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Introduction
Vitamins are an organic chemical
compound which the body requires in
small amounts for the metabolism and
to protect your health.
Vitamins assist the body in
functioning properly by helping in
the formation of hormones, blood
cells, nervous-system chemicals and
genetic growth. An over dose can be
harmful to your health.
1
The Body & Vitamins
The body can only produce one
vitamin naturally by itself. This
is vitamin D. All other vitamins
that the body requires to
function properly have to be
derived from the diet. Lack of
vitamins can have a serious
affect on your health and may
end in metabolic and other
dysfunctions.
2
Origin of the word VITAMIN
• Casimir Funk, a Polish biochemist, isolated an antiberberi substance from rice polishing.
– Named it vitamine
• An amine
• Vital for life
• Originally it was thought these necessary compounds
were all amines. Since they were vital to our health they
became known as
“vital amines”, ie. vitamines.
• When it was discovered that some were not amines, i.e.,
not ' --ines', the name was changed to
vitamins
3
What are Vitamins?
• Vitamins are micronutrients (nutritionally
important organic compounds required in very
small amounts).
• Plants and animals synthesize vitamins.
– Vitamins form through biochemical life processes of the
plants and animals we eat.
Examples:
– Most mammals can synthesize vitamin C; not humans
and primates.
– No mammal can synthesize B vitamins but rumen
bacteria do.
• Some function as vitamins after undergoing a
chemical change
4
– Provitamins (e.g., β-carotene to vitamin A).
Vitamin Groups
Vitamins are divided up into two
main groups which are fatsoluble vitamins and watersoluble vitamins. Fat-soluble
vitamins are usually found in
foods that contain fat.
The body stores the fat soluble
vitamins and because of this,
people don’t usually need to
make a special effort to include
them in their diet.
5
Vitamin Groups
Water soluble vitamins can’t
be stored in the body for a
long time and have to be
replenished everyday.
In some cases when it’s not
possible to obtain these
vitamins in a regular diet,
they have to be acquired by
other vitamin supplements.
6
Vitamin classification
7
Name(Letter)
RDI
Retinol (A)
5000 IU
Calciferol (D)
400 IU
Tocopherol (E)
30 IU
Phylloquinone (K)
70 g
Classification, Requirements, Absorption
Water-soluble
• Absorbed at the small
intestine.
• Absorption often highly
regulated by either other
vitamins or binding proteins in
the small intestine.
• Transported away from small
intestine in blood.
• Typically not stored; instead,
kidney filters excess into urine
– Thus, important to get these
vitamins daily.
– Toxicities almost unheard of.
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Name(Letter) RDA
(mg)
Thiamin (B1)
1.5
Riboflavin (B2)
1.7
Niacin (B3)
Pantothenic acid (B5)
Pyridoxine (B6)
2
10
2
Biotin (B7)
Folic acid (B9)
Cobalamin (B12)
Ascorbic acid (C)
0.3
0.4
6 g
60
Classification, Requirements, Absorption
Oil-soluble
Name(Letter)
RDI
Retinol (A) 5000 IU
Calciferol (D)
Tocopherol (E)
400 IU
30 IU
Phylloquinone (K)
70 g
• Absorbed with dietary fat in small intestine
• 40-90% absorption efficiency
• Absorption typically regulated by need
– need absorption
• Transported away from small intestine in chylomicra via
blood and lymph (depending on size)
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What do vitamins do?
•
•
•
•
Metabolically they have diverse functions as:
Coenzymes (B vitamins)
Hormones (retinoic acid, vitamin D)
Modulators or regulators of growth and
development (retinoic acid, folic acid)
• (apparently non-specific) antioxidants (Vitamins
C and E)
10
Coenzymes, Cofactors, and
Prosthetic groups
• Vitamins bind the enzyme either loosely or
tightly:
– Coenzymes are lost upon dialysis because they bind
the enzyme loosely.
– When they bind enzymes tightly, they are considered
prosthetic group.
– The term cofactor includes such compounds but also
includes other molecules such as metal ions that may
be necessary for enzyme activity.
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Cofactors and coenzymes
12
13
Vitamin A
Compounds with 20-carbon structure.
Contain a methyl substituted cyclohexenyl ring (-ionone ring),
and an isoprenoid side chain with either a hydroxyl group, and aldehyde group, a
carboxylic acid group, or an ester group (retinyl ester) at the terminal C15.
All-trans-retinal
Retinol
14
11-cis-retinal
Retinoic Acid
Can’t be reduced to retinol or retinal in the body.
Vitamin A
1.
Active vitamin A- Preformed vitamin A can be obtained
either directly from foods that are substantial in vitamin
A (beef liver, fish liver oils, egg yolks and butter)
•
•
2.
Provitamin A- or from provitamins, substances that are
transformed into vitamins in the body
•
•
•
15
The active form of vitamin is retinol, an alcohol which can be
converted to other forms (e.g. vitamin A esters) for storage in liver
and tissues.
much the body's vitamin A is stored in the liver as retinyl palmitate
Beta-carotene is the most abundant and widespread provitamin A.
Beta-carotene comes from a group of compounds called the
"carotenoids ".
Dark-green leafy vegetables (spinach) and yellow-orange
fruits (apricots and mango) and vegetables (carrots, yellow
squash and sweet potatoes) are high in beta-carotene and other
carotenoids .
Vitamin A: Biological functions
• Vitamin A (retinal) is an essential precursor for formation of the visual
pigment, rhodopsin, in the retina of the eye. Retinal plays an important
role in vision, especially night vision.
• Helps regulate cell development.
• Promotes the proper growth of bones and teeth. Bone cells (osteoblasts
and osteoclasts) depend on vitamin A for their normal functioning.
• Boosts the body's immune system helping to increase our resistance to
infectious diseases.
• Is important in the formation and maintenance of healthy hair, skin and
mucous membranes.
• Vitamin A holds an important place in sexual reproduction. Adequate
levels of vitamin A are needed for normal sperm production. The female
reproductive cycle requires sufficient amounts of vitamin A.
