862
c h a p t e r 36
Biochemistry of Hemostasis
Reduced ( hydroquinone
)
Epoxide (quinone )
V itam in K -D ep en d e n t
C a r b o x y la se
R ea ctio n
Adapted from Suttie , 1994
F IG U R E 3 6 -2 0
Vitamin K action in protein Glu carboxylation. The formation of Gla from
Glu requires abstraction of a H ion from the
y
position of Glu in the
proteins that is followed by the addition of a carboxyl group from CO
2
.
The proteins to be carboxylated are marked by a propeptide that serves as a
signal for the vitamin K-dependent carboxylase.
factors. Vitamin K is a substituted naphthoquinone. A
methyl group at the 2-position and a prenyl (or phytyl)
side chain of varying length is required for the vitamin to
be active. The two primary sources of vitamin K in hu-
mans are green plants and intestinal flora. The designation
“dietary factor K” derives from the initial description of
the vitamin as the “koagulations vitamin.” The active form
of the vitamin is the reduced hydroquinone form, not the
quinone form found in the diet. In the vitamin K-dependent
carboxylase reaction, the vitamin is converted from the hy-
droquinone to an epoxide (Figure 36-20). During this reac-
tion, an intermediate O- anion is involved in the removal
of a hydrogen from the
y
-carbon of the Glu residues in the
vitamin K-dependent protein. This stereospecific reaction
requires O
2
and the hydroquinone for the formation of the
Glu anion. Reaction of the Glu anion with CO
2
leads to
the formation of the
y
-carboxy Glu. A propeptide of 23
amino acids at the N-terminal end of vitamin K-dependent
proteins provides the signal for carboxylation of the Glu
residues in the “precursor” Gla domain of the protein on
the ribosome. The propeptide is cleaved from the precursor
molecule prior to secretion.
In the absence of vitamin K or in the presence of an-
tagonists of vitamin K action, uncarboxylated vitamin
K-dependent proteins are synthesized and secreted from
the liver. The residues that would be Gla in the nor-
mal, posttranslationally modified proteins are ordinary
Glu residues. These proteins do not bind Ca2+ normally
and do not bind to the surface of the phospholipids in the
cell membranes. The structure of the Gla domains with
bound Ca2+ ions is shown in Figure 36-21.
F IG U R E 3 6 -2 1
(Also see color figure.) Structure of a Gla domain of prothrombin. The Gla
domains of the vitamin K-dependent proteins contain between 9 and 12
carboxyglutamic acid residues. Prothrombin contains 10 Gla residues: pairs
of adjacent Gla residues at positions 6 and 7; 19 and 20; and 25 and 26, and
four other Gla residues at positions 14, 16, 29, and 32. Factor VII also
contains 10 Gla residues in the same positions as in prothrombin. Factor IX
contains 12 Gla residues. Because factor IX has an additional amino acid
between its amino terminal Tyr and the first Gla residue, the numbering of
the residues is different from prothrombin and factor VII. However, the
pattern is similar with pairs of adjacent Gla residues at positions 7 and 8; 20
and 21; and 26 and 27. The six other Gla residues are at positions 15, 17,
30, 33, 36, and 40. The last two Gla residues account for 12 Gla residues in
factor IX. Factor X contains 11 Gla residues: pairs of adjacent Gla residues
at positions 6 and 7; 19 and 20; and 25 and 26, with five other Gla residues
at positions 14, 16, 29, 32, and 39. Protein C contains 9 Gla residues: pairs
of adjacent Gla residues at positions 6 and 7; 19 and 20; and 25 and 26, and
the other Gla residues at positions 14, 16, and 29. In the color versions of
this figure, the cartoon structures show helices in magenta,
fS
sheets in
yellow,
ft
bends in blue, and Ca2+ ions in gray. The space-filling dot
surfaces for the residues of the Gla domains are standard CPK colors.
The physiological expression of oral anticoagulant
action is an increase in the time required for clotting
in the prothrombin time assay. The slowing of all of
the reactions that lead to the formation of thrombin is
the direct result of the reduced concentrations of the
vitamin K-related proteins in the reaction complexes on
the membrane surface. The effects of oral anticoagulant
blockage on the carboxylation reaction are common to all
vitamin K-related proteins.
Action of Warfarin and Other
Vitamin K Antagonists
Vitamin K antagonists were first identified by K. R Link
while he was investigating a hemorrhagic disease in
