxide to the lungs. White blood cells help fight infection. Platelets are tiny blood cells that help stop bleeding. Platelets are produced in bone marrow. Platelets circulate in the blood in an inactive, resting form for an average of 10 days. The exterior of a platelet, its membrane, is coated with receptors that are targeted by various proteins and this membrane is a key element in clotting.
If there is any injury, the circulatory system must work to stop the leak, and repair the ‘pipe’. That is when the coagulation cascade begins. In the cascade, a series of actions take place, where blood is transformed from a liquid into what becomes eventually becomes a blood clot.
The success of this system all depends on the vitamin K. Vitamin K is a necessary ingredient in the circulation of blood throughout the body. The presence of vitamin K is necessary both for the coagulation capacity that it is well known for, but also for the fluidity and the anti-coagulant properties, which is not as well known (Jesty & Beltrami, 2005).
Vitamin K is well known for its role in coagulation, and it can be a common misconception that if one takes a vitamin K supplement, the additional amount of vitamin K would result in your blood coagulating excessively, or ‘over clotting’, plugging your veins and you would die.
This outcome is impossible and the following information will help provide a background on the science of coagulation, the function of vitamin K, and why it is impossible to ‘plug up’ or ‘over clot’ your blood.
ANTI-COAGULATION
Typically, blood maintains its fluidity as it circulates throughout the body. This is referred to as an anti-thrombogenic or anti-coagulant state. This fluidity is no accident, but is due to a precise and balanced regulatory control by the anticoagulant system. The system includes certain processes and mechanisms to keep the coagulation cascade in check (Colvin, 2004), and actively works to prevent excess coagulation, which could produce widespread, unnecessary clots. It is also involved in localizing a clot, so that it only forms where needed at the site of an injury. The main anticoagulant mechanisms naturally present in the body include the following:
Platelets.
Platelets participate in the anticoagulant pathways. They secrete several inhibitors which serve to terminate the procoagulant process. For example, platelets release the enzyme plasminogen, which degrades the clot, and initiates tissue repair (Lanzer & Topol, 2013).
Vessel Wall.
Another mechanism is the vessel wall, which contains endothelial cells. Endothelial cells line the wall of the blood vessel and secrete coagulation inhibitors, like protein S on an ongoing basis (Stern et al, 1985; Fair et al, 1986). These inhibitors can rapidly inactivate the enzymes that promote coagulation enzymes. Some of the primary coagulation inhibitors secreted by the endothelial cells are discussed below:
One primary inhibitor is thrombomodulin, which is a membrane protein and is expressed on the surface of endothelial cells and serves as a cofactor for thrombin. Thrombin is a key enzyme of coagulation, however thrombomodulin can convert thrombin to an anticoagulant (Moussa, 2011; Yasuda, 2016). Thrombin also participates as an anti-coagulant by activating plasminogen to plasmin, which degrades clots while stimulating the production of other enzymes that inhibit coagulation.
Endothelial cells also secrete PA1-1 (plasminogen activator inhibitor) which helps convert plasminogen to plasmin, and creates resistance to clotting (Phillips et al, 1984). Plasmin will dissolve the old fibrin at injury sites and any fibrin which may be deposited in normal vessels, as well as fibrin blood clots (van Hinsbergh et al, 1985; Sakata et al, 1985).
Endothelial cells also produce tissue factor pathway inhibitor (TFPI), an anticoagulant protein. Tissue factor pathway inhibitor acts as a potent, natural inhibitor of the coagulation pathway, inhibiting major enzymatic complexes that would initiate the coagulation cascade (Broze, 1995; Bauer & Zwicker, 2003).
The endothelium also regulates the activities of circulating blood cells. When undisturbed, the endothelium actively prevents platelets from gathering together. Interactions between endothelial cells, platelets, and white blood cells are prohibited. These interactions are mediated by a wide array of soluble compounds and adhesion molecules (Celi & Lorenzet, 1997).
The endothelial cells also provide receptors for proteins in the blood that interfere with clot formation, (protein C), and promotes protein C pathway effects (Esmon, 1987; Esmon, 1989). Protein C is a major anti-coagulant. This means that unwanted clots are not formed, and embolisms don’t cause injury (McCarron, Lee & Wilson, 2017).
Circulating Elements
There are chemical elements circulating through the bloodstream that are anticoagulant forces.
A naturally occurring anticoagulant protein produced in the liver and circulating through the body, is antithrombin (AT), which functions as a mild blood thinner. It acts like a police protein that prevents you from clotting too much. Antithrombin binds coagulation factors, such as thrombin, which then form inert complexes, inactivating their clotting potential. This action is greatly enhanced (a thousand fold) by the presence of heparin, a substance formed by connective tissue cells (More et al, 1993; Liu & Rodgers, 1996; Broze, 1995; Stern et al, 1985; Shuman, 1986; Casu et al, 1981; Lindahl et al, 1980, Marcum & Rosenberg, 1984; Tollefsen & Pestka, 1985).
Also circulating are Protein C and S, which are proteins in the blood that help regulate blood clot formation. They work in concert as a natural anticoagulant system, mainly by inactivating clotting factors V and VIII, which are required for clot generation. Protein C works in conjunction with Protein S.
Vitamin K
Vitamin K has an important role in regulating the anti-coagulation system, and keeping the blood fluid (Espana et al., 2005). While vitamin K is primarily known for its role in clotting, it is also a key component in preventing blood clots. There are anticoagulation proteins which all depend on sufficient amounts of Vitamin K to be functional and, thus, active. These elements are known as protein C, protein S, and protein Z. (Esmon et al 1987: Lobato-Mendizabal & Ruiz-Arguelles, 1990; Grober et al, 2015).
Note: Vitamin K’s number one job is anti-clotting!!!
In the presence of Vitamin K, these proteins are carboxylated, which means that carboxyglutamic acid is brought into the protein and modified into Gla. Gla has an affinity for calcium ions (Friedman & Przysiecki, 1987; Vermeer, 1990). The calcium induces structural changes in the Gla domain which facilitate its interaction with the surface membrane of the cell (Freedman et al, 1996). This is a key mechanism for both the anti-coagulant and the coagulant systems.
The carboxylation and resulting changes in the Gla domain of the proteins are essential for them to be functional and active (Suttie, 1993). If the active forms of the protein are low, due to insufficient vitamin K, than there can be increased coagulation in the blood vessels which can result in abnormal clotting, sometimes with devastating consequences (Dahlback, Villoutreix, 2005).
Protein C
Protein C is a potent vitamin K-dependent, anticoagulant protein. The activation of protein C and the components involved have been termed the protein C pathway. The protein C pathway serves as a major system for controlling thrombosis or blood clots by inhibiting blood clotting factors V and VII, which limits coagulation. The essential components of this pathway involve the enzyme thrombin, thrombomodulin, the endothelial cell protein C receptor (EPCR), protein C and protein S (Esmon, 2003; Kisiel et al, 1977).
Thrombomodulin is a membrane protein, which is expressed on the surface of endothelial cells. Thrombomodulin serves as a cofactor for thrombin, which is the principal enzyme of clotting. When thrombomodulin binds to thrombin, thrombin is inactivated, and cleared from circulation more than 20 times faster than free thrombin, thus reducing the risk of clots forming. Essentially, thrombin is converted to an anticoagulant, instead of a coagulant, inhibiting its clotting and cell activation potential.
Also the formation of the thrombin-thrombomodulin complex augments protein C activation, leading to a 1000-fold increase in the production of activated Protein C (APC), which is a potent anticoagulant. APC degrades clotting factors, limits coagulation and prevents clot formation in areas of undamaged endothelium.
Protein C also indirectly degrades fibrin within a clot.
Protein S
Protein S (PS) was randomly discovered in the 1970s as a new vitamin K-dependent plasma protein in Seattle and therefore, named after the city of its discovery (Di Scipio et al, 1977). Protein S is a natural anti-coagulant with multiple biologic functions. The best characterized function of Protein S is its role as a cofactor to Protein C. The action of Protein C is enhanced when bound to Protein S (Stern et al, 1985; Fair et al, 1986).
Activated protein C (APC) requires protein S as part of the Protein C pathway (Castoldi & Hackeng, 2008). Whenever procoagulant forces are locally activated to form a clot, protein S participates in controlling clot formation (Soare & Popa, 2010; Castoldi et al, 2010; Marlar et al, 2011). As a cofactor, protein S helps to inactivate two clotting factors, FVa and FVIIIa, which dampens and shuts off clotting. A deficiency in the level of protein C or protein S is associated with an excessive tendency to form clots (Marlar et al 1982; Takashi et al, 1986).
The importance of protein C and protein S is demonstrated by the fact that deficiency of either is associated with increased risk of thrombosis or clotting (Bertina, 1985; Comp et al, 1984).
Protein Z
This is the most recently described component of the anticoagulant system. Protein Z is a vitamin K-dependent protein, which functions as a cofactor that dramatically enhances the inhibition of some coagulation factors. For example, in the presence of protein Z, the ability to inhibit clotting factor Xa is increased 100-fold ( Corral et al, 2007).
Thus, there is a potent system of anti-coagulant forces present in the body, maintaining the fluidity of blood and inhibiting clot formation throughout the body. Vitamin K is a necessary component, enhancing the effectiveness of the anti-coagulant system. With the presence of sufficient vitamin K, the proteins C, S, and Z are carboxylated and active.
If there was insufficient vitamin K available, the anti-coagulant proteins may be insufficiently carboxylated, with fewer of the proteins being active, leading to an increased risk of clots. Ironically, while Vitamin K is well known for its role in helping blood to form clots, it is also very necessary to prevent clots from forming (Esmon et al, 1987: Lobato-Mendizabal & Ruiz-Arguelles, 1990; Grober, et al, 2015) .
The anti-coagulant system is not triggered not is it resolved by vitamin K. The anti-coagulant system functions regardless of the presence of vitamin K, however, its effectiveness in preventing clots is improved with sufficient amounts of vitamin K.
Note: you are primarily in an anti-coagulant state, with your blood circulating
throughout your body, and vitamin K makes that happen.
COAGULATION
Typically, the anti-coagulation system maintains the fluidity of blood as it circulates throughout the body, and actively prevents clots from forming. However, if there is a threat to the integrity of the vascular system, then the body responds by forming blood clots, to ensure survival. The threats could consist of a trauma, injury, inflammation, infectious agents, or pathological situations such as surgery. These threats trigger a cascade of biochemical reactions known as the coagulation cascade.
Coagulation Cascade
The coagulation cascade is a pathway containing many biochemical steps that lead to the formation of blood clots during tissue injury. The clotting mechanism involves a multistep cascade of enzymes, whose jobs are mostly to activate the next enzyme in the cascade. In this cascade, the blood-clotting proteins assemble into a complex on the membranes of platelets and on endothelial cells in the vessel wall. These complexes allow the factors to efficiently contact one another to become activated and participate in clot formation.
There are many steps to this complex cascade.
1. First the blood vessel gets smaller, also known as vasoconstriction, where the damaged blood vessels narrow to reduce blood loss. Vasoconstriction is produced by vascular smooth muscle cells and is the blood vessel’s first response to injury. Blood vessels are lined with smooth muscle cells called endothelial cells, and these cells trigger the muscles to contract. Normally, the endothelial cells express molecules that inhibit platelet adherence and activation while platelets circulate through the blood vessels. However, when injured, the damaged cells release endothelin, promoting vessel constriction in an attempt to limit blood loss.
In this initiation phase, the blood comes in contact with a substance under the endothelial cells, called collagen, which activates circulating platelets and pro-coagulant proteins. Platelets floating in the blood are attracted to collagen and move to the site of the injury.
2. The second critical step is the formation of a platelet plug. As the platelets are attracted to collagen, they stick to each other and to fibers in the blood vessels and they form a plug that temporarily blocks blood flow. This platelet plug is a temporary patch over the leak.
There are three steps to platelet plug formation; platelet adherence, activation, and aggregation.
Adherence. The endothelial cells in the vessel wall begin to secrete the von Willebrand factor. Von Willebrand Factor (VWF) is a protein that acts as a bridge and binds to the exposed collagen, serving as the site for platelets to adhere to each other and to the disrupted vessel surface (Heemskerk et al, 2002). VWF also causes the platelets to change form, growing adhesive filaments and extensions that adhere to the collagen on the endothelial wall.
Activation. After platelet adherence occurs, the sub-endothelial collagen binds to receptors on the surface membrane of the platelet, which activates them. When activated, the platelets aggregate and expose sites that coagulation factors can bind to.
Inside each platelet are storage spaces called granules. The platelet contains three types of internal granules: alpha granules, dense granules, and lysomes. The alpha granules and dense granules move to the surface of the platelet, and fuse with the platelet membrane. Each of these granules are rich in certain chemicals that have an important role in platelet function. During platelet activation, the chemicals inside the granules are pushed out into the bloodstream via a process called degranulation.
These chemicals signal other platelets to come and pile onto the clot. For example, dense granules contain large quantities of calcium ions and adenosine diphosphate ADP). ADP is a key mediator in activating platelet aggregation and clot formation. Upon release from the platelet, ADP stimulates other platelets to activate. Calcium helps provides an important surface for various coagulation factors to assemble on.
Another secretion is thromboxane, which causes platelets to change shape from spherical to stellate. The platelets undergo a morphological change by assuming an irregular surface, forming numerous pseudopods and drastically increasing their surface area (Andrews & Berndt, 2004). This provides more area for the assembly of activated coagulation factors,
The alpha granules contain many proteins, including fibrinogen, thrombospondin, fibronectin, and von Willebrand factor.
Subsequently, there is an extensive formation of procoagulant enzymes on the membrane sites provided by activated platelets, monocytes, other circulating blood cells, as well as the damaged endothelium. These enzymes allow the next step in forming a platelet plug, aggregation, to happen. Platelets, therefore, act as vehicles to concentrate and potentiate coagulation reactions on the damaged vessels.
Aggregation. The final step of platelet plug formation is the aggregation of the platelets into a barrier-like plug. Receptors on the platelets bind and hold the platelets together and anchor them to the damaged endothelium. The completed plug will cover the damaged components of the endothelium, seal the injured area, and will stop blood from flowing out of it. If the wound is large enough, blood will not coagulate until the fibrin mesh from the coagulation cascade is produced, which strengthens the platelet plug. If the wound is minor, the platelet plug may be enough to stop the bleeding without the coagulation cascade.
3. The formation of a fibrin clot is the final step in the coagulation cascade. The platelet plug temporarily stops bleeding and is a very helpful emergency response. However, a platelet plug is only a temporary fix, as it is weak. It can’t last on its own. A fibrin clot is needed. A fibrin clot is a good, strong patch over a hole in a blood vessel. It is normally enough to stop the bleeding completely.
In the final step of coagulation, thrombin is generated, which is needed to produce fibrin. The activated platelet surface helps produce a large-scale thrombin burst, leading to the conversion of fibrinogen to fibrin (Monroe et al, 1996; Hoffman et al, 1995).
Fibrin threads wind around the platelet plug at the damaged area of the blood vessel, forming an interlocking network of fibers and a framework for the clot. This net of fibers traps and holds platelets, blood cells, and other molecules tight to the site of injury, functioning as the initial fibrin clot. This temporary fibrin clot can form in less than a minute and slows blood flow. It takes approximately sixty seconds until the first fibrin strands begin to intersperse among the wound. After several minutes, the platelet plug is completely formed by fibrin and blood loss is stemmed. This fibrin mesh is the base material for eventual healthy skin.
Think of this process as a spider web made of duct tape.
Then platelets in the clot begin to shrink, tightening the clot and drawing together the vessel walls to initiate the process of wound healing. Usually, the whole process of clot formation and tightening takes less than a half hour (Heemskerk et al, 2002; Kalafatis, et al, 1997; Davie, 2003). The result is a sturdy scab to protect the area as you heal (Hall, 2010). When it is no longer needed, the body gets rid of the fibrin clot.
Fibrinolysis is the tightly regulated system that digests already formed clots and/or prevents clots from growing. Fibrinolysis occurs simultaneously with the initiation of clot formation, limiting thrombosis to the local area of injury, and beginning the processes of clot revision, vascular damage repair, and ultimately, vessel recanalization (Davis et al, 2011). In this system, a series of blood proteins disengage the activated blood-clotting system, and begin the process to dissolve the clot.
The fibrinolytic system involves the conversion of plasminogen to plasmin. As the clot is formed, it also contains elements that will dissolve the clot. As an example, plasmin is trapped within the clot. The first key molecule in the cascade of the fibrinolytic system is plasmin, which is produced in an inactive form, plasminogen, in the liver.
Tissue plasminogen activators (TPA) are released from injured vessel walls. TPA activates plasminogen within the clot to plasmin. During fibrinolysis, plasmin degrades the fibrin mesh into small fragments. These fragments possess anticoagulant properties that inhibit thrombin and cause platelet dysfunction which inhibits aggregation (Krutsch, 2011). Plasmin also interferes with coagulation by destroying clotting factors VIII and V, from the fibrinogen group of factors.
After several days, the fibrin is dissolved, replaced by a permanent framework of scar tissue that includes collagen, the platelets subsequently degenerate into an amorphous mass, and thus, healing is complete.
Clotting Factors
The clotting factors are the group of related proteins that are in constant circulation in the blood or present in tissues of the blood vessels. At least 12 substances called clotting factors or tissue factors take part in the cascade of chemical reactions that eventually create a mesh of fibrin within the blood. They are numbered with Roman numerals (I, II, III, IV, V, VII, VIII, IX, X, XI, XII, XIII) and given a common name as well. The factors are numbered according to the order in which they were discovered and not according to the order in which they react. Each of the clotting factors has a very specific function. The clotting factors work together, with one triggering the other.
The first 4 of the 12 originally identified factors are referred to by their common names, i.e., fibrinogen (factor I), prothrombin (factor II), tissue factor (factor III), and calcium (factor IV). Clotting factor VI no longer exists. The more recently discovered clotting factors (e.g. prekallikrein and high-molecular-weight kininogen) have not been assigned Roman numerals..
Clotting factor III is also known as platelet tissue factor (TF). It is present in endothelial cells and helps initiate fibrin formation.
Calcium.
Clotting factor IV, calcium, is a co-factor for several other clotting factors such as III, VII, IX and X (Arthus & Pages, 1890; Arthus & Pages, 1891). Calcium is needed for those clotting factors to bind to membrane surfaces and to assemble together into clots. But that can only happen with the help of vitamin K. In the presence of vitamin K, those clotting factors become carboxylated, activating them. When calcium binds to carboxylated proteins it leads to a shape change that allows the protein to bind to membranes, such as the membranes of platelets and vessel walls.
Also, the calcium helps in assembling the clotting factors on the membrane surfaces of the activated blood platelets. The clotting factors are anchored to the platelet membrane surfaces through calcium bridges (Elodi & Elodi, 1983). The calcium ions essentially form a bridge between the clotting factor and the activated platelet surface. This is how the clotting process is kept localized at the original site of vessel injury.
Much of blood clotting is a result of blood-clotting proteins assembling into a complex on the membranes of platelets and endothelial cells; within these complexes, the factors can efficiently contact one another to become activated and participate in clot formation, as in the clotting cascade. These clotting factors together activate prothrombin to produce thrombin, which is central to coagulation. The thrombin then produces fibrin to fibrinogen, which produces the strongest clot. Calcium acts as a catalyst for this reaction.
Without carboxylation, these bridges will not form, and the adherence of the clotting factors to the injured cell walls does not take place. When levels of vitamin K are low, blood will take longer to clot. If calcium levels are also deficient, you may bleed to death.
Calcium is so important that the body precisely controls the amount of calcium in cells and blood. The body moves calcium out of bones and into the blood as needed, to maintain a steady level of calcium in the blood. If people do not consume enough calcium from their diet, then too much calcium is mobilized from the bones, weakening them. Osteoporosis can result (Doolittle, 2016). Most people gain enough calcium from their diets and taking a calcium supplement is not recommended.
Clotting factors can also be classified into three groups; the Fibrinogen Family, the Contact family, and Vitamin K dependent proteins family.
1.Fibrinogen Family:
Fibrinogen is clotting Factor 1. During a vascular injury thrombin converts fibrinogen to fibrin and subsequently to a fibrin-based blood clot.
Factor V is a protein made in the liver. When Factor V is activated it interacts with coagulation factor X. Together they form a complex that converts helps convert fibrinogen into fibrin.
Factor VIII is an essential blood clotting protein, also known as anti-hemophilic factor (AHF).
Factor XIII is an enzyme that crosslinks fibrin. A deficiency affects clot stability. This factors causes clots to dissolve slowly and carefully.
2.Contact Family: one of two major pathways for triggering the clotting cascade
Factor Xi or plasma thromboplastin is a form of factor XIa, one of the enzymes of the coagulation cascade.
Factor XII is also known as Hageman factor, is a protein that seems to be involved in later stages of clot formation (Renne, et al 2005).
Hmwk is one of four proteins which initiate the contact activation pathway (also called the intrinsic pathway) of coagulation. The other three are factor Xii, Xi, and prekallikrein.
Prekallikrein (PK) also known as the Fletcher Factor, participates in the surface-dependent activation of blood coagulation and fibrinolysis.
3.Vitamin K Dependent clotting proteins:
Vitamin K was originally identified for its role in the process of blood clot formation, though we know it is involved in much more than coagulation. The letter K was derived from the German word "koagulation". As with the anti-coagulant proteins, Vitamin K is a necessary cofactor for the critical clotting proteins.
With the presence of vitamin K, these clotting proteins become carboxylated. Amino acids are the building blocks of proteins. In this carboxylation process, the amino acid Glu, is converted into gamma-carboxyglutamate acid or Gla. Gla makes it possible for the protein to bind metals such as calcium. The presence of Gla residues in the vitamin K dependent clotting factors are essential for their function in the clotting process (Suttie, 1993; Monroe, 2010).
These Gla residues enable the protein to bind to membrane surfaces. Glu is a weak calcium chelator and Gla is a much stronger one, so the carboxylation of the protein substantially increases the calcium binding capacity of the protein (Esmon et al, 1975a; Nowak & Grzubowska-Chelbowczyk, 2014; Shearer, 1995). When the Gla domain binds calcium, it undergoes an important shape change, and is transformed from a random coil to an ordered structure. The Gla residues orient inward and together with calcium ions, form the interior core, and expose a binding site. Without the calcium, the shape change would not happen, and the binding site of the protein would not be exposed which is an important part of the clotting process (Stenflo, 1999; Nelsestuen et al, 2000; Soriano-Garcia et al, 1992).
Calcium binding by the Gla residues is also essential for activating and anchoring circulating blood clotting factors at the site of injury. The Gla domain directly binds to cell membrane surfaces through calcium ion bridges (Furie & Furie, 1992; Freedman et al, 1995; Freedman et al, 1996; Berkner & Runge, 2004; Kalafatis et al, 1994). The presence of calcium is required for the vitamin K dependent clotting factors, such as prothrombin to be activated and trigger the clotting process to form the enzyme thrombin (Rawala-Sheikh et al, 1992).
An essential element in the success of coagulation cascade is the assembly of the various clotting complexes in an orderly fashion on the surface of the platelets and endothelial cells. Within these complexes, the factors can efficiently contact one another to become activated and participate in clot formation. The GLA domains provide the critical link, localizing the clotting factors to the platelet surface (Magnusson et al, 1974).
The vitamin K dependent clotting factors are Factor II, VII, IX, and X. (Stenfloet al, 1934; Nelsestuen et al, 1974; Magnusson et al, 1974; Zytkovicz et al, 1975; Vermeer et al, 1978; Olson, 1984). These four coagulation proteins need to be carboxylated or activated in order to become biologically functional. These proteins have in common the requirement to be post-translationally modified by carboxylation of glutamic acid residues, forming Gla, in order to become biologically active. Vitamin K enables those clotting factors to clot the blood quickly (Askim, 2001; Bern, 2004; Theuwissen et al, 2012; Shearer & Newman, 2008).
