Download Enzyme cofactors

Enzyme cofactors
T. Kučera
(based on an older presentation by
J. Novotná)
Enzyme cofactors
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non-protein, low-molecular enzyme “component”
“co-catalyst” required for the activity of the enzyme
helper molecules in the enzymatic reaction
holoenzyme (active) “consists” of
– apoenzyme (the enzyme without the cofactor – inactive)
– coenzyme – the cofactor
• reaction partner of the substrate
• or reactant regenerating the enzyme
• cofactors
– tightly bound to the enzyme – prosthetic groups
• regenerated in the same reaction (isomerases)
• or by a reaction with a different substrate
– loosely bound – coenzymes
• regenerated in reaction with a different enzyme
• inorganic
– metal ions (Mg2+, Cu+, Mn2+, Zn2+, Fe2/3+, Mo)
• organic
– heterocycles, often containing phosphate
– many contain a nucleotide component
– often vitamins or their derivatives
• water soluble – B, C, fat soluble – E, K
Cofactors by the reaction
• of oxidoreductases
– NAD, NADP, FAD, FMN, ubiquinone, biopterin,
lipoamide, glutathion, FeS-clusters, heme, metal ions
• of transferases of 1C-residues
– THF, biotin, S-adenosylmethionine
• of transferases of other carbon residues
– TDP, CoA
• of transferases of other groups
– ATP, PAPS, UDP, CDP, cobalamin,
pyridoxal phosphate
Cofactor
Vitamin
Enzyme reaction (example)
Thiamin diphosphate
Thiamin, B1
Oxidative decarboxylation
FAD, FMN
Riboflavin, B2
Redox reactions (transfer of 2H)
NAD+, NADP+
Niacin, B3
Redox reactions (transfer of H−)
Coenzyme A
Pantothenic acid, B5
Metabolism of FA (transfer of acyl)
Pyridoxalphosphate
Pyridoxin, B6
Transamination, decarboxylation
Carboxybiotin
Biotin, H, B7
Carboxylation (CO2)
Tetrahydrofolate
folic acid, B9
Transfer of a 1C-group
Ascorbate
Vitamin C
Hydroxylation reactions (colagen)
Menaquinon
Vitamin K
Transfer of carbonyl group and
electrons
Lipoamide
Lipoic acid
Transfer of electrons and acyl groups
Cobalamin
Cobalamin, B12
Isomerization, methyl transfer
NAD (NADP )
+
+
Redox reaction of NAD
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RH2 + NAD+ → NADH + H+ + R
(de)hydrogenaion of the C—O or C—N bond
proton released to the solution
example: lactate dehydrogenase (lactate + NAD+ →
→ pyruvate + NADH +H+)
FAD, FMN
Vitamin B2
FMN → ATP-dependent phosphorylation of
riboflavine
FAD → further reaction with ATP – transfer of
AMP to FMN
Reduction and reoxidation
of FMN and FAD
Examples of enzymes
with FAD and FMN
acyl-CoA dehydrogenase
succinate dehydrogenase
glycerol-3-phosphate dehydrogenase
FAD
FAD
FAD
NADH-reductase (complex I)
FMN
Coenzyme Q
• electron transport chain
• human – CoQ10 (Q = quinone head , 10 = number of isoprene units
in the hydrophobic chain)
Reduced form
Oxidized form
ATP
+ P ─ P ─ adenosine (ADP)
inorganic phosphate (Pi)
+ P ─ adenosine (AMP)
inorganic pyrophosphate (PPi)
• transport of chemical energy within the cell
• phosphorylation in signal transduction
Pyridoxalphosphate (PLP)
• prosthetic group of aminotransferases
• cofactor of all transamination reactions and of some
decarboxylations and deaminations of amino acids
Pyridoxine
(vit. B6)
Pyridoxal-5-phosphate (PLP)
Pyridoxamine-5-phosphate
Thiaminpyrophosphate
(diphosphate)
TPP (TDP)
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active form of the vitamin B1 (the first discovered vitamin)
contains substituted heterocycles pyrimidine and thiazol
reactions
– reversible cleavage on the C—C bond connecting C=O group with the vicinal
reactive group (usually —COOH or —OH)
– transfer of 2C-residues in transketolase reactions in the pentosephosphate
pathway
– oxidative decarboxylation of α-ketoacids and formation of aldehydes
(pyruvate → acetaldehyde), cofactor of multienzyme complexes PDH, -KGDH,
BCADH
Tetrahydrofolate (THF)
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Active form of folic acid (vitamin B9)
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donor of 1C units of all oxidation levels except CO2
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coenzyme of transferases. N5,N10-THF carries methylene or methenyl
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occurs e.g. in the synthesis of nucleotides and nucleic acids
Tetrahydrofolate (THF)
Coenzyme A
• acyl carrier
• the thiol reacts with the carboxyl forming a thioester
• e.g. transfer of FA from the cytoplasm to the mitochondrion
• high-energy compound participating in many metabolic reactions
(-oxidation of FA, citric acid cycle, biosynthesis of lipids…)
S-adenosylmethionine
COOH
NH3+
NH3+
CH
COOH
CH2
CH
CH2H
CH2 +
CH2
ATP
+
NH2
H3C S C
N
2
O
H3C S
OH OH
N
N
N
+
HPO42- +
H2P2O72-
Biotin
• cofactor of carboxylation reactions
• prosthetic group of acetyl-CoA carboxylase and other
ATP-dependent carboxylases
• covalently attached to the apoenzyme through the
ε‑amino group of lysine
Lipoic acid
• prosthetic group, transfer of
hydrogen and acyl
• amidic bond to the ε‑amino group of
lysine (lipoamide)
• oxidative decarboxylation of
‑ketoacids (PDH, -KGDH,
BCADH)
Vitamin K
• Vitamin K1 (phyloquinone) – plant
origin (redox cofactor of
phofotosystem I)
• Vitamin K2 (menaquinone) –
bacterial origin (large intestine,
bacterial electron transport chain)
• K1 and K2 have different functions
Fyloquinone
Vitamin K1
Vitamin K2
Menaquinone
– K1 – blood clotting
– K2 – metabolism of the bones and
of the vascular walls, cellular
growth
Synthetic vit. K derivatives
Vitamin K - function
• cofactor of hepatic microsomal glutamate carboxylase
– formation of carboxyglutamate residues in prothrombin and
coagulation factors VII, IX and X (post-translation modification)
• carboxylated glutamate chelates Ca2+, enabling the binding of
coagulation factors to membranes
• forms Ca2+ binding site also e.g. in osteocalcin
Tocopherol, vitamin E
α-tocopherol quinone formed by oxidation from
‑tokopherol is a cofactor in synthesis of mitochondrial
unsaturated FA
Chromanol ring | hydrophobic alifatic side chain
Vitamin C
• hydroxylation of proline and lysine residues (colagen)
• synthesis of colagen – prolyl hydroxylase, lysyl hydroxylase and
lysyl oxidase contain Fe2+ and ascorbate as cofactors
• metabolism of tyrosine in the brain
Ions as cofactors
Ion
Example of an enzyme
Cu2+
Cytochrome oxidase, catalase
Fe2+ a Fe3+
Cytochromes, hydroxylases
Mg2+
Glucose-6-phosphatase, hexokinase,
DNA-polymerase
Mn2+
Arginase
Zn2+
Alcohol dehydrogenase, DNA polymerase,
carbonic anhydrase
Se
Glutathion peroxidase
FeS proteins (Fe2S2)
NADH dehydrogenase, succinate dehydrogenase
Ferredoxins
• FeS proteins – Fe2S2, Fe4S4, transfer of electrons
– redox reactions in the respiratory chain
Metaloporphyrins - heme
• compounds derived from the cyclic
tetrapyrrole - porphyrin
• complexes with metal ions
– heme – Fe, transport of O2, transfer
of electrons
– chlorophyll – Mg2+ (Zn2+),
photosynthesis
Metaloporphyrins - cobalamin
(vitamin B12)
• chemically the most complex
vitamin
• structurally similar to heme – the
centrally chelated metal is Co
• in human 2 metabolically active
forms: methylcobalamin and
deoxyadenosylcobalamin
• reactions
– cytoplasmic methylation of
homocysteine to methionine
– mitochondrial methylmalonylCoA mutase reaction
(methylmalonyl-CoA →
succinyl-CoA)