Vitamin K
Introduction
Vitamin
K is a group of fat-soluble vitamins essential primarily for normal
blood coagulation, bone metabolism, and the regulation of several vitamin
K-dependent proteins. The name “Vitamin K” originates from the German word Koagulationsvitamin,
reflecting its originally discovered role in blood clotting.
Vitamin
K acts as an essential cofactor for the enzyme responsible for the γ-carboxylation
of specific glutamate residues in vitamin K-dependent proteins. This
post-translational modification is necessary for the biological activity of
several coagulation factors and proteins involved in bone and vascular
metabolism.
General
Characteristics
- Vitamin
K is a fat-soluble vitamin.
- It
is absorbed in association with dietary fats.
- Bile
salts are important for its intestinal absorption.
- It
plays an essential role in the synthesis and activation of several
blood-clotting proteins.
- Vitamin
K is required for the post-translational modification known as γ-carboxylation.
- It
is involved in bone metabolism and vascular biology.
- The
body has relatively limited stores of vitamin K; therefore, prolonged
deficiency may affect physiological functions.
- Vitamin
K is recycled through the vitamin K cycle.
Major
Forms of Vitamin K
Vitamin
K occurs in several forms.
1.
Vitamin K₁ (Phylloquinone)
Vitamin
K₁, also called phylloquinone, is the major dietary form of vitamin K.
Major
Sources
- Green
leafy vegetables
- Spinach
- Kale
- Broccoli
- Cabbage
- Vegetable
oils
Phylloquinone
is synthesized by plants and is particularly abundant in chloroplast-containing
green plant tissues.
2.
Vitamin K₂ (Menaquinones)
Vitamin
K₂ refers to a group of compounds collectively known as menaquinones (MK-n).
Different
menaquinones are classified according to the length of their isoprenoid side
chains.
Examples
include:
- MK-4
- MK-7
- MK-8
- MK-9
Menaquinones
are associated with:
- Fermented
foods
- Certain
animal-derived foods
- Bacterial
synthesis
Some
menaquinones can also be produced by microorganisms present in the human
intestinal tract.
3.
Vitamin K₃ (Menadione)
Vitamin
K₃, or menadione, is a synthetic compound.
It
is a provitamin form and differs from naturally occurring vitamin K₁ and K₂.
Menadione
is not generally used as a routine nutritional form of vitamin K for humans
because of potential toxicity concerns.
Chemical
Nature
Vitamin
K compounds contain a common:
2-Methyl-1,4-naphthoquinone
Ring
The
different forms of vitamin K differ mainly in their side chains.
The
quinone structure allows vitamin K to participate in oxidation-reduction
reactions that are essential for the vitamin K cycle.
Absorption
and Transport
Vitamin
K is absorbed primarily in the small intestine.
The
absorption process involves:
- Dietary
fat
- Bile
salts
- Formation
of mixed micelles
- Intestinal
uptake
- Incorporation
into chylomicrons
After
absorption, vitamin K is transported through the lymphatic system and
circulation.
Phylloquinone
from dietary sources is transported initially in chylomicrons.
Vitamin
K Cycle
The
biological function of vitamin K depends upon a continuous recycling mechanism
known as the:
Vitamin
K Cycle
The
cycle involves the conversion of vitamin K between different oxidation states.
Simplified
Pathway
Vitamin
K Quinone
↓
Vitamin
K Hydroquinone
↓
γ-Carboxylation
Reaction
↓
Vitamin
K Epoxide
↓
Vitamin
K Quinone
The
regeneration of active vitamin K is essential because the vitamin participates
repeatedly in the carboxylation process.
Role
of Vitamin K in γ-Carboxylation
Vitamin
K acts as a cofactor for the enzyme:
γ-Glutamyl
Carboxylase
This
enzyme converts specific glutamate (Glu) residues into:
γ-Carboxyglutamate
(Gla)
The
reaction can be represented as:
Glutamate
Residue
↓
Vitamin
K-dependent γ-carboxylation
↓
γ-Carboxyglutamate
(Gla) Residue
The
Gla residues enable proteins to bind calcium ions effectively.
This
calcium-binding property is essential for the biological activity of several
proteins.
Vitamin
K-Dependent Proteins
Several
important proteins require vitamin K-dependent γ-carboxylation.
Coagulation
Proteins
Vitamin
K is necessary for the functional activation of:
- Factor
II (Prothrombin)
- Factor
VII
- Factor
IX
- Factor
X
These
proteins are essential components of the blood coagulation system.
Anticoagulant
Proteins
Vitamin
K is also required for:
- Protein
C
- Protein
S
- Protein
Z
Thus,
vitamin K is involved not only in procoagulant pathways but also in the
regulation of anticoagulant mechanisms.
Role
in Blood Coagulation
Vitamin
K is essential for normal haemostasis.
The
vitamin-dependent coagulation factors contain γ-carboxyglutamate residues that
allow them to bind calcium ions.
This
calcium-dependent interaction enables coagulation proteins to associate with
phospholipid surfaces during the coagulation process.
Therefore,
vitamin K deficiency can impair the synthesis of fully functional clotting
factors.
Role
in Bone Metabolism
Vitamin
K is involved in the activation of several proteins present in bone.
One
important vitamin K-dependent protein is:
Osteocalcin
Osteocalcin
is produced by osteoblasts.
Vitamin
K-dependent γ-carboxylation modifies osteocalcin and contributes to its
calcium-binding properties.
Another
vitamin K-dependent protein associated with bone and soft tissues is:
Matrix
Gla Protein (MGP)
MGP
has been studied for its role in regulating mineralization processes.
Role
in Vascular Biology
Vitamin
K-dependent proteins are also present in vascular tissues.
Matrix
Gla Protein (MGP) is one of the important proteins associated with regulation
of mineralization in soft tissues.
Research
has investigated the relationship between vitamin K status, MGP activation, and
vascular calcification.
However,
the exact clinical benefits of vitamin K supplementation for cardiovascular
disease prevention continue to be investigated.
Major
Functions of Vitamin K
1.
Blood Coagulation
Vitamin
K is essential for the activation of several coagulation factors.
These
include:
- Factor
II
- Factor
VII
- Factor
IX
- Factor
X
2.
Activation of Protein C and Protein S
Vitamin
K is also required for proteins involved in anticoagulant regulation.
These
include:
- Protein
C
- Protein
S
Thus,
vitamin K contributes to the balance between coagulation and anticoagulation.
3.
Bone Metabolism
Vitamin
K is involved in the activation of osteocalcin and other vitamin K-dependent
proteins associated with skeletal tissue.
4.
Regulation of Mineralization
Vitamin
K-dependent proteins such as Matrix Gla Protein participate in the regulation
of mineralization in bones and soft tissues.
5.
Cellular Functions
Vitamin
K-dependent proteins have been identified in several tissues, suggesting
broader biological roles beyond classical blood coagulation.
Research
continues to investigate these additional functions.
Dietary
Sources of Vitamin K
Vitamin
K₁ (Phylloquinone)
Major
dietary sources include:
- Spinach
- Kale
- Broccoli
- Cabbage
- Lettuce
- Green
leafy vegetables
- Soybean
oil
- Canola
oil
Vitamin
K₂ (Menaquinones)
Sources
include:
- Fermented
foods
- Certain
cheeses
- Fermented
soybean products
- Some
animal-derived foods
The
fermented soybean food natto is particularly rich in menaquinone-7
(MK-7).
Vitamin
K Deficiency
Vitamin
K deficiency is relatively uncommon in healthy adults consuming a balanced
diet.
However,
deficiency can occur under certain conditions.
Causes
of Deficiency
- Inadequate
dietary intake
- Fat
malabsorption
- Biliary
obstruction
- Liver
disease
- Prolonged
use of broad-spectrum antibiotics
- Disorders
affecting lipid absorption
- Long-term
parenteral nutrition without adequate supplementation
- Certain
medications
Vitamin
K Deficiency in Newborns
Newborn
infants have relatively low vitamin K stores.
Several
factors contribute to this vulnerability, including:
- Limited
placental transfer
- Low
vitamin K content in breast milk
- Initially
limited intestinal bacterial production
For
this reason, newborn vitamin K prophylaxis is used in many healthcare systems
to prevent vitamin K deficiency bleeding.
Clinical
Consequences of Deficiency
Vitamin
K deficiency may result in impaired activation of clotting factors.
Consequences
may include:
- Increased
tendency toward bleeding
- Prolonged
clotting times
- Reduced
activity of vitamin K-dependent coagulation factors
One
laboratory parameter that may be affected is:
Prothrombin
Time (PT)
Vitamin
K deficiency may prolong prothrombin time because Factor VII has a relatively
short half-life.
Vitamin
K and Anticoagulant Drugs
Certain
anticoagulant medications interfere with the vitamin K cycle.
The
best-known example is:
Warfarin
Warfarin
inhibits:
Vitamin
K Epoxide Reductase Complex (VKOR)
This
enzyme is responsible for recycling vitamin K into its active form.
Inhibition
of VKOR reduces the availability of active vitamin K and decreases the
functional activation of vitamin K-dependent clotting factors.
Vitamin
K Cycle and Warfarin
Simplified
mechanism:
Vitamin
K Epoxide
↓
VKOR
↓
Reduced
Vitamin K
↓
γ-Glutamyl
Carboxylase
↓
Activation
of Vitamin K-dependent Proteins
Warfarin
inhibits VKOR, thereby reducing vitamin K recycling.
Vitamin
K Toxicity
Toxicity
from naturally occurring dietary forms of vitamin K₁ and K₂ is rare.
No
clear evidence of toxicity has been established from normal dietary consumption
of phylloquinone or menaquinones in healthy individuals.
However:
- Synthetic
forms may have different safety profiles.
- Menadione
(Vitamin K₃) can produce toxicity at inappropriate doses.
Therefore,
supplementation should be used according to appropriate nutritional and
clinical recommendations.
Important
Differences Between Vitamin K₁ and K₂
|
Characteristic |
Vitamin K₁ |
Vitamin K₂ |
|
Chemical
Name |
Phylloquinone |
Menaquinones |
|
Major
Source |
Plants |
Fermented
and animal foods |
|
Common
Dietary Source |
Green
leafy vegetables |
Fermented
foods |
|
Side
Chain |
Phytyl
side chain |
Isoprenoid
side chain |
|
Biological
Role |
Coagulation
and other functions |
Coagulation,
bone and other functions |
|
Examples |
Phylloquinone |
MK-4,
MK-7, etc. |
Important
Vitamin K-Dependent Proteins
|
Protein |
Major Function |
|
Prothrombin
(Factor II) |
Blood
coagulation |
|
Factor
VII |
Blood
coagulation |
|
Factor
IX |
Blood
coagulation |
|
Factor
X |
Blood
coagulation |
|
Protein
C |
Anticoagulant
regulation |
|
Protein
S |
Anticoagulant
regulation |
|
Osteocalcin |
Bone
metabolism |
|
Matrix
Gla Protein |
Regulation
of mineralization |
Vitamin
K Metabolism at a Glance
Dietary
Vitamin K
↓
Intestinal
Absorption
↓
Chylomicron
Transport
↓
Tissue
Uptake
↓
Vitamin
K Quinone
↓
Vitamin
K Hydroquinone
↓
γ-Carboxylation
of Proteins
↓
Vitamin
K Epoxide
↓
Vitamin
K Epoxide Reductase (VKOR)
↓
Recycled
Active Vitamin K
Mechanism
of Action
Step
1
Vitamin
K is converted into its reduced active form.
Step
2
Reduced
vitamin K serves as a cofactor for γ-glutamyl carboxylase.
Step
3
Specific
glutamate residues in target proteins are converted into γ-carboxyglutamate
residues.
Step
4
The
modified proteins gain the ability to bind calcium ions.
Step
5
Calcium
binding allows appropriate interaction with phospholipid surfaces and other
molecular components.
Important
Examination Points
One-Line
Facts
- Vitamin
K is a fat-soluble vitamin.
- The
name K originates from Koagulation.
- Vitamin
K₁ is called phylloquinone.
- Vitamin
K₂ refers to menaquinones.
- Vitamin
K₃ is known as menadione.
- Green
leafy vegetables are rich in vitamin K₁.
- Vitamin
K₂ occurs in fermented foods and other sources.
- Vitamin
K is required for γ-carboxylation.
- The
enzyme involved is γ-glutamyl carboxylase.
- Vitamin
K-dependent proteins contain γ-carboxyglutamate (Gla) residues.
- Vitamin
K-dependent clotting factors include II, VII, IX and X.
- Protein
C and Protein S are also vitamin K-dependent.
- Osteocalcin
is a vitamin K-dependent protein associated with bone.
- Matrix
Gla Protein is involved in mineralization regulation.
- Vitamin
K deficiency may cause bleeding.
- Prothrombin
time may be prolonged during deficiency.
- Warfarin
interferes with the vitamin K cycle.
- Warfarin
inhibits vitamin K epoxide reductase.
- Newborns
are particularly susceptible to vitamin K deficiency.
- Natural
vitamin K₁ and K₂ have low toxicity under normal dietary conditions.
Quick
Comparison of Forms
|
Vitamin |
Name |
Major Source |
|
K₁ |
Phylloquinone |
Green
plants |
|
K₂ |
Menaquinone |
Bacteria
and fermented foods |
|
K₃ |
Menadione |
Synthetic
form |
Academic
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