Vitamin K-dependent carboxylase: Evidence for a semiquinone radical intermediate

Nanosecond laser flash photolysis has been used to produce and identify the vitamin K semiquinone (radical) from vitamin K dihydroquinone and to observe its formation and decay in the presence of vitamin K-dependent carboxylase (epoxidase). The activity of vitamin K-dependent carboxylase is not decreased by exposure to the laser. Absorbance of the semiquinone is proportional to enzyme concentration and is stimulated by a synthetic substrate, PheLeuGluGluIle. Stabilization of the semiquinone is observed in the presence of the enzyme. The semiquinone is rapidly destroyed in the presence of inhibitors of vitamin K-dependent carboxylase and vitamin K epoxidase.     

Vitamin K-dependent gamma-glutamyl carboxylase activity in the chick embryonic chorioallantoic membrane.

During embryonic development of the chick, the onset of calcium transport by the chorioallantoic membrane (CAM) is concomitant with the appearance of a calcium-binding protein (CaBP). The development-specific expression of the CaBP in the CAM is inhibited by vitamin K antagonism in ovo with the anticoagulant, warfarin. However, the CaBP remains immunologically detectable in the CAM of warfarin-treated embryos, suggesting the presence of a precursor form of the CaBP. Previously, we have demonstrated that CaBP expression in CAM organ cultures is inducible by vitamin K. Furthermore, the CaBP contains several residues of the modified amino acid, gamma-carboxyglutamic acid (gamma-CGlu), which has been shown to be formed by vitamin K-dependent carboxylation of glutamic acid in several plasma clotting proteins. This study reports the presence of a post-translational, vitamin K-dependent gamma-glutamyl carboxylase activity in the CAM. Our results show that explants of CAM incorporate H14CO3 in an age-specific and vitamin K-dependent manner. Incorporation of H14CO3 by the CAM is further potentiated by warfarin treatment of the embryos, presumably owing to an elevation of the amount of endogenous uncarboxylated protein precursor(s). Among the subcellular (nuclear, mitochondrial, microsomal, and soluble) fractions of the CAM, only microsomes exhibit specific incorporation of of H14CO3 into gamma-CGlu. The CAM microsomal carboxylation activity is post-translational, vitamin K-dependent, specific for prenylated homologs of vitamin K, sensitive to warfarin, and appears to be unrelated to the activities of biotin-dependent carboxylases or phosphoenolpyruvate carboxykinase. Optimal carboxylation activity occurs after incubation of the microsomes with H14CO3 for 60 min at 37 degrees C in the presence of over 100 microgram of vitamin K1/ml.     

Vitamin K-dependent carboxylase: the carboxylation of exogenous substrates in different systems

Two types of solid-phase carboxylase, SPC-II and SPC-X, have been prepared from the livers of warfarin-treated cows. Their enzymatic activities were compared with substrate-free carboxylase in microsomes from normal cows and substrate-bound carboxylase in microsomes from warfarin-treated cows. A number of exogenous substrates for carboxylase have been purified and tested. We found that large substrates, such as descarboxyprothrombin, are carboxylated only by substrate-free carboxylase and not by the substrate-bound enzyme. No differences in apparent Km values between solid-phase carboxylases II and X were observed.     

Vitamin K-dependent carboxylase: Synthesis of an inhibitor of the glutamyl binding site

Liver microsomes contain a vitamin K and O₂-dependent carboxylase that converts peptide-bound glutamyl residues to γ-carboxyglutamyl residues. The peptide Boc-O-phospho—Ser-O-phospho—Ser—Leu-OMe has now been synthesized. This peptide inhibits the carboxylation of endogenous protein precursors by a detergent-solubilized preparation of the carboxylase and is an apparent competitive inhibitor of the carboxylation of Phe—Leu—Glu—Glu—Leu.     

Vitamin K-dependent carboxylase. Requirements of the rat liver microsomal enzyme system.

