COENZYMES
What are enzymes and how do they work in
Biochemistry
• Brief explanation of enzyme catalyzed reactions (how enzymes lower
activation energy, substrate binding to active site of enzyme)
• Do all enzymes need a coenzyme or cofactors ?
• Why do some enzymes need a coenzyme ?
Enzymes
• Enzymes are biological substance that act as catalyst in biochemical reactions
that occur in a living organism.
• They are mostly proteins, but some are RNA molecules called Ribonucleases
(RNAse) or ribozymes (large subunit of rRNA, RNA splicing enzyme etc).
• As catalyst they increase the rate of biochemical reactions by lowering the
activation energy
Enzyme classification
1. Oxidoreductases: catalyze oxidation/reduction reactions
2. Transferases: transfer a functional group (e.g. amino group or phosphate
group)
3. Hydrolases: catalyze the hydrolysis of various bonds
4. Lyases: break various bonds by means other than hydrolysis and
oxidation
5. Isomerases: catalyze isomerization changes within a single molecule
6. Ligases: join two molecules with covalent bonds.
7. Translocases: catalyze the movement of ions or molecules across
membranes.
Enzyme Activity
How substrates bind to Enzymes
COENZYMES
• Coenzymes are complex organic molecules that are required to enhance
enzyme activities.
• Coenzymes are usually used, interchangeably with Cofactors. It is important
to differentiate between the two at the early onset of the lecture.
• Cofactor is an umbrella term that describes molecules that are required to
enhance enzyme activities, i.e needed for the enzyme to perform optimally.
• When the molecule is an organic molecule (usually with complex structure) it
is referred to as a C0ENZYME (such as NAD, FAD, TPP, Biotin, CoA)
• When the molecule is inorganic cation or metal ions (such as Mg, Zn, Mn),
they are referred to as co-factors.
• Therefore, all Co-enzymes are Co-factors but not all Co-factors are Coenzymes.
• Coenzymes are a sub-type of Cofactors.
Coenzymes
• One important distinguishing feature of coenzymes is that they carry
molecules or transfer molecules during the enzyme catalyzed reactions e.g,
transfer carboxyl, hydroxyl, formyl, Acetyl etc; or serve as electron carriers
(NAD, FAD)
• Cofactors do not transfer any molecule in an enzyme catalyzed reaction but
rather stabilize the enzyme/substrate during catalysis, e.g DNA Polymerase
containing Mg2+, uses the Magnesium ions to stabilize the negatively
charged structure of DNA (multiple PO42- grps in the structure), during DNA
polymerization.
Other terminologies related to Coenzymes
• Holo-enzymes: Enzyme along with its cofactors is called Holoenzymes.
e.g Hexokinase bound to Magnesium ion (Mg2+).
• Apo-enzymes: An enzyme (inactive) without the cofactor (or the
cofactor removed) is called Apo-enzyme. e.g Hexokinase alone
separated from the Mg2+ is an apo-enzyme.
• Prosthetic group: they are coenzymes (organic cofactors) that are
tightly (covalently) bound to an enzyme.
• Allosteric enzymes:
Coenzyme, Apoenzyme and Holoenzyme
Allosteric enzyme
• Allosteric enzymes are enzymes
with additional binding site,
different from the active site, to
which a regulatory/effector
molecule can bind.
QUESTION?
• The following are components of a Holoenzyme
• a. Apoenzyme + Coenzyme
• b. Apoenzyme + Cofactors
• c. Apoenzyme + Prosthetic group
• d. All of the above
Question
Which of the following statement is/are
correct;
a. Coenzymes = Apoenzymes +
Holoenzymes
b. Holoenzymes = Coenzymes +
Cofactors
c. Apoenzymes = Cofactor +
Holoenzymes
d. Holoenzymes = Apoenzymes +
Coenzymes
Vitamins acting as Coenzymes
• Some vitamins act as coenzymes when they participate in the
biochemical processes such as catabolism, anabolism or production of
energy.