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Role of Vitamin A in Vision
1. When the rhodopsin is exposed to light it is bleached releasing the
11-cis-retinal from opsin.
2. Isomerization of the cis-isomer of retinal to all-trans-retinal, causes
conformational changes in rhodopsin, hyperpolarization of the retinal
rod cell, and extremely rapid transmission of electrical activity to the
brain via the optic nerve
3. Trans-retinal is isomerized to cis-retinal in the dark, which
associates with opsin to regenerate rhodopsin.
All trans retinol
= main circulating
form of Vit A
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Visual Pigment
Vitamin A: Deficiency symptoms
1.
Night blindness" - lessened ability to see in dim light.
2. Increased susceptibility to infection and cancer and anemia
equivalent to iron deficient.
3. Prolonged lack of vitamin A
(keratinization of the cornea, a condition known as
xerophthalmia).
4. Abnormal bone development in fetal and neonatal life.
5. Various congenital defects.
Retinol and its precursors are used as dietary
supplements to prevent the above symptoms.
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Vitamin A: Toxicity
•
•
•
•
•
19
Skeletal malformations
spontaneous fractures
internal hemorrhages
loss of appetite
slow growth or weight loss.
Vitamin D: Types and Sources
• Vitamin D2 (ergocalciferol) is derived
from plants and irradiated yeast and
fungi.
• Vitamin D3 (cholecalciferol) is
synthesized in the body when skin is
exposed to sunlight
– Cholesterol + sunshine = Vitamin D3
– “sunshine vitamin” – UV-B rays (5-10
minutes arms and legs, mid-day sun).
• We can obtain vitamin D3 from foods like
milk, fortified cereals, tuna, salmon and
fish oils.
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Sunlight
Vitamin D2
(Ergocalciferol)
Ergosterol
(in plants)
Diet
Sunlight
7-Dehydrocholesterol
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Vitamin D3
Cholecalciferol)
Activation of Vitamin D
• Vitamin D2 and vitamin D3 are
biologically inactive but can
have equal biological activity:
• Both can be converted first to
calcifediol in the liver and then
to calcitriol, also known as
1,25-dihydroxycholecalciferol,
in the kidneys.
• Calcitriol, which is the most
active form of vitamin D3, is
then transported via a carrier
protein to the various sites in
the body where it is needed.
Calcitriol is also called
1,25-dihydroxy vitamin D3,
or (1,25-(OH)2D3.
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1,25-dihydroxyvitamin D3
Conversion of 25-(OH)D3 to its
biologically active form, calcitriol,
occurs through the activity of a
specific D3-1-hydroxylase present
in the proximal convoluted tubules
of the kidneys, and in bone and
placenta. Cytochrome P450, O2
and NADPH are needed.
25-hydroxyvitamin D3
In the liver is hydroxylated at the
25 position cholecalciferol by a
specific D3-25-hydroxylase
generating 25-hydroxy-D3 [25(OH)D3] which is the major
circulating form of vitamin D.
23
Vitamin D Functions:
Hormone for Calcium and Phosphate regulation
• Nerves and muscles must function properly; calcium is vital
for nerve cell transmissions and muscle fiber contractions.
• Calcitriol functions in concert with parathyroid hormone
(PTH) and calcitonin to regulate serum calcium and
phosphorous levels by affecting:
– Dietary calcium absorption from the small intestine.
– Urinary calcium excretion
– Bone calcium metabolism
• There is evidence that vitamin D (specifically, vitamin D3) is
involved in regulation of the body's immune system.
• Vitamin D is essential for normal insulin secretion by the
pancreas and therefore control of blood sugar levels.
24
Vitamin D: Deficiency symptoms
– Rickets (bone deformities in children)
– Osteoporosis
– Osteomalacia (weak bones)
25
Vitamin D: Toxicity
• Nausea, thirst, loss of appetite, stupor
• Hypercalcemia: calcium gets deposited
in soft tissues, Arteries and kidneys.
26
27
Vitamin E
Four of the eight vitamin E molecules are called tocopherols (alpha, beta,
gamma and delta)). alpha-tocopherol is the most biologically active in
humans.
a-Tocopherol
is the most potent
of the tocopherols.
28
Functions
• Vitamin E in the form of alpha-tocopherol is
an important fat-soluble antioxidant,
scavenging oxygen free radicals, lipid peroxy
radicals and singlet oxygen molecules before
these radicals can do further harm to cells.
[Free radicals are very reactive atoms or
molecules that typically possess a single
unpaired electron.]
29
Free Radicals - the Metabolic Oxidizers
Free radical = unpaired electron very reactive
.
OH
OH
.
OH
OH
Oxygen radicals: Hydroxy (HO•) / Peroxy (HOO•)
30
An antioxidant is a chemical so easily
oxidized itself that it protects others from oxidation.
OH
and / or
Double Bond
eg. Vitamin A
31
Phenol
eg. Vitamin E or C
Vitamin E (deficiency)
• Deficiency: rare in adults usually due to impaired fat
absorption or transport; seen usually in children (anemia,
edema in infants)
• Excess: very safe below 800 IU/day
• Source:
– Vitamin E is present in animal fats, meat, green vegetables,
nuts/seeds.
– Alpha-tocopherol is found in a number of vegetable oils, including
safflower and sunflower. It is also found in wheat germ. Soybean
and corn oils contain mainly gamma-tocopherol.
• The major site of vitamin E storage is in adipose tissue.
• Estimated requirements: 5mg/day = 0.6mg/day of
unsaturated fat.
• Uses:
• Hemolytic anemia in premature infants, unresponsive to B12, Fe
and folic acids.
– Macrocytic megaloblastic anemia seen in children with severe proteincalorie malnutrition.
32
Vitamin K
• The "K" in vitamin K comes from the German word
"koagulation," which refers to blood clotting (coagulation).
• Vitamin K is essential for the functioning of several proteins
involved in normal blood clotting. Vitamin K is needed for the
body to make four of the blood's coagulation factors,
including prothrombin (also known as factor II), proconvertin
(factor VII), Christmas factor (factor IX) and the StuartPower factor (factor X).
33
Vitamin K1
• Naturally occurring vitamin K is absorbed from the
intestines only in the presence of bile salts and other lipids
through interaction with chylomicrons. Therefore, fat
malabsorptive diseases can result in vitamin K deficiency.
• Present in green leafy vegetables like lettuce, parsley,
spinach and various greens (beet and mustard). Broccoli
and certain vegetable oils (soybean, cottonseed, and olive).
are also a good source of vitamin K1.
__
34
Vitamin K2
• Vitamin K2 is a group of compounds called the "menaquinones."
• Synthesized by intestinal bacteria "n" can be 6, 7 or 9 isoprenoid
groups.
•
• Vitamin K2, which is the most biologically active form of vitamin K, is
found in egg yolks, butter, liver, cheddar cheese and yogurt.
• It has been suggested that products like yogurt, may help to increase
the functioning of these useful bacteria.
__
35
_______
Vitamin K3
• The synthetic (man-made)
vitamin K3 is water soluble
and absorbed irrespective
of the presence of
intestinal lipids and bile.
Uses : essential cofactor in blood clotting.
Deficiency: Rare, (bruising/bleeding in infants).
Excess: Dangerous if taking anti-coagulants.
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Vitamin K cycle
GLU
residue
R
NH
CH
O2 + CO2
CH2
CH2
R
+
H2O +H
CO2 -
CH
CO2CO2-
C= O
R
R
K(red)
K(epox)
vitamin K
reductase
epoxide
reductase
K(ox)
37
NH
CH CH2
carboxylase
C= O
GLA
residue
D
i
e
t
coumarins

Ca

Thrombin Activation
vWF
WOUND
collagen
endothelium

Thrombin
Pro-Thrombin
platelet
Va
Xa


 
Ca

 


Ca

 
Gla
Gla
Gla Gla
S
S
S
S
proteolytic cut
PL surface
ProNH2
NH2
COOH
COOH
C
i
r
c
u
l
a
t
i
o
n
38
The common pathway
*Xa
Va
prothrombin
Common
pathway
39
V
fibrinogen
*thrombin
XIII
CLOT
XIIIa
Fibrin monomer
Fibrin polymer
Vitamin B Complex
• Originally thought to be one vitamin, BUT
Vitamin Chemical name
B1
B2
B3
B4
B5
B6
B7
B8
B9
B10
B11
B12
40
Thiamine
Riboflavin
Nicotinamide (niacin)
Adenine (no longer considered a vitamin)
Pantothenic acid
Pyridoxine
Biotin
Folacin (folic acid)
Folacin (folic acid)
p-aminobenzoic acid (PABA) / H1
L-carnitine / b-hydroxy-g-trimethylammonium butyrate
Cyanocobalamin
41
Chemical structure
pyrimidine + thiazole
Thiamine
MgATP2-
Thiamine
diphosphotransferase
MgAMP-
42
TPP
O
O
H3C
CH2
N
H3C
CH2
H
O
P
O
O
+
N
C
N
CH2
P
O
S
acidic H+
NH2
thiamine pyrophosphate (TPP)
Thiamine pyrophosphate (TPP) is a derivative of
thiamine (vitamin B1).
Nutritional deficiency of thiamine leads to the
disease beriberi.
It affects especially the brain, because TPP is
required for CHO metabolism, and the brain
depends
on glucose metabolism for energy.
43
O
Thiamine Deficiency (B1) Beriberi
Wet beriberi – dilated cardiomyopathy
Due to peripheral dilation of arterioles
Dry beriberi – peripheral neuropathy, atrophy
Wernicke Korsakoff (see alcohol)
44
Alcohol, Wernicke Korsakoff syndrome:
Ataxia (inability to coordinate muscular movements due to nervous disorders)
and confusion
Memory loss/confabulation (to fill in gaps in memory by fabrication)
Alcohol dilated
cardiomyopathy
Opthalmoplegia – can’t follow light source
Nystagmus-involuntary jerking of the eye
45
Sources
• Widely distributed.
• Brewers' yeast is very rich source.
• Cereal grains are rich sources, especially in
germ and seed coat.
• Fresh green, leafy plants
• Animal products (especially egg yolk, liver,
kidney) are good sources.
• Synthetic vitamin is usually available as
thiamin hydrochloride.
46
Vitamin B2 or Riboflavin
• Yellow, crystalline compound with yellow-green
fluorescence in aqueous solution.
• Only sparingly soluble in water.
• Stable in acid or neutral, but not alkaline solutions.
• Unstable in light.
• Riboflavin is phosphorylated in the intestine to
generate FMN (riboflavin 5’-phosphate) by the
action of Flavokinase.
• FMN then reacts with ATP, yielding FAD:
FMN + ATP
+ ppi
FADFAD
synthetase
ppi = inorganic pyrophosphate.
47
Chemical structure
• Isoalloxazine ring system = dimethylbenzene + pteryn
• Ribitol (Reduced ribose) attached to N10
Riboflavin
48
Chemical structure and atom numbering of
the flavin mononucleotide
49
FMN
FAD
5
1
50
5
1
dimethylisoalloxazine
O
H
C
C
N
O

H3C
C
C
C
NH
H3C
C
C
C
C
C
H
N
H
C
+
2e +2H
O
N
H
N
H3C
C
C
C
NH
H3C
C
C
C
C
C
H
CH2
FAD
N
O
N
H
CH2
HC
OH
HC
OH
HC
OH O
H2C
C
O
P
O-
Adenine
O
O
P
O-
O
Ribose
FADH2
HC
OH
HC
OH
HC
OH O
H2C
O
P
O-
Adenine
O
O
P
O
Ribose
O-
FAD (Flavin Adenine Dinucleotide is derived from
the vitamin riboflavin. The dimethylisoalloxazine ring
system undergoes oxidation/reduction.
FAD is a prosthetic group, permanently part of E3.
- + 2 H+  FADH
Reaction:
FAD
+
2
e
51
2
52
Riboflavin Functions
• Essential constituent of the
– Flavoproteins
– Flavin mononucleotide (FMN)
– Flavin adenine dinucleotide (FAD)
• These play key roles in hydrogen transfer
reactions associated with
– Glycolysis
– TCA cycle
• Oxidative phosphorylation.
53
Deficiency symptoms
1.
Inappetence, poor growth,
vomiting, skin eruptions and
eye abnormalities in pigs.
•
•
•
•
Cheilosis/Angular stomatitis
(fissure at the angle of the
mouth)
Localized seborrheic dermatitis
of the face
Vascular changes in the cornea
Purple smooth tongue due to
loss of tongue papillae
(Glossitis).
Cheilosis/Angular stomatitis
2. Poor growth and "curled toe
paralysis" in chicks.
•
54
The toes frequently curl inward
and they may be unable to
stand.
Glossitis
Dietary Sources
• Dairy products
• organ meats (liver and heart) but not
muscle meat.
• Green leafy plants (especially alfalfa)
• Yeast and animal products
• Cereals are poor sources so poultry fed
cereal-based rations should receive
supplemental riboflavin.
55
Niacin = Vitamin B3
• Beta pyridine carboxylic acid
• Two forms: Nicotinic acid and Nicotinamide.
Nicotinamide is the amide derivative of nicotinic acid.
• In most animal species (including humans) niacin can be
synthesized from the essential amino acid, tryptophan.
• Both forms contain a pyridine ring.
Nicotinic acid
56
Nicotinamide
NAD+
Functions:
Active coenzymes:
nicotinamide-adenine
dinucleotide (NAD+)
nicotinamide-adenine
phosphate (NADP+).
Both are extremely
important in hydrogen
transfer reactions
catalyzed by
dehydrogenase
enzymes.
NADP+
ATP synthesis, from
oxidation of primary
fuels (glucose, fatty
acids and to a lesser
extent, amino acids)
(NAD+)
Also important in
reductive
biosynthesis (NADP+)
57
58
59
Deficiency symptoms
1. Pellagra in farm animals and humans (fiery inflammation of tongue, mouth and upper
esophagus).
2. Poor growth, enteritis and dermatitis.
3. Occurs in people who subsist mainly on corn which is low in both niacin and
tryptophan
4. The signs of pellagra include dermatitis, diarrhea, dementia (the three Ds) and loss of
tongue papillae.
Sources of B3
Most non-corn-based diets contain adequate amounts of nicotinamide or its
precursor, tryptophan.
60
vitamin B5 / pantothenic acid
• Chemical nature
• Dipeptide derivative of the amino acid Balanine and a butyric acid derivative.
61
Coenzyme A and Acetyl coenzyme A
• Essential constituent of coenzyme A, Pantothenic acid combines with
ATP and cysteine in the liver to generate CoA-SH.
• CoA-SH transfers activated acyl groups, R-(C=O)-, such as acetyl
group by binding them as a thioester. Acyl transfer is important in the
TCA cycle and de novo fatty acid synthesis.
62
Vitamin B5 deficiency
• Deficiency symptoms
• 1. Poor growth, diarrhea, loss of hair,
characteristic "goose-stepping" in pigs.
• 2. Poor growth and feather development,
dermatitis in chickens.
• Sources
• Widely distributed in plants (especially legumes
and cereal) and animal products.
• Deficiency has been observed in pigs fed a low
protein (14%) corn-soybean ration fortified with
minerals and vitamins except pantothenic acid.
63
Lipoic Acid &
DiHydroLipoic Acid (DHLA)
lipoic acid = Internal disulfur of 6,8-dithiooctanoic acid.
Lipoic Acid (LA) is part of a redox pair.
oxidized form
reduced form
64
S
CH2
CH2
S
lipoic acid
CH
O
CH2 CH2 CH2 CH2 C
Lipoamide
includes a
dithiol that
undergoes
oxidation/
reduction.
lipoamide
NH
NH (CH2)4 CH
C
O
2e + 2H+
HS
CH2
CH2
HS
O
CH
CH2 CH2 CH2 CH2 C
dihydrolipoamide
65
lysine
NH
NH (CH2)4 CH
C
O
S
CH2
CH2
S
CH
lipoic acid
O
CH2 CH2 CH2 CH2 C
lipoamide
lysine
NH
NH (CH2)4 CH
C
O
2e + 2H+
The carboxyl at the end of lipoic acid's hydrocarbon
HS CH
chain forms
an2 amide bond to the e-amino group of a
CH2
lysine
residue of E2, yielding NH
lipoamide.
HS
CH
O
A long flexible arm, including hydrocarbon chains
CH2 CH2 CH2 CH2 C NH (CH2)4 CH
of lipoate and the lysine R-group, links each dithiol
C O
of lipoamide to one of two lipoate-binding domains
of
E2.
66
Structure
PDH = Pyruvate
dehydrogenase
complex
67
Lipoic acid
• Alpha Lipoic acid is a natural substance
found in certain foods and also produced
in the human body.
• Alpha Lipoic acid is a disulfide
compound found naturally in
mitochondria as the coenzyme for
pyruvate dehydrogenase and aketoglutarate dehydrogenase.
68
The coenzyme function for pyruvate dehydrogenase
and a-ketoglutarate dehydrogenase
69
Pyruvate dehydrogenase complex (PDH)
The reaction is:
Pyruvate + NAD+ +CoASH
PDH
Acetyl CoA + NADH + H+ + CO2
5 non-protein molecules
(coenzymes) required for this
enzyme catalyzed reaction are:
NAD+ and CoASH (coenzyme
A); (these are present in the
equilibrated reaction formula,
as can be seen above)
TPP (thiamine pyrophosphate),
Lipoic acid and FAD (flavin
adenein dinucleotide)
participate in the reaction but
do not show up in the
equilibrated reaction formula.
E1 = Pyruvate dehydrogenase
E2 = Dihydrolipoamide acyltransferase
70
E3 = Dihydrolipoamide dehydrogenase
O
In the overall reaction
catalyzed by the
Pyruvate
Dehydrogenase
complex, the acetic
acid generated is
transferred to
coenzyme A.
C CH3
Coenzyme A-SH + HO
acetic acid
O
Coenzyme A-S
C
CH3 + H2O
acetyl-CoA
H
The final electron
acceptor
is NAD+.
71
O
H
H
C
C
NH2
+
N
O
NH2
2e + H
+
N
R
R
NAD+
NADH
Sequence of reactions catalyzed by Pyruvate
Dehydrogenase complex:
1. The keto C of pyruvate reacts with the carbanion
of TPP on E1 to yield an addition compound.
The electron-pulling (+) charged N of the thiazole
ring promotes CO2 loss. Hydroxyethyl-TPP
remains.
2. The hydroxyethyl carbanion on TPP of E1 reacts
with the disulfide of lipoamide on E2. What was
the keto C of pyruvate is oxidized to a carboxylic
acid, as the lipoamide disulfide is reduced to a
dithiol.
72
The acetate formed by oxidation of the
hydroxyethyl is linked to one of the thiols of the
reduced lipoamide as a thioester (~).
Sequence of reactions (continued)
3. Acetate is transferred from the thiol of
lipoamide to the thiol of coenzyme A, yielding
acetyl CoA.
4. The reduced lipoamide, swings over to the E3
active site.
Dihydrolipoamide is reoxidized to the
disulfide, as 2 e- + 2 H+ are transferred to a
disulfide on E3 (disulfide interchange).
5. The dithiol on E3 is reoxidized as 2 e- + 2 H+
are transferred to FAD.
73
The resulting FADH2 is reoxidized by electron
transfer to NAD+, to yield NADH + H+.
View an animation of the Pyruvate Dehydrogenase
reaction sequence.
O
H3C
C
S
CoA
acetyl-coenzyme A
Acetyl CoA, a product of the Pyruvate
Dehydrogenase reaction, is a central compound in
metabolism.
74
The "high energy" thioester linkage makes it an
excellent donor of the acetate moiety.
glucose-6-P
Glycolysis
pyruvate
fatty acids
acetyl CoA
oxaloacetate
ketone bodies
cholesterol
citrate
Krebs Cycle
Acetyl CoA functions as:
 input to Krebs Cycle, where the acetate moiety
is further degraded to CO2.
 donor of acetate for synthesis of fatty acids,
75
ketone bodies, & cholesterol.
Mechanism of the reaction catalyzed by
PDH complex
76
E1 uses TPP to release CO2 and produce
HydroxyethylTPP (HETPP)
77
E2 uses lipoic acid to transfer the
hydroxyethyl group from TPP to CoASH in
order to produce AcetylCoA
78
79
80
6. PYRIDOXINE (vitamin B6)
B6 is involved in:
Amino acid metabolism
Breakdown of glycogen
Synthesis of epinephrine (adrenaline) and norepinephrine (noradrenaline)
Synthesis of globular proteins
Conversion of certain fatty acids
Synthesis of niacin (vitamin B3) from the amino acid tryptophan.
• Three interconvertible forms (Vitamers) exist in
tissues:
– Pyridoxine (alcohol) (PN)
– pyridoxal (aldehyde) (PL)
– pyridoxamine (amine) (PM)
• All can easily convert to each other and to the
active form.
81
Chemical nature
Pyridoxal (PL)
82
Pyridoxamine (PM)
Pyridoxine
Pyridoxol (PN)
Each of these forms can be phosphorylated at position 5 to form:
PLP, PMP, and PNP.
Active form
• Active functional form is pyridoxal
phosphate (PLP) and pyridoxamine
phosphate (PMP).
• For absorption, the “phosphorylated”
form must be hydrolyzed to
“dephosphorylated” form by the enzyme
alkaline phosphatase in the intestine.
• In the portal vein Vit B6 is present as PL,
PM, PN.
• In the liver they are converted back to
phosphorylated forms. This conversion is
catalyzed by the ATP requiring enzyme,
pyridoxal kinase.
83
Pyridoxal phosphate (PLP)
• PLP and PL account for 90% of the
total B6 in the blood.
• In the blood B6 is transported both
in the plasma and the RBCs.
• In the blood PLP is hydrolyzed to
PL because only free PL gets inside
the cells.
• In muscle and other tissues, PL is
converted back to PLP by a reversible
reaction with the help of alkaline
phosphatase and pyridoxal kinase.
Functions
80-90% of body vit B6 is present in the muscles, most of it in PLP (coenzyme) form
bound to glycogen phosphorylase. Only 1 mol or less is present in the blood,
FUNCTIONS: A cofactor for enzymes involved in:
•Transamination reactions required for the synthesis and catabolism of
the amino acids.
•Decarboxylation reactions.
•Glycogenolysis as a cofactor for glycogen phosphorylase.
84
Vitamin-Coenzymes in Amino
Acid Metabolism
• Vitamin B-6 : pyridoxal
phosphate
– Enzymes that bind amino
acids use PLP as
coenzyme for binding
• Transaminases
• Amino acid
decarboxylases
• Amino acid deaminases
85
Covalent bonds of a-amino acids made labile by
their binding to PLP-containing enzyme
In the reactions of amino acid metabolism, the formyl (CHO) group of PLP
condenses with a-NH2 group of an amino acid and forms a Schiffs base. This
linkage weakens or labilizes all the bounds around the a-carbon of the amino acid.
The specific bond of an amino acid that is broken depends on the particular
enzyme to which PLP is attached.
86
Mechanism of catalyzed reaction
87
Biosynthesis of Amino Acids:
Transaminations
Amino Acid1 +a-Keto Acid2
NH3 +
-
O 2 CCH 2 CH 2 CHCO 2 -
Glutamate
Amino Acid2 +a-Keto Acid1
O
R-CCO 2 -
+
Pyridoxal phosphate (PLP)Dependent Aminotransferase
O
O 2 CCH 2 CH2 CCO 2 -
a-Ketoglutarate
88
+
NH2
R-CHCO 2 -
Transaminations: Role of PLP
CO2 H
CHO
CH2 OPO3-2
HO
H3 C
CH2 OPO3-2
HO
N
H3 C
+
H
-
C
NH3
+
N CHCH2 CH2 CO2-
N
+
H2 O
H
O 2 CCH 2 CH 2 CHCO 2 -
Tautomerization
CO2 -
O
-
O 2 CCH 2 CH 2 CCO 2 -
N CCH2 CH2 CO2-
CH2 NH2
HO
H3 C
89
CH2
CH2 OPO3-2
N
+
H
CH2 OPO3-2
HO
H2 O
H3 C
N
+
H
Decarboxylation reactions
Formation of -aminobutyric acid (GABA) •
from glutamate and formation of Serotonin.
Formation of neurotransmitters in the nervous •
system: norepinephrine, dopamine, histamine.
90
Deficiency
• Food sources:
– In animal foods major forms are PL and and PM along with their
phosphorylated forms.
– In plants PN.
– Bananas, beans, lentils, walnuts, salmon, chicken, beef, whole grain breads
and cereals, soybeans, liver, eggs, dairy products are excellent sources.
• Requirements:
– The requirement for vitamin B6 in the diet is proportional to the level of
protein consumption ranging from 1.4 - 2.0 mg/day for a normal adult.
– During pregnancy and lactation the requirement for vitamin B6 increases
approximately 0.6 mg/day.
• TOXICITIES:
– Megadoses of B6 (daily doses of >500mg) are used to treat pms symptoms.
They can cause neurotoxocity and photosensitivity in some individuals.
• Deficiencies: are rare and usually are related to an overall deficiency of
all the B-complex vitamins.
• Certain drugs form complexes with PL and PLP
– Penicillamine (used to treat rheumatoid arthritis and cystinurias).
– Isoniazid (the hydrazide derivative of isonicotinic acid) is the primary drug for
chemotherapy of tuberculosis.
91
7. BIOTIN
It is
sometimes
called vitamin
H and also
coenzyme R.
• Biotin is relatively small, bicyclic (two-ring)
compound formed from a tetrahydrothiophene
(thiophene) ring
,
• and a second ring, which contains a ureido group.
• The thiophene ring also has a valeric acid side chain.
• Although eight different stereoisomers of biotin exist,
only one stereoisomer is found naturally and to have
biologically activity as a coenzyme. It is called d-(+)biotin, D-biotin or simply biotin.
92
Biotin Cycle
Biotin cycle: the chain of chemical reactions involved in the use and reuse of the
vitamin biotin. One important role of biotinidase is
1. To separate or free biotin from proteins to which it is bound in foods. Biotin in its
free form can then be used by the body.
2. Biotinidase lets the body recycle or reuse the biotin over and over again so that
we do not need to consume large amounts of this vitamin in our diets.
•Within cells, the carboxylases
(pyruvate carboxylase, acetyl-CoA
carboxylase, methycrotonyl-CoA
carboxylase, propionyl-CoA
carboxylase) are biotinylated via
holocarboxylase synthetase. Biotin
and apocarboxylases are the
substrates. ATP and magnesium also
participate in the reaction.
Biotinidase deficiency is a treatable,
inherited metabolic disorder in which the
body cannot process the vitamin biotin in a
normal manner.
93
Holocarboxylase
In humans, the four holocarboxylases are : acetyl-CoA carboxylase,
propionyl-CoA carboxylase, pyruvate carboxylase and betamethylcrotonyl-CoA carboxylase. Biotin is chemically bonded in each of these
enzymes via an amide linkage between the carboxyl group of the valeric acid
side-chain in biotin and the epsilon-amino group of the lysine residue in the
apocarboxylase.
94
The enzyme that catalyzes the formation of this covalent bond is called
holocarboxylase synthetase.
Biotin (functions)
Coenzyme for several reactions involving CO2 fixation into various •
compounds e.g.
Acetyl CoA to malonyl CoA
(acetyl CoA carboxylase)
- initial step in de novo fatty
acid synthesis.
Pyruvate to oxaloacetate
(pyruvate carboxylase)
95
Propionyl CoA to methylmalonyl CoA
(propionyl CoA carboxylase)
Deficiency symptoms
• Rare because of widespread distribution in feeds
and significant lower gut synthesis.
• Can be induced by eating raw egg white
– The fact is that nature created the egg in such a way
that its yolk is very rich in biotin. One of the highest
concentration in nature. Eat the egg whole together
with the egg white and you will be fine.
– Egg whites contain a glycoprotein called "avidin"
which binds biotin - one of the B vitamins - very
effectively. The cooking process deactivates the
avidin in the egg, much the same way it deactivates
every other protein in the egg white.
• Biotin deficiency is chief cause of fatty liver and
kidney syndrome.
This baby developed severe
biotin deficiency during
intravenous feeding without
biotin.
Sources
• Yeast, rice, soybeans, peanuts, fish
(herring and mackerel), mushrooms and
bananas, safflower meal, liver and milk
are rich sources.
96
Aajonus Vonderplanitz,
in his book “We Want to
live” is a strong proponent
of raw eggs.
How Biotin Works
1- Biotin carrier protein
2- Biotin carboxylase
3- Transcarboxylase
97
VITAMIN B12 (cobalamin)
•
Vitamin B12, is also called cobalamin,
cyanocobalamin and
hydroxycobalamin.
•
It is built from : A nucleotide and a
complex tetrapyrrol ring structure (corrin
ring) and a cobalt ion in the center.
Vitamin B12 is synthesized exclusively
by microorganisms (bacteria, fungi and
algae) and not by animals and is found in
the liver of animals bound to protein as
methycobalamin or 5'deoxyadenosylcobalamin.
•
•
•
•
•
When R is cyanide (CN), vitamin B12
takes the form of cyanocobalamin.
In hydroxycobalamin, R equals the
hydroxyl group (-OH).
In the coenzyme forms of vitamin B12,
R equals an adenosyl group in
adenosylcobalamin.
R equals a methyl (-CH3) group in
methylcobalamin.
98
• Known as the "red" vitamin because it exists as
a dark red crystalline compound, Vitamin B12 is
unique in that it is the only vitamin to contain
cobalt (Co3+) metal ion, which by the way, gives it
the red color.
• The vitamin must be hydrolyzed from protein in
order to be active.
• Intrinsic factor, a protein secreted by parietal
cells of the stomach, carries it to the ileum where Dorothy Crowfoot Hodgkin
it is absorbed.
(1910-1994)
• It is transported to the liver and other tissues in
the blood bound to transcobalamin II.
• It is stored in the liver attached to
transcobalamin I.
– It is released into the cell as
Hydroxocobalamin
In the cytosol it is converted to
methylcobalamin.
– Or it can enter mitochondria and be
converted to 5’-deoxyadenosyl cobalamin.
Dr. Stadtman in her lab
99
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Absorption of Vitamin B-12 (Fig. 10-10)
100
Functions
•
Only two reactions in the body require vitamin B12 as a
cofactor:
1. During the catabolism of fatty acids with an odd number of
carbon atoms and the amino acids valine, isoleucine and
threonine the resultant propionyl-CoA is converted to
succinyl-CoA for oxidation in the TCA cycle.
– methylmalonyl-CoA mutase, requires vitamin B12 as a cofactor in
the conversion of methylmalonyl-CoA to succinyl-CoA.
– 5'-deoxyadenosine derivative of cobalamin is required for this
reaction
2. The second reaction catalyzed by methionine synthase
converts homocysteine to methionine
– This reaction results in the transfer of the methyl group from N5methyltetrahydrofolate to hydroxycobalamin generating
tetrahydrofolate and methylcobalamin during the process of the
conversion.
101
102
Methionine and Folate cycles are interrelated
Methionine cycle
Folate cycle
Methionine
THF
CH2-THF
SAM
MS
methyl
transferases
B12
CH3-THF
Methyl
CH3- acceptor
Homocysteine
SAH
CBS
B6
cystathionine
B6
103
Methyl
acceptor
cysteine
Transulfuration
pathway
Vitamin-Coenzymes in Amino
Acid Metabolism
• Vitamin B-12
– Catabolism of BCAA
• Methyl-malonyl CoA
mutase (25-9 &10)
104
Deficiency symptoms
• Pernicious anemia in humans (inability to absorb B12
because of lack of gastric intrinsic factor).
• Neurological disorders due to progressive demyelination
of nerve cells.
– This results from increase in methylmalonyl-CoA.
– Methylmalonyl-CoA is a competitive inhibitor of malonyl-CoA in
fatty acid biosynthesis.
– Can substitute malonyl-CoA in any fatty acid bisynthesis and
create branched-chain fatty acid altering the architecture of
normal membrane structure of nerve cells.
• Sources
– Synthesized only by microorganisms, so traces only are present
in plants; liver is a rich source.
– B12 is found in organ and muscle meats, fish, shellfish, dairy
products, eggs and in fortified foods like breakfast cereals.
105
106
9. FOLIC ACID (folacin)
• Folacin includes several derivatives of folic acid
(monopteroylglutamic acid).
• Active functional form is tetrahydrafolic acid.
• Folic acid is obtained primarily from yeasts and leafy vegetables as
well as animal liver. Animal cannot synthesize PABA nor attach
glutamate residues to pteroic acid, thus, requiring folate intake in the
diet.
107
Structure
Folic acid exists in a polyglutamate form. Intestinal mucosal cells
remove some of the glutamate residues through the action of the
lysosomal enzyme, conjugase.
108
Structure
Folic acid
PABA (vitamin Bx)
109
Folic acid is reduced
within cells (principally the
liver where it is stored) to
tetrahydrofolate (THF or
H4folate) through the
action of folate reductase
[or dihydrofolate
reductase (DHFR) ] which
is an NADPH-requiring
enzyme.
Folate
110
Dihydrofolate
Tetrahydrofolate
Active center (N5 and N10)
111
112
• Active center of tetrahydrofolate (THF). The N5 position is the site of
attachment of methyl and formimino groups, the N10 the site for
attachment of formyl group and that both N5 and N10 bridge the
methylene and methenyl groups.
folate conversions
113
Function
• Carrier of one-carbon (e.g. methyl) groups that
are added to, or removed from, metabolites such
as histidine, serine, methionine, and purines.
– Role of N5,N10-methylene-THF in dTMP synthesis is
the most metabolically significant function for this
vitamin.
– Vitamin B12 and N5-methyl-THF in the conversion of
homocysteine to methionine is important in helping
cells to regenerate needed THF.
114
Participation of H4folate in dTMP synthesis
______Deoxyuridine______________
________Deoxythymidine
____Monophosphate (dUMP)_______________Monophosphate (dTMP)_______
115
Vitamin-Coenzymes in Amino
Acid Metabolism
• Folacin:
Tetrahydrofolate
(THF)
– Carrier of single
carbons
•
•
•
•
•
116
Donor & receptor
Glycine and serine
Tryptophan degradation
Histidine degradation
Purine and pyrimidine
synthesis
Deficiency symptoms
• Identical to those for vitamin B12 deficiency.
• Effect of folate deficiency on cellular processes is upon DNA
synthesis.
– Impairment in dTMP synthesis
– Cell cycle arrest in S-phase of rapidly proliferating cells, especially
hematopoietic cells.
• The result is megaloblastic leukemia as for vitamin B12 deficiency.
• The inability to synthesize DNA during erythrocyte maturation leads
to abnormally large erythrocytes termed macrocytic anemia.
Deficiency is rare due to the adequate presence of folate in food.
•Poor dietary habits as those of chronic alcoholics
•Impaired absorption or metabolism or an increased demand for the vitamin.
•Pregnancy
•folate will nearly double by the third trimester of pregnancy
Certain drugs such as anticonvulsants and oral contraceptives can
impair the absorption of folate.
117
Ascorbic Acid Structure
OH
O
HO
HO
OH
(AscH2)
118
O
Vitamin C (Chemical nature)
• It is derived from glucose via uronic acid pathway. Enzyme
L-gluconolactone oxidase is reponsible for conversion of
gluconolactone to ascorbic acid.
• This enzyme is absent in primates, including humans, some bats….
• The active form is ascorbic acid itself.
1’
4’
2’
5’
6’
3’
6’
5’
4’
1’
3’
119
2’
AscH2 is a Di-acid
OH
O
HO
OH
O pK = 4.1
1
OH
HO
AscH2
O
HO
O
OH
O
OH
AscH
O
pK2 = 11.8 HO
O
O
2-
O
Asc
At pH 7.4, 99.95% of vitamin C will be present
as AscH ; 0.05% as AscH2 and 0.004% as Asc2. Thus, the antioxidant chemistry of vitamin C is
the chemistry of AscH .
120
OH
O
HO
HO
O
Forms of
Ascorbate
OH
AscH2
+H
+
+
-H pK = 4.1
OH
OH
O
HO
O
AscH-
O
+H+
-e
O
HO
OH
O
-H+ pK = 11.8
AscH
+H
OH
O
Asc2
+
OH
-H
+
pK = -0.86
OH
O
HO
O
O
-e
O
HO
O
OH
O
Asc
O
-e
O
O
O
O
O DHA O
-H2O +H2O
OH
HO
HO
HO
O
+H2O
HO
OH
HO OH
DHAA (2)
-H2O
O
O
O
OH
HO OH
DHAA (1) (>99%)
(pK ~ 8-9)
121
OH
HO
OH
O
-e -2H+
O
OH
O
HO
O
-e
+
+e +2H
HO AscH OH
HO
O
O
C
O
O
+H2O
Asc
O
O
C
H C OH
C O
H C OH
HO C H
CH2OH
CH2OH
L-xylonic
acid
2,3-diketo-Lgulonic acid
CH2OH
O
O
H C OH
122
+
C
O
OH
C
OH
C
HO C
CH2OH
L-xylose
OH
C O
HO C H
C O
O
O DHA O
OH
H C OH
O DHA O
O
+e
2
OH
HO
H C OH
HO C
OH
oxalic acid
CH2OH
L-threonic
acid
O
C
OH
HO C H
+
H C OH
HO C H
CH2OH
L-lyxonic acid
Ascorbate
Falling
Apart
-
AscH is a Donor Antioxidant
OH
O
HO
OH
+
OH
O
AscH
O
HO
O
O
R
+ RH
O
O
Asc
AscH- donates a hydrogen atom (H or H+ + e-) to an
oxidizing radical to produce the resonance-stabilized
tricarbonyl ascorbate free radical. AscH has a pKa of
-0.86; thus, it is not protonated in biology and will be
present as Asc-.
123
Ascorbate, Summary
Ascorbate is a versatile, water soluble, donor,
antioxidant.
Thermodynamically, it can be considered to be the
terminal, small-molecule antioxidant.
OH
O
HO
OH
+
OH
O
AscH
124
O
HO
O
O
R
+ RH
O
O
Asc
125
VITAMIN C
• Vitamin C is L-ascorbic acid, which is a colorless,
crystalline acid with strong reducing properties.
• Functions
• Vitamin C has antioxidant properties similar to those of
vitamin E,
– Protects cells from free radicals.
– Protects iron from oxidative damage, thus enhancing iron
absorption in the gut.
• The main function is as a reducing agent.
– It has the potential to reduce cytochrome a and c of the
respiratory chain and molecular oxygen and nitrates.
• It is required for various hydroxylation reactions e.g.
proline to hydroxypoline for collagen synthesis (see next
slide).
126
Hydroxylation of proline and lysine residues in
collagen
• Vitamin C is required for the maintenance
of normal connective tissue as well as for
wound healing since synthesis of
connective tissue is the first event in
wound tissue remodeling.
127
Other activities
• Several other metabolic reactions require
vitamin C as a cofactor:
• The catabolism of tyrosine and the synthesis of
epinephrine from tyrosine and the synthesis of
the bile acids.
• It is also believed that vitamin C is involved in
the process of steroidogenesis.
• The adrenal cortex contains high levels of
vitamin C which are depleted upon
adrenocorticotropic hormone (ACTH) stimulation
of the gland.
128
Roles in the body
Sources
• Citrus fruits and green leafy vegetables
• Vitamin C is readily absorbed and so the primary cause of vitamin C
deficiency is poor diet and/or an increased requirement.
Deficiency symptoms
1. Scurvy
–
–
–
–
–
–
Bleeding gums
Small red spots on skin
Rough skin
Wounds fail to heal
Weak bones and teeth
Anemia and infections
2. Stress (e.g., infections, smoking)
– Mechanism unknown, but vitamin C requirements increase during stress
3. Common cold?
4. Disease prevention?
– Cancer, heart disease
129
Some activated carriers in metabolism
Carrier molecule in
activated form
Group carried
Vitamin precursor
ATP
Phosphoryl
NADH and NADPH
Electrons
Nicotinate (niacin)
FADH2
Electrons
Riboflavin (vitamin B2)
FMNH2
Electrons
Riboflavin (vitamin B2)
Coenzyme A
Acyl
Pantothenate
Lipoamide
Acyl
Thiamine pyrophosphate
Aldehyde
Thiamine (vitamin B1)
Biotin
CO2
Biotin
Tetrahydrofolate
One-carbon units
Folate
S-Adenosylmethionine
Methyl
Uridine diphosphate
glucose
Glucose
Cytidine diphosphate
130
diacylglycerol
Phosphatidate