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)