2. Cofactors
A cofactor is a non-protein chemical compound or metallic ion
that is required for an enzyme's role as a catalyst.
• If it tightly bound, it is called a prosthetic group
• Enzyme without prosthetic group (cofactor)
• Apoenzyme
• Enzyme with prosthetic group (cofactor)
• Holoenzyme
3. Coenzyme
A coenzyme is defined as an organic molecule that binds to the
active sites of certain enzymes to assist in the catalysis of a reaction
4. Coenzyme classification
• Metabolite coenzymes: Coenzymes that can be synthesized from
common metabolites are referred to as metabolite coenzymes.
Ex: ATP, and nucleotide sugars such as uridine diphosphate glucose
(UDP-glucose) has a central function in phosphoryl-group transfer,
as occurs in reactions catalyzed by kinases. Pyrophosphoryl and
adenylyl (AMP) groups also can be transferred from ATP to
substrates
• Vitamin-derived coenzymes: Those that cannot be synthesized
and are derived from vitamins are known as vitamin-derived
coenzymes.
• Ex: All of the water-soluble vitamins and two of the fat-soluble
vitamins, A and K, function as cofactors or coenzymes. ...
• The active forms of riboflavin, vitamin B2, are the coenzymes
flavin mononucleotide (FMN) and flavin adenine dinucleotide
(FAD)
5.
6. Catalysis by Coenzymes
Some common coenzymes
Transfers
•Oxidation/Reduction
• Nicotinamide Adenine
Dinucleotide (NAD+)
• Flavin Adenine Dinucleotide
(FAD)
Hydride ion (H-)
Electrons
7. Catalysis by Coenzymes
Some common coenzymes
• Acyl groups and others
– Coenzyme A
– Lipoic acid
– Thiamine pyrophosphate
– Pyridoxal phosphate
– 5’-Deoxyadenosyl-cobalamin
Transfers
Acyl groups
Acyl groups
Aldehydes
Amino groups and
amines
Alkyl groups
10. NAD+ and NADP+
• These compounds were the first coenzymes to be recognized.
• Derived from the vitamin niacin (nicotinic acid).
• Severe deficiency of niacin (vitamin B3) causes the disease
pellagra.
12. • The phosphate is important in enzyme recognition.
• Namely, NAD+ is used only in oxidative reactions of
catabolism whereas NADPH is used in the reductive
biosynthesis reactions of anabolism.
• The pyridine nucleotides function as electron carriers in
oxidation-reduction reactions
• NAD+ is used in reactions where an alcohol group is
converted to a ketone or aldehyde group.
13. General redox
reaction catalyzed
by alcohol
dehydrogenase (EC
1.1.1.1)
Alcohol dehydrogenases are a class of zinc enzymes, which
catalyse the oxidation of primary and secondary alcohols to the
corresponding aldehyde or ketone by the transfer of a hydride
anion to NAD+ with release of a proton:
14. Examples of Coenzyme Catalysis
Nicotinamide adenine dinucleotide and alcohol dehydrogenase
N
N N
H
N
NH2
O
OH
OH
CH2
O
P
O
P
O
H2C
O
OH OH
N
C
O
NH2
O
O
O
O
[H-
]
N
C
O
NH2
R
H
H
NADH
NAD+
15.
16. FAD/FADH2
• Flavins, are fundamental catalytic cofactors that are responsible
for the redox functionality of a diverse set of proteins.
• FAD+ is flavin adenine dinucleotide.
• These are cofactors and are involved in various metabolic
processes
• They are also involved in various oxidation-reduction reactions.
FADH:
• Flavin adenine dinucleotide.
• High energy electron carrier used to transport electrons
generated in Glycolysis and Krebs Cycle to the Electron
Transport Chain.
When NAD+ and FAD+ accept electrons, they are reduced to
NADH and FADH2, respectively.
23. Flavin adenine dinucleotide (FAD) is an important redox cofactor
involved in many reactions in metabolism. The fully oxidized form,
FAD, is converted to the reduced form, FADH2 by receiving two
electrons and two protons.
24.
25.
26.
27.
28.
29.
30. Coenzyme A
• Also known as CoA, SHCoA, CoASH is a coenzyme, notable for
its role in the synthesis and oxidation of fatty acids, and the
oxidation of pyruvate in the citric acid cycle.
• Coenzyme A is a coenzyme containing pantothenic acid,
adenosine 3-phosphate 5-pyrophosphate, and cysteamine;
involved in the transfer of acyl groups, notably in
transacetylations.
• Coenzyme A is naturally synthesized
from pantothenate (vitamin B5), which is found in food such as
meat, vegetables, cereal grains, legumes, eggs, and milk.
• In humans and most living organisms, pantothenate is an
essential vitamin that has a variety of functions
32. Fatty acid synthesis
• coenzyme A is, in chemical terms, a thiol, it can react
with carboxylic acids to form thioesters, thus functioning as
an acyl group carrier.
• It assists in transferring fatty acids from
the cytoplasm to mitochondria. A molecule of coenzyme A
carrying an acyl group is also referred to as acyl-CoA. When it
is not attached to an acyl group, it is usually referred to as
'CoASH' or 'HSCoA'
33.
34. Biotinis hexahydro-2-oxo-1H-thieno(3,4-d)imidazole-4-
pentanoic acid.
• Biotin is a water-soluble B-vitamin, also called vitamin B7 and
formerly known as vitamin H or coenzyme R.
• Biotin is a coenzyme for carboxylase enzymes, involved in the
synthesis of fatty acids, isoleucine, and valine, and in
gluconeogenesis.
35. • Biotin is necessary for cell growth, the production of
fatty acids, and the metabolism of fats and amino
acids.
• Biotin assists in various metabolic reactions
involving the transfer of carbon dioxide.
• It may also be helpful in maintaining a steady blood
sugar level.
• Biotin is often recommended as a dietary
supplement for strengthening hair and nails, though
scientific data supporting this outcome are weak.
• Nevertheless, biotin is found in many cosmetics and
health products for the hair and skin.
36. Cofactor biochemistry
D-(+)-Biotin is a cofactor responsible for carbon
dioxide transfer in several carboxylase enzymes:
•Acetyl-CoA carboxylase alpha
•Acetyl-CoA carboxylase beta
•Methylcrotonyl-CoA carboxylase
•Propionyl-CoA carboxylase
•Pyruvate carboxylase
37. • Biotin is important in fatty acid synthesis, branched-chain
amino acid catabolism, and gluconeogenesis.
• It covalently attaches to the epsilon-amino group of specific
lysine residues in these carboxylases.
• This biotinylation reaction requires ATP and is catalyzed by
holocarboxylase synthetase.
38. • Pyruvate carboxylase uses a covalently
attached biotin cofactor which is used to catalyze the ATP–
dependent carboxylation of pyruvate to oxaloacetate.
39. Biotin is involved in the action of four carboxylases
• Acetyl-CoA carboxylase, which catalyses the binding
of bicarbonate to acetyl-CoA to form malonyl-CoA in the
synthesis of fatty acids;
• Pyruvate carboxylase, which is involved in
gluconeogenesis; synthesis
of phosphoenolpyruvate (PEP) from pyruvate.
• Pyruvate carboxylase is involved in the In fatty acid
synthesis, biotin is required by the enzyme that forms
malonyl CoA from acetyl-CoA
• Methylcrotonyl-CoA carboxylase, which catalyses an
essential step in the metabolism of leucine;
• Propionyl-CoA carboxylase, which catalyses essential
steps in the metabolism of amino acids, cholesterol, and
fatty acids.
40. Biological importance:
Recent evidence emerged that biotin also plays unique roles
in cell signaling, epigenetic regulation of genes, and
chromatin structure
41. Thiamine pyrophosphate (TPP or ThPP), or thiamine
diphosphate (ThDP), or cocarboxylase is a thiamine (vitamin
B1) derivative which is produced by the enzyme thiamine
diphosphokinase.
Thiamine pyrophosphate is a cofactor that is present in all
living systems, in which it catalyzes
several biochemical reactions
42. TPP works as a coenzyme in many enzymatic reactions,
such as:
•Pyruvate dehydrogenase complex
•Pyruvate decarboxylase in ethanol fermentation
•Alpha-ketoglutarate dehydrogenase complex
•Branched-chain amino acid dehydrogenase complex
•2-hydroxyphytanoyl-CoA lyase
•Transketolase
43. Reaction mechanisms
In several reactions, including that of pyruvate dehydrogenase,
alpha-ketoglutarate dehydrogenase, and transketolase, TPP
catalyses the reversible decarboxylation reaction (aka cleavage of a
substrate compound at a carbon-carbon bond connecting
a carbonyl group to an adjacent reactive group—usually
a carboxylic acid or an alcohol). It achieves this in four basic steps:
1.The carbanion of the TPP: nucleophilically attacks the carbonyl
group on the substrate. (This forms a single bond between the TPP
and the substrate.)
2.The target bond on the substrate is broken, and its electrons are
pushed towards the TPP. This creates a double bond between the
substrate carbon and the TPP carbon and pushes the electrons in
the N-C double bond in TPP entirely onto the nitrogen atom,
reducing it from a positive to neutral form.
44. 3. In what is essentially the reverse of step two, the electrons
push back in the opposite direction forming a new bond
between the substrate carbon and another atom. (In the case
of the decarboxylases, this creates a new carbon-hydrogen
bond. In the case of transketolase, this attacks a new substrate
molecule to form a new carbon-carbon bond.)
4. In what is essentially the reverse of step one, the TPP-
substrate bond is broken, reforming the TPP ylid and the
substrate carbonyl.
