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Enzyme cofactors
T. Kučera(based on an older presentation by
J. Novotná)
Enzyme cofactors
• 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
• 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 riboflavineFAD → further reaction with ATP – transfer of AMP to FMN
Reduction and reoxidationof FMN and FAD
Examples of enzymeswith FAD and FMN
acyl-CoA dehydrogenase FAD
succinate dehydrogenase FAD
glycerol-3-phosphate dehydrogenase 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
• transport of chemical energy within the cell• phosphorylation in signal transduction
+ P ─ P ─ adenosine (ADP)
+ P ─ adenosine (AMP)
inorganic phosphate (Pi)
inorganic pyrophosphate (PPi)
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)
• 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)• Active form of folic acid (vitamin B
9)
• donor of 1C units of all oxidation levels except CO2
• coenzyme of transferases. N5,N10-THF carries methylene or methenyl
• 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
CH
COOH
NH3+
CH2
CH2
SCH3
+ ATP
N
NN
NO
CH
COOH
NH3+
CH2
CH2
S+
CH3 CH2
OH OH
NH2
+ HPO42- + H2P2O7
2-
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 lysin‑ e
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 K1
Vitamin K2
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– K1 – blood clotting– K2 – metabolism of the bones and
of the vascular walls, cellular growth
Synthetic vit. K derivatives
Fyloquinone
Menaquinone
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 methylmalonyl-
CoA mutase reaction (methylmalonyl-CoA → succinyl-CoA)
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