Vitamin K is required in an enzymatic reaction which carboxylates glutamyl residues in a microsomal protein precursor of plasma prothrombin to form gamma-carboxyglutamic acid residues. The partial requirements of this microsomal, vitamin K-dependent carboxylase system have been determined. A requirement of the system for cytosolic factors appears to be due primarily to the presence of reduced pyridine nucleotides or a reduced pyridine nucleotide-generating system in the cytosol. The hydroquinone of vitamin K has been demonstrated to be the enzymatically active form of the vitamin. When vitamin K1 hydroquinone is added to the carboxylase system, no NAD(P)H is needed for maximum activity. The carboxylase activity is half-maximally stimulated by 0.25 mug of vitamin K1/ml in the presence of cytosolic components but requires at least 10 times as much vitamin when microsomes are incubated in a cytosol-free buffer. Menadione is inactive as a vitamin source in this system, and the carboxylase activity is inhibited by the 2-chloro analog of vitamin K1 and by Warfarin. The ATP analog, AMP-P(NH)P, inhibited the carboxylase activity, but a dependence on exogenous ATP or an ATP-generating system could not be demonstrated. Carboxylase activity was found to be dependent on an O2-containing gas phase, and upon the HCO3- concentration.     

Vitamin K-dependent carboxylase from calf liver: studies on the steady-state kinetic mechanism

The steady-state kinetic mechanism of vitamin K-dependent carboxylase from calf liver has been investigated by initial-velocity measurements with varying concentrations of two carboxylase substrates and constant, nonsaturating concentrations of the other two substrates. With all combinations of the varied substrates tested linear kinetics were obtained with lines intersecting on the left side of the 1/v axis in double-reciprocal plots. Thus the carboxylase has a sequential reaction mechanism which includes the quinternary complex of the enzyme with its four substrates. A mechanism with the ordered steady-state addition of all substrates to the enzyme accords well with the results. A totally random mechanism was excluded but the alternative possibility remained that part of the substrates are added in a rapid-equilibrium random reaction. Experiments with saturating constant concentrations of sodium bicarbonate and varying concentrations of the other substrates suggest that bicarbonate (CO2) is either the first or, more probably, the last substrate bound to the enzyme.     

Vitamin K-dependent carboxylase. Control of enzyme activity by the "propeptide" region of factor X

A liver microsomal enzyme catalyzes the vitamin K-dependent posttranslational carboxylation of specific glutamyl residues of a limited number of plasma proteins to gamma-carboxyglutamyl residues. The intracellular precursor forms of these proteins are known to contain a homologous basic amino acid-rich propeptide region between the signal peptide region and the amino terminus of the mature protein. This region of the precursor protein has been implicated as a possible recognition site for the carboxylase enzyme. A 20-residue peptide containing the octadecapropeptide of human clotting factor X has now been shown to strongly stimulate the activity of the enzyme toward a noncovalently linked substrate. This stimulatory effect is seen at less than micromolar concentrations and is accompanied by a decrease in the Km of the glutamic acid substrate. These observations raise the possibility that the catalytic activity of other enzymes involved in protein processing may be regulated by a portion of their normal substrates.     

The role of sulfhydryl groups in the ribulose- 1,5-bisphosphate carboxylase and oxygenase reactions.

Ribulose-l,5-bisphosphate carboxylase (E.C. 4.1.1.39) isolated from Chromatium strain D contains 64 free cysteinyl -SH groups per mol (M_r 5.11 × 10⁵) as determined using three different titrants: p-[¹⁴C]chloromercuribenzoate, the Ellman reagent, and [¹⁴C]iodoacetamide.Distribution of -SH groups in the two constituent subunits (A and B) isolated from spinach and Chromatium ribulose-1,5-bisphosphate carboxylases was determined to be for spinach, 9 in A and 3 in B; and for Chromatium, 7 in A and 1 in B.The relationship between the numbers of -SH groups blocked vs residual activities of both the ribulose-1,5-bisphosphate carboxylase and oxygenase reactions was examined by titration with p-chloromercuribenzoate. In both spinach and Chromatium enzymes, antisigmoidal curves were obtained for the degree of the enzyme activity loss in relation to the numbers of -SH groups masked. However, at alkaline pH the Chromatium enzyme shows a sharp decline in both carboxylase and oxygenase activities, apparently due to the alkali dissociation of the enzyme molecule accompanied by its structural deformation. The functional role of -SH groups in the ribulose-1,5-bisphosphate carboxylase molecule is discussed in relation to two constituent enzyme reactions, and it is concluded that in both enzyme sources the active sites are probably the same for the two reactions.     

Vitamin K-dependent carboxylase: influence of the "propeptide" region on enzyme activity

The liver microsomal vitamin K-dependent carboxylase catalyzes the post-translational conversion of specific glutamyl to gamma-carboxyglutamyl (Gla) residues in precursor forms of a limited number of proteins. These proteins contain an amino-terminal extension (propeptide) that is presumed to serve as an enzyme recognition site to assure their normal processing. The free, noncovalently bound propeptide has also been shown to stimulate the in vitro activity of this enzyme. This peptide has now been shown to lower the app Km of a low-molecular-weight Glu site substrate while having no influence on the app Km of the other substrates, vitamin KH2, O2, and CO2/HCO3-. Propeptide addition was shown to have no influence on the ratio of the two products of the enzyme, Gla and vitamin K-2,3-epoxide. Stimulation of carboxylase activity by the propeptide from human factor X was observed in a number of rat tissues and in the liver of a number of different species. Stability of the enzyme in crude microsomal preparations was greatly enhanced by the presence of propeptide. These observations are consistent with the hypothesis that this region of the protein substrates for the carboxylase not only serves an enzyme recognition or docking function but also modulates the activity of the enzyme by altering the affinity for one of its substrates.     

Vitamin K-dependent carboxylase: requirements for carboxylation of soluble peptide and substrate specificity.

Rat liver microsomes contain a triton X-100 solubilizable vitamin K-dependent carboxylase activity that converts specific glutamyl residues of precursor proteins to γ-carboxyglutamyl residues. This activity has been studied utilizing synthetic peptides as substrates for the enzyme. When compared to the carboxylation of the endogenous microsomal precursors, the peptide carboxylase activity is more sensitive to the action of various inhibitors, and requires a higher concentration of vitamin K for maximal activity. The apparent K_m for the peptide Phe-Leu-Glu-Glu-Leu was found to be 4 mM. Substrate specificity depends on residues adjacent to the carboxylated Glu residues and macromolecular recognition sites.     

Vitamin K-dependent carboxylase: Possible artifact of analysis due to a pyridine nucleotide-dependent carboxylation

Addition of pyridine nucleotides to a microsomal system which is commonly used to study the vitamin K-dependent microsomal carboxylase promoted carboxylation of unknown endogenous compounds. Upon gel filtration, the carboxylated products were found to be of lower molecular weight (MW range 180–650) than the peptide substrate of the vitamin K-dependent carboxylase. Synthesis of these products was not inhibited by vitamin K antagonists nor did pyridine nucleotides stimulate carboxylation of the peptide substrate for vitamin K-dependent carboxylation in the absence of vitamin K. Thus the reaction appears to be mediated by a different enzyme. Dialysis of the microsomal system removed this pyridine nucleotide-stimulated carboxylation and activated the vitamin K-dependent carboxylation and epoxidation reactions. These data point out a possible artifact in the routine study of this enzyme and suggest that dialysis should be carried out prior to studying these two vitamin K-dependent reactions.     

Pyruvate is a by-product of catalysis by ribulosebisphosphate carboxylase/oxygenase

Pyruvate is a minor product of the reaction catalyzed by ribulosebisphosphate carboxylase/oxygenase from spinach leaves. Labeled pyruvate was detected, in addition to the major labeled product, 3-phosphoglycerate, when 14CO2 was the substrate. Pyruvate production was also measured spectrophotometrically in the presence of lactate dehydrogenase and NADH. The Km for CO2 of the pyruvate-producing activity was 12.5 microM, similar to the CO2 affinity of the 3-phosphoglycerate-producing activity. No pyruvate was detected by the coupled assay when ribulose 1,5-bisphosphate was replaced by 3-phosphoglycerate or when the carboxylase was inhibited by the reaction-intermediate analog, 2'-carboxyarabinitol 1,5-bisphosphate. Therefore, pyruvate was not being produced from 3-phosphoglycerate by contaminant enzymes. The ratio of pyruvate produced to ribulose bisphosphate consumed at 25 degrees C was 0.7%, and this ratio was not altered by varying pH or CO2 concentration or by substituting Mn2+ for Mg2+ as the catalytically essential metal. The ratio increased with increasing temperature. Ribulose-bisphosphate carboxylases from the cyanobacterium Synechococcus PCC 6301 and the bacterium Rhodospirillum rubrum also catalyzed pyruvate formation and to the same extent as the spinach enzyme. When the reaction was carried out in 2H2O, the spinach carboxylase increased the proportion of its product partitioned to pyruvate to 2.2%. These observations provide evidence that the C-2 carbanion form of 3-phosphoglycerate is an intermediate in the catalytic sequence of ribulose-bisphosphate carboxylase. Pyruvate is formed by beta elimination of a phosphate ion from a small portion of this intermediate.     

Ribulose bisphosphate carboxylase/oxygenase from Thiocapsa roseopersicina

d-Ribulose-1,5-bisphosphate carboxylase/oxygenase has been purified 80-fold from malate-grown Thiocapsa roseopersicina by salting out the enzyme from the high-speed supernatant between 68–95% saturation with respect to (NH₄)₂SO₄, gelfiltration through Sephadex G-100, and DEAE-cellulose chromatography followed by sedimentation into a 14–34% glycerol gradient. The specific activity of enzyme for the carboxylase reaction was 2.45 mol RuBP-dependent CO₂ fixed/min · mg protein (at pH 8.0 and 30° C) and for the oxygenase reaction was 0.23 mol RuBP-dependent O₂ consumed/min · mg protein (at pH 8.6, and 25° C). The enzyme, which was ultracentrifugally homogeneous in the presence of 4 and 10% v/v glycerol, was stable for at least one year at-80° C in the presence of 10% glycerol. S_{20, w} values obtained in the presence of 4 and 10% glycerol were 19.3 and 16.2, respectively. The enzyme contained both large (53,000-daltons) and mixed small subunits (15,000- and 13,500-daltons).Borate-dependent inactivation of the enzyme by 2,3-butadione, which was greatly reduced in the presence of the product 3-phosphoglycerate, suggested that one or more arginines are at the active site.Abbreviations DTT dithiotreitol - RuBP d-ribulose-1,5-bisphosphate - SDS sodium dodecylsulfate - TCA trichloroacetic acid - TEMBDG buffer (pH 8.0 at 25°C) containing 20 mM Tris, 1 mM disodium EDTA · 2 H₂O, 10 mM MgCl₂·6 H₂O, 50 mM NaHCO₃, 0.1 mM DTT and 10% glycerol (v/v)     

Vitamin K-dependent carboxylase: effect of ammonium sulfate on substrate carboxylation and on inhibition by stereospecific substrate analogs

(1) High concentrations of ammonium sulfate may stimulate the carboxylase activity of bovine liver microsomes about 10-fold. This effect results from an increase of the Vmax, whereas neither the apparent Km for a number of substrates nor the Ki for substrate analogs is affected. (2) The effect of ammonium sulfate was only found in substrates lacking the pro-sequence. No effect was measurable on the carboxylation of pro-PT28 and endogenous precursor proteins. (3) If the pro-fragment was added as a peptide not covalently bound to a carboxylatable substrate, the carboxylation thereof was only slightly affected and ammonium sulfate remained active as a stimulator of carboxylase activity. (4) S-MeTPT is a much stronger inhibitor of carboxylase activity than is R-MeTPT. (5) The inhibition of carboxylase by the methylated tripeptides is competitive and independent of the type of substrate. Also pro-PT28, which contains the full pro-sequence, could be inhibited completely. (6) On the other hand the carboxylation of endogenous protein precursors could only be partly inhibited by the substrate analogs: even at high concentrations of S-MeTPT a residual endogenous substrate carboxylation of about 30% was left.