enzymes cofactors 2012
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Enzymes-cofactorsDr. Mamoun Ahram
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Resources
Biochemistry. 5th edition. Berg JM, Tymoczko JL, Stryer L. New
York: W H Freeman; 2002.
8.1.1 Many Enzymes Require Cofactors for Activity
http://www.ncbi.nlm.nih.gov/books/NBK22380/ Section 9.2 Making a Fast Reaction Faster: Carbonic
Anhydrases
http://www.ncbi.nlm.nih.gov/books/NBK22599/
The Medical Biochemistry Pages Introduction to Vitamins and Minerals
http://themedicalbiochemistrypage.org/vitamins.php
http://www.ncbi.nlm.nih.gov/books/NBK22380/http://www.ncbi.nlm.nih.gov/books/NBK22599/http://themedicalbiochemistrypage.org/vitamins.phphttp://themedicalbiochemistrypage.org/vitamins.phphttp://www.ncbi.nlm.nih.gov/books/NBK22599/http://www.ncbi.nlm.nih.gov/books/NBK22599/http://www.ncbi.nlm.nih.gov/books/NBK22599/http://www.ncbi.nlm.nih.gov/books/NBK22380/http://www.ncbi.nlm.nih.gov/books/NBK22380/http://www.ncbi.nlm.nih.gov/books/NBK22380/ -
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Cofactors
What are cofactors?
What are coenzymes?
When are coenzymes called prosthetic groups and
when are they cosubstrates?
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Classification of cofactors
Cofactors
Metals
Metallo-proteins
Metal-associated
proteins
Small organicmolecules
Coenzymes cosubstrates
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Apoenzymes vs. holoenzymes
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Vitamin derivatives as cofactors
What are vitamins?
Why are they important?
What are the two classes of vitamins?
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Vitamin C
Ascorbic acid
Example: prolyl
hydroxylase
synthesizes 4-hydroxyproline
(collagen)
An antioxidant
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Thiamin (vitamin B1)
Active form: thiamin pyrophosphate, TPP
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Pyruvate dehydrogenase complex
Decarboxylation of pyruvate into acetyl CoA by the
pyruvate dehydrogenase complex
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-ketoglutarate dehydrogenase
Decarboxylation of -ketoglutarate into succinyl CoA
by -ketoglutarate dehydrogenase
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Riboflavin (vitamin B2)
The precursor for the coenzymes flavin adenine
mononucleotide (FMD) and flavin adenine
dinucleotide (FAD)
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Flavoproteins
Protiens that require FMN or FAD as cofactors
Redox reactions
FAD and FMN are prosthetic groups
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Succinate dehydrogenase
Oxidation of succinate into fumarate by succinate
dehydrogenase
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Pyruvate dehydrogenase complex
FAD is a prosthetic group for the pyruvate
dehydrogenase complex
It does not change in the overall reaction
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Niacin (vitamin B3)
Precursor of nicotinamide adenine dinucleotide(NAD+) and nicotinamide adenine dinucleotidephosphate (NADP+)
Cofactors for numerous dehydrogenases
Cosubstrates
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Pantothenic acid (vitamin B5)
Precursor of alanine and pantoic acid
Synthesis of coenzyme A (CoA)
Metabolism of carbohydrate, fats and proteins
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Coenzyme A
Coenzyme A transports acetyl groups from one
substrate to another
via reactive thioester bond
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Pyruvate dehydrogenase complex
Decarboxylation of pyruvate into acetyl CoA by the
pyruvate dehydrogenase complex
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Citrate synthase
Condensation of acetyl CoA and oxaloacetate into
citrate by citrate synthase
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Pyridoxal phosphate (vitamin B6)
Pyridoxal, pyridoxamine and pyridoxine are
precursors of active form
Metabolism of amino acids
pyridoxal
phosphate
PyridoxaminePyridoxalPyridoxine
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Aminotransferases
All aminotransferases contain the prosthetic group
pyridoxal phosphate (PLP)
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Examples
Aspartate aminotransferase
Alanine aminotransferase
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Biotin
Carboxylation reactions
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Pyruvate carboxylase
condensation of CO2 to pyruvate forming oxaloacetate
by pyruvate carboxylase
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Acetyl CoA carboxylase
Carboxylation of acetyl CoA to malonyl CoA by acetyl
CoA carboxylase initiating fatty acid synthesis
Biotin is a prosthetic group
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Cobalamin (vitamin B12)
Composed of a tetrapyrrol ring
structure and a cobalt ion in the
center
Two significant reactions
linking fatty acid metabolism
to sugar metabolism by
rearranging methylmalonyl
CoA to succinyl CoA
the formation of methionine
from homocysteine by
methylation
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Folic acid
Reduced within cells to tetrahydrofolate (THF) by
dihydrofolate reductase (DHFR), an NADPH-requiring
enzyme
THF derivatives carry and transfer of one carbon unitsduring biosynthetic reactions, especially of nucleotides
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Lipoic acid
Not a dietary requirement in humans
Not a vitamin
A co-factor in pyruvate dehydrogenase and -
ketoglutarate dehydrogenase
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Metals
EnzymeMetalCarbonic anhydraseZn2+CarboxypeptidaseZn2+
HexokinaseMg2+Glutathione peroxidaseSeSuperoxide dismutaseMn2+
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Example
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