Coenzymes, vitamins and trace elements 209. Petr Tůma Eva Samcová

Transcription

1 Coenzymes, vitamins and trace elements 209 Petr Tůma Eva Samcová

2 History and nomenclature of enzymes 1810, Gay-Lussac made an experiment with yeats alter saccharide to ethanol and CO 2 Fermentation From greek: en = in, zyme = yeats Historical names- pepsin, ptyalin substrate + -asa : ureasa, sacharasa Classification of enzymes X.X.X.X. class, subclass, subsubclass, order lactate dehydrogenase 1. oxidoreductases 1.1. donor e - -CH-OH acceptor e - NAD + Reaction: Lactate + NAD + pyruvate + NADH

3 Specificity of enzymes Substrate specificity absolute (urease) relative (group) glucose/fructose (hexokinase), ethanol/methanol (ADH) different affinity (K M ) Reaction specificity Reaction of amino acids deamination, decarboxylation, transamination Dependent on coenzymes and apoenzymes Stereospecificity D-monosaccharides, L-amino acids

4 Classes of Enzymes

5 Chemical composition of enzymes Ribozymes RNA with catalytic activity (ribosomes) Simple enzymes: only proteins peptidases Trypsin(endopeptidase) cleaves peptide bonds next to Arg, Lys Chymotrypsin(endopeptidase) cleaves peptide bonds next to Tyr, Trp, Leu Complex enzymes : holoenzyme = apoenzyme + cofactor Cofactors: Metal ion : Zn 2+, Mn 2+, Mg 2+, Fe 2+, Cu 2+ Coenzymes: NAD +, ubiquinone, ATP, Coenzyme A Prosthetic group: heme, FAD, lipoamide

6 Cofactors Many enzyme-catalyzed reactions involve the transfer of electrons or groups of atoms from one substrate to another. Molecules which act as temporary acceptors of the group being transferred are named coenzymes. Coenzymes cooperate with many different enzymes. Soluble coenzymes are bound like substrates, undergo a chemical change and then released for regeneration. Prosthetic groups are tightly bound to the enzyme and remain associated with it during the reaction Carbonic anhydrase Phenylalanine hydroxylase

7 Metal ions and trace elements Metal ions are not coenzyme in the same sense as FAD, FMN, NAD + NADP but are required as cofactors in two-thirds of all enzymes Stabilize natural conformation of active sites Some participate in redox reactions Some polarize chemical bonds in substrate Carbonic anhydrase (Zn 2+ ) Chelates are organometallic coordination complexes (complex between iron and protoporphyrin IX heme) Good example of a metal functioning as a Lewis acid ( zinc accepts electron pair to vacant orbital) is found in carbonic anhydrase

8 Trace elements daily requirements

9 Redox coenzymes Dehydrogenases pyruvate + NADH/H + lactate + NAD + NAD + Nicotinamide adenine dinucleotide NADP + Nicotinamide adenine dinucleotide phosphate Monooxygenases phenylalanine + O 2 + NADH/H + tyrosine + H 2 O+NAD +

10 Redox coenzymes Flavin coenzymes FMN flavin mononucleotide FAD flavin dinucleotide Prosthetic groups Dehydrogenases, oxidases, monooxygenases Ubiquinone/Ubiquinol Coenzym Q (during reduction,the quinone form is converted to hydroquinone) Respiratory chain

11 Cytochromes Heme coenzymes Fe 2+ Fe 3+ Electron-transport heme proteins Redox cofactors in the respiratory chain (complex II cytochrom b 560 ) Complex III in respiratory chain (coenzyme Q-cytochrome c oxidoreductase) Unlike in hemoglobin, the iