Vitamins and

Metals as
Coenzymes
Outline..
• Metals as Cofactors
• Vitamins as Coenzymes

• Exceptions….
– Cofactors/ Coenzymes – non vitamins
– Metals/ Vitamins – not coenzymes
From Enzymes…
• Some enzymes ‘require no’ chemical groups for
activity other than their amino acid residues.
• Others ‘require an additional chemical’ component
called a cofactor (inorganic ions) or coenzyme
(complex organic or metalloorganic molecule).
• Cofactors: additional chemical component inorganic
ions:
Fe2+, Mg2+, Mn2+ or Zn2+.
• Coenzyme: complex organic or metallorganic
molecule, transient carriers of specific function
groups.
Vitamins
Types of Cofactors in Enzymes
Some definitions..
• Holoenzyme : A complete, catalytically active enzyme
together with its bound coenzyme and/or metal ions)
– Polypeptide portion of enzyme (apoenzyme)
– Nonprotein prosthetic group (cofactor)

• The prosthetic group may be organic (such as a

vitamin, sugar, or lipid) or inorganic (such as a metal
ion).
• Prosthetic groups are bound tightly to proteins and
may even be attached through a covalent bond.
Example:
• Heme is an organic compound containing
iron that is the coenzyme for a variety of
enzymes known as the cytochromes.
• It’s also the prosthetic group of hemoglobin,
the protein in RBC that bind and transports
oxygen (hemoglobin, myoglobin).
• Some compounds referred to as coenzymes
are really cosubstrates. They bind and leave
in the same way as other substrates.

• They are not really coenzymes, but they are

present in the cell in very much smaller
concentration than most substrates (µmolar
rather than mmolar), and are involved in the
large number of reactions, so that they turn
over rapidly.
MICROnutrients
• Structure
– Individual Units
– NOT linked together – i.e. NOT building blocks like
for glucose, fatty acids, amino acids
• Function
– Do NOT yield energy when broken down
– Assist enzymes and other biological factors that
release energy from CHO, Pro, Fats = coenzymes
• Amounts needed
– Measured in micrograms or milligrams, not grams
Metals as Co-factors
 Cofactors
• Cofactors are
bound to the
enzyme for it to
maintain the
correct
configuration of
the active site
– Metal ions
– Organic compounds
– Organometallic
compounds
Metal ion catalysis
• 1/3 of known enzymes require metal ions for catalysis

Metal ions can …

• Electrostatically stabilizing or shielding negative

charges.
• Act much like a proton but can be present in high
concentration at neutral pH and can have multiple
positive charges
• Act to bridge a substrate and nucleophilic group.
• Bind to substrates to insure proper orientation.
• Participate in oxidation/reduction mechanisms
through change of oxidation state.
 Ion Examples of enzymes containing this ion
Cupric Cytochrome oxidase
Ferrous or Ferric Catalase
Cytochrome(via Heme)
Nitrogenase
Hydrogenase
Magnesium Glucose 6-phosphatase
Hexokinase
Manganese Arginase
Molybdenum Nitrate reductase
Nickel Urease
Selenium Glutathione peroxidase
Zinc Alcohol dehydrogenase
Carbonic anhydrase
DNA polymerase

http://en.wikipedia.org/wiki/Cofactor_%28biochemistry%29
Exceptions..
1. Although chromium deficiency causes impaired
glucose tolerance, no human enzyme that uses this
metal as a cofactor has been identified.
2. Iodine is also an essential trace element, but this
element is used as part of the structure of thyroid
hormones rather than as an enzyme cofactor.
3. Calcium is another special case, in that it is required
as a component of the human diet, and it is needed
for the full activity of many enzymes: such as nitric
oxide synthase, protein phosphatases or adenylate
kinase, but calcium activates these enzymes in
allosteric regulation, often binding to these enzymes
in a complex with calmodulin. Calcium is therefore a
cell signaling molecule, and not usually considered
as a cofactor of the enzymes it regulates
Vitamins as
Co-enzymes
Coenzymes
• A coenzyme is required
by some enzymes
– An organic molecule
bound to the enzyme by
weak interactions /
Hydrogen bonds
– Most coenzymes carry
electrons or small groups
– Many have modified
vitamins in their structure
The vitamins are grouped into two categories based on
their solubility:
1. Water soluble vitamins
2. Fat soluble vitamins
• Water soluble vitamins which include
i. Vitamin B complex, e.g.
– Thiamine (vitamin B1)
– Riboflavin (vitamin B2)
– Niacin (vitamin B3)
– Pantothenic acid (vitamin B5)
– Pyridoxine (vitamin B6)
– Biotin
– Folic acid
– Cobalamin (vitamin B12)
ii. Vitamin C or ascorbic acid.
• Fat soluble vitamins, which include
– Vitamin A or retinol
– Vitamin D or cholecalciferol
– Vitamin E or tocopherol
– Vitamin K.
 Vitamin classification
VITAMINS

WATER SOLUBLE FAT SOLUBLE

Vitamin A (Retinol, beta-carotene)

Non-B-Complex Vitamin D (Cholecalciferol)
Vitamin K (Phylloquinones,
Vitamin C (Ascorbic Acid) Menaquinones)
B-Complex Vitamin E (Tocopherols)

Energy Releasing Hematopoietic Other

Vitamin B1 (Thiamine) Folic Acid Vitamin B6 (Pyridoxin)

Vitamin B2 (Riboflavin) Vitamin B12 Pyridoxal
Vitamin B3 (Niacin) Pyridoxamine
Biotin
Pantothenic Acid
• In metabolism, coenzymes are involved in both
group-transfer reactions
• coenzyme A and adenosine triphosphate (ATP),
and redox reactions and redox reactions
• coenzyme Q10 and nicotinamide adenine
dinucleotide (NAD+).
• Coenzymes molecules are often vitamins or are
made from vitamins.
• Many coenzymes contain the nucleotide adenosine
as part of their structures, such as ATP, coenzyme A
and NAD+.
 Vitamins and derivatives:
Coenzyme Vitamin Additional Chemical groups
component transferred
NAD+ and NADP+ Niacin (B3) ADP Electrons

Coenzyme A Pantothenic Acid ADP Acetyl and acyl groups

(B5)
Tetrahydrofolic Folic acid (B9) Glutamate Methyl, formyl, methylene,
acid residues (one C-units)
Menaquinone Vitamin K None Carbonyl and electrons

Ascorbic Acid Vitamin C None Electrons

Coenzyme Riboflavin (B2) Amino acids Electrons

,FAD+, FMN
Biocytin Biotin

Thiamine Thiamin (B1) Aldehyde

pyrophosphate
 Non-vitamins:
Coenzyme Chemical group transferred
Adenosine triphosphate Phosphate group
S-Adenosyl methionine Methyl group
Coenyzme Q Electrons
Coenyzme M Methyl group
Coenzyme B Electrons
Methanofuran Formyl group
THIAMINE
Functions
• Thiamine is required mainly for carbohydrate metabolism
• Thiamine pyrophosphate (TPP) is a coenzyme involved in several
enzymatic reactions mainly for oxidative decarboxylation and
transketolase reactions as follows:

1 TPP is a coenzyme for pyruvate dehydrogenase complex which

catalyzes the conversion of pyruvate into acetyl CoA by oxidative
decarboxylation.

Acetyl-CoA is a precursor for the synthesis of the neurotransmitter

acetylcholine and also for the synthesis of myelin. Thus, thiamine
is required for the normal functioning of the nervous system.
2 TPP is a coenzyme for α-ketoglutarate dehydrogenase which catalyzes
the conversion of α-ketoglutarate to succinyl-CoA in TCA cycle.

3 TPP is a coenzyme for the enzyme transketolase, in the pentose

phosphate pathway of glucose oxidation.
Riboflavin (Vitamin B2)
• Energy yielding metabolism – CHO
• Coenzyme flavin mononucleotide (FMN)
and flavin adenine dinucleotide (FAD)
• Electron Carriers in redox reactions
– Oxidases
• Deficiency disease: Seborrheic Dermititis
– non fatal
When the enzyme contain riboflavin, it is usually
covalently bound at the active site,
whereas riboflavin phosphate and FAD are not
normally covalently bound to the enzyme
BUT they are very tightly bound and can be
thought of as prosthetic group.
Riboflavin and the
coenzymes: FMN and FAD
Deficiency Manifestations
• Riboflavin deficiency is quite rare as it has a wide distribution in food stuffs.
It is usually seen along with deficiencies of other vitamins of B-complex group.
It is most commonly seen in chronic alcoholics.

• The characteristic symptoms of riboflavin deficiency are:

– Cheilosis: Fissures at the angles of the mouth,
– Glossitis: Inflammation of the tongue that
becomes swollen and magenta colored
– Dermatitis: Rough and scaly skin
Niacin (Vitamin B3)
• Can be made from the amino acid tryptophan in the body –
not a vitamin
• Nicotinic acid and nicotinamide, have the biologic activity of
niacin
• Coenzymes NAD (nicotinamide adenine dinucleotide) and NADP
(Nicotinamide adenine dinucleotide phosphate)
• Redox reactions
• Deficiency: Pellagra is characterized by a photosensitive
dermatitis
• Hartnup disease is a rare genetic condition – pellagra (defect
of the membrane transport mechanism for
tryptophan)
Unlike flavins, the nicotinamide nucleotide
coenzymes do not remain bound to the
enzyme, but act as substrates: binding to the
enzyme, undergoing reduction, and leaving.

The reduced coenzymes is then reoxidized:

either by reaction with another enzyme, or by
way of the mitochondrial electron chain.
Pantothenic acid
• Acyl group metabolism when acting as the
pantetheine functional moiety of coenzyme A
or acyl carrier protein (ACP)
• CoA takes part in reactions of the citric acid
cycle, fatty acid synthesis and oxidation,
acetylations, and cholesterol synthesis.
• ACP participates in fatty acid synthesis.
• Deficiency is unknown (vitamin is widely
distributed in all foodstuffs)
Pantothenic acid is formed by a combination of pantoic acid and β-alanine
(Active form)

Active forms of pantothenic acid are:

• Coenzyme-A (CoA-SH)
• Acyl carrier protein (ACP).
Pantothenic acid and coenzyme A.
FUNCTIONS
• Pantothenic acid is a component of coenzyme-A (CoASH) and acyl
carrier protein (ACP). The thiol (-SH) group of CoA-SH and ACP acts as a
carrier of acyl groups.
• Coenzyme-A participates in reactions concerned with:
– Reactions of citric acid cycle
– Fatty acid synthesis and oxidation
– Synthesis of cholesterol
– Utilization of ketone bodies.
• ACP participate in reactions concerned with fatty acid synthesis.
Pyridoxine (Vitamin B6)
Vitamin B6 consists of a mixture of three different closely related
pyridine derivatives namely:
1. Pyridoxine
2. Pyridoxal
3. Pyridoxamine.
All the three have equal vitamin activity, as they can be
interconverted in the body.
Active Form of Vitamin B6 Pyridoxal phosphate (PLP) is the
active form of vitamin B6. PLP is formed from phosphorylation of all
three forms of vitamin B6.
Functions
• Active form of vitamin B6, pyridoxal phosphate (PLP) acts as coenzyme in large
number of reactions of amino acid metabolism.

For example:
– Transamination
– Decarboxylation
– Non-oxidative deamination
– Trans-sulfuration
– Condensation reactions of amino acids.
Transamination reactions:
Transamination reactions are catalyzed by transaminases and PLP acts as
coenzyme converting amino acid to keto acid, e.g. aspartate transaminase
(AST) and alanine transaminase (ALT).

Decarboxylation reactions:
PLP acts as coenzyme in decarboxylation of some amino acids. The amino
acids are decarboxylated to corresponding amines.
i) Formation of GABA
ii) Formation of serotonin and melatonin
iii) Formation of histamine
iv) Formation of dopamine

Non-oxidative deamination: Hydroxyl group containing amino acids (serine,

threonine) are non-oxidatively deaminated to α-keto acids and ammonia,
which requires PLP.
Cobalamin (Vitamin B12)
• Synthesized exclusively by microorganisms, found
only in foods of animal origin, no plant sources
• Three Vitamin B12-Dependent Enzymes
• Methylmalonyl CoA mutase, leucine
aminomutase, and methionine synthase

• Deficiency
• Pernicious anemia arises when vitamin B12
deficiency blocks the metabolism of folic acid, leading
to functional folate deficiency. (Pancreatic
insufficiency)

• Impairment of methionine synthase in B12 deficiency

results in the accumulation of methyl-
tetrahydrofolate—the “folate trap.”
Vitamin B12 (cobalamin)

R = CN– in cyanocobalamin;
R = OH- in hydroxocobalamin;
Tetrahydrofolate Is a Carrier of One-Carbon Units

• Tetrahydrofolate can carry one-carbon

fragments attached to N-5 (formyl, formimino,
or methyl groups), N-10 (formyl group), or
bridging N-5 to N-10 (methylene or methenyl
groups).
• 5-Formyl-tetrahydrofolate is more stable than
folate as folinic acid in the synthetic
(racemic) compound leucovorin.

Tetrahydrofolate Folinic acid

Inhibitors of Folate Metabolism Provide
Cancer Chemotherapy & Antibacterial &
Antimalarial Drugs
• Thymidylate synthase and dihydrofolate reductase
are especially active in tissues with a high rate of cell
division.
• Methotrexate, an analog of 10-methyl-
tetrahydrofolate, inhibits dihydrofolate reductase and
has been exploited as an anticancer drug.
• The dihydrofolate reductases of some bacteria and
parasites differ from the human enzyme;
• inhibitors of these enzymes can be used as
antibacterial drugs, eg, trimethoprim, and
antimalarial drugs, eg, pyrimethamine.
Biotin
• Biotin is widely distributed in many foods as
biocytin
• Synthesized in intestinal flora in excess of
requirements.
• Functions to transfer carbon dioxide in a
small number of carboxylation reactions.
• Deficiency is unknown except among people
maintained for many months on parenteral
nutrition
Biotin is a coenzyme of carboxylase reactions, where it is a carrier of CO2.
Some of the carboxylation reactions requiring biotin are given below.
• Conversion of acetyl-CoA into malonyl-CoA catalyzed by acetyl-CoA
carboxylase in fatty acid synthesis.
• Conversion of pyruvate into oxaloacetate, catalyzed by pyruvate
carboxylase in gluconeogenesis.
• Conversion of propionyl-CoA to D-methyl malonyl- CoA catalyzed by
propionyl-CoA carboxylase in the pathway of conversion of propionate to
succinate
• It is also involved in the catabolism of branched chain amino acid catalyzed
by β-methyl-crotonyl- CoA carboxylase.
Ascorbic Acid (Vitamin C)
• Ascorbic acid is a vitamin for only some
species
• Vitamin C Is the Coenzyme for Two
Groups of Hydroxylases
– copper-containing hydroxylases and
– α-ketoglutarate-linked iron-containing
hydroxylases
• As a reducing agent
• Oxygen radical quencher
• During hydroxylation, the Cu+ is oxidized to
Cu2+; reduction back to Cu+ specifically
requires ascorbate, which is oxidized to
monodehydroascorbate.

• A number of iron-containing, ascorbate-

requiring hydroxylases share a common
reaction mechanism in which hydroxylation of
the substrate is linked to decarboxylation of
α-ketoglutarate
• Functions:
• Helps to form fibrous structural protein of
connective tissues – collagen
• Enhances the immune system

• Deficiency: Scurvy include

– skin changes,
– fragility of blood capillaries,
– gum decay,
– tooth loss and
– bone fracture, many of which can be attributed to
deficient collagen synthesis.
Other coenzymes
• Moves freely within mitochondrial membrane.
• Carries electrons.
• Coenzyme Q10 (CoQ 10) or ubiquinone is essentially
a vitamin or vitamin-like substance.
– Found in small amounts in a wide variety of foods.
– It is synthesized in all tissues.
– The biosynthesis of CoQ10 from the amino acid tyrosine
requires at least eight vitamins and several trace elements.
• Coenzyme Q10 is the coenzyme for at
least three mitochondrial enzymes
(complexes I, II and III) as well as enzymes
in other parts of the cell.

• CoQ10 has been studied in its reduced form

as a potent antioxidant.

2H++2e

Coenzyme Q
Coenzyme Q
Function:
A vital 'transport agent' in the use of oxygen to
generate energy, especially in the heart
muscle and liver (highest E requirements)

Supplements do aid in many forms of heart

disease, gum disease, BUT >> slows
Parkinson’s / Alzheimer’s/ aging, relieves
allergies, boosts athletic performance and
immunity, aids weight loss
• CoQ10 deficiency may be caused by
1. Insufficient dietary CoQ10.
2. Impairment in CoQ10 biosynthesis.

• HMG-CoA reductase inhibitors used to treat elevated

blood cholesterol levels by blocking cholesterol
biosynthesis also block CoQ10 biosynthesis.