• All the water-soluble enzymes can act as coenzymes.
• Vitamins A and K are example of fat-soluble vitamins that act as
coenzymes.
TPP (Thiamine/Vit B1), FADH2 (Riboflavin/Vit B2)
and NADH (Niacin/B3) as Coenzymes
NADPH as Coenzyme
• NADPH as Coenzyme
in Hexose
Monophosphate
shunt (HMP)
Biochemical Functions of NADPH
1. NADPH serve as coenzyme in many biosynthetic reactions e.g fatty acids
synthesis, Cholesterol synthesis, HMP.
2. Essential coenzyme in HMP for synthesis of pentose sugars (ribose and
deoxyribose) which serve as component of DNA and RNA.
3. It helps to regenerate reduced glutathione in presence of superoxide
dismutase, peroxidase and glutathione reductase.
4. NADPH alongside glutathione maintain membrane integrity of erythrocytes
5. It is essential for scavenging reactive oxygen species (ROS).
6. It is required for detoxification of drugs by microsomal P450 enzymes in the
liver.
7. It is required to maintain iron in reduced form (ferrous) inside Hb, and
prevent accumulation of met-haemoglobin.
8. It helps to maintain the transparency of eye lens for clear vision.
Coenzyme A (Vit B5)
• Coenzyme A is essential for the metabolism of fatty acid, amino acids,
carbohydrates, and other biological molecules.
• It contains pantothenic acid (PA).
• PA also participates in the fatty acid synthesis as an acyl-carrier
protein (ACP) cofactor.
Coenzyme A (CoA) in Beta-Oxidation of fatty acids
Vitamin B6 (Pyridoxal phosphate) as Coenzyme
• Vitamin B6: Coenzyme in form of
pyridoxal phosphate (PLP) and
pyridoxamine phosphate (PMP)
act as cofactors for many
enzymes, e.g. aminotransferases
and syntases,
Biotin (Vit B7) as a Coenzyme
• Biotin act as coenzyme in
several carboxylation reactions
of fatty acids and amino acids to
facilitate their metabolism.
Enzyme-biotin
HCO3- + ATP
1
ADP + Pi
Enzyme-biotin-CO2O
Acetyl CoA Carboxylase is made
up of 2 components
1. Biotin carboxylase
2. Carboxyl transferase
ll
CH3-C-SCoA
acetyl-CoA
2
Enzyme-biotin
O
ll
-O C-CH -C-SCoA
2
2
malonyl-CoA
Vit B12 (Cobalamin) and Folic acid (Vit B9) as coenzyme
Vit C (Ascorbic acid) as Coenzyme
• Ascorbic acid is a coenzyme of hydroxylases. They hydroxylate lysine
and proline to form hydroxylysine and hydroxyproline. The
hydroxylation strengthens the integrity of collagen structure;
• They also act as coenzyme during hydroxylation of cholesterols to
form bile acids, as well as tyrosine hydroxylation to form the hormone
noradrenaline.
VITAMINS
COENZYMES
ENZYME
Vitamin B1 (Thiamine)
Thiamine pyrophosphate (TPP)
Pyruvate dehydrogenase complex, α-KG
dehydrogenase complex,
Pyruvate carboxylase
Vitamin B2 (Riboflavin)
Flavin mononucleotide (FMN) or Flavin Adenine
Dinucleotide (FAD)
Acyl CoA dehydrogenase, Pyruvate
dehydrogenase complex.
Vitamin B3 (Niacin)
Nicotinamide Adenine Dinucleotide (NAD) or
Nicotinamide Adenine Dinucleotide Phosphate
(NADP)
Hydroxyacyl CoA dehydrogenase.
Glyceraldehyde 3-phosphate dehydrogenase.
G6PD (NADPH), AcylCoA dehydrogenase
(NADPH)
Vitamin B6 (Pyridoxine)
Pyridoxal phosphate or Pyridoxamine
ALT, AST
Vitamin B12
(Cyanocobalamin)
Methylcobalamin or
deoxyadenosylcobalamin.
Methylmalonyl CoA mutase (Metabolism of
BCAA). Converts malonyl CoA to Succinyl CoA.
DNA methyl transferase
Folic acid
Tetrahydrofolate
Tetrahydrofolate reductase, Thymidylate
synthase. DNA methyl transferase
Biotin
Biotin
Acetyl CoA Carboxylase, Pyruvate Carboxylase
Pantothenic acid
Coenzyme A
Acetyl CoA Synthase, Pyruvate dehydrogenase
complex, α-KG dehydrogenase complex,
Vitamin C (Ascorbic acid)
Ascorbic acid
Lysine hydroxylase, Proline hydroxylase
VITAMINS
COENZYMES
FUNCTIONS
Vitamin B1 (Thiamine)
Thiamine pyrophosphate (TPP)
Decarboxylation reaction,
Oxidation/reduction reaction
Vitamin B2 (Riboflavin)
Flavin mononucleotide (FMN) or Flavin Oxidation/reduction reaction
Adenine Dinucleotide (FAD)
Vitamin B3 (Niacin)
Nicotinamide Adenine Dinucleotide
(NAD) or Nicotinamide Adenine
Dinucleotide Phosphate (NADP)
Oxidation/reduction reactions
Vitamin B6 (Pyridoxine)
Pyridoxal phosphate or Pyridoxamine
Transamination reactions
Vitamin B12 (Cyanocobalamin)
Methylcobalamin or
deoxyadenosylcobalamin.
DNA synthesis, maturation of RBC in
bone marrow.
Folic acid
Tetrahydrofolate
Carrier of formyl group
Biotin
Biotin
Carboxylation reactions
Pantothenic acid
Coenzyme A
Carrier of acyl groups
Vitamin C (Ascorbic acid)
Antioxidant, hydroxylation t-reaction
in collagen formation
Vitamin A & K as Coenzymes
VITAMINS
COENZYMES
FUNCTIONS
Vitamin A
Retinoic acid
Regulates normal cell
development.
Vitamin A aldehyde form retinol;
serves as a cofactor for apoproteins
found in the eye.
Responsible for vision in dim light,
and color vision in the retina.
Vitamin K
Menaquinone
Transfer carboxyl grp in
Carboxylases.
Vitamin K is a coenzyme for the
synthesis of blood coagulation
Factors II, VII, IX and X.
It is also a coenzyme in the
glutamic acid carboxylation
Common Metal ion (Cofactors)
Enzymes containing the metal ions as cofactor
Magnesium
Hexokinase, Glucose-6-phosphatase, DNA Polymerase
Manganese
Arginase, Pyruvate Carboxylase, superoxide dismutase
(SOD)
Zinc
Alcohol dehydrogenase, DNA polymerase, carbonic
anhydrase, Alkaline phosphatase
Iron (Ferrous or Ferric)
Catalase, Cytochromes (a, b and c), Succinate
dehydrogenase
Copper II (Cupric)
Cytochrome C oxidase, Tyrosinase, SOD
Cofactors
Questions
1. A coenzyme that is essential for the metabolism of fatty acid, amino acids,
carbohydrates, and other biological molecules
(a) Biotin (b) Coenzyme A (c) Prostaglandin
2. Cofactor for hydroxylases
(a) Biotin (b) Ascorbic acid © Pantothenic acid
3. Coenzyme involved in carboxylation reactions of fatty acids and amino acids.
(a) Biotin (b) Ascorbic acid © NADH
4. Coenzyme (fat-soluble vitamin) in carboxylase enzymes.
(a) Vitamin A (b) Vitamin E © Vitamin K
5. A mineral that acts as cofactor
(a) manganese (b) Aluminium © potassium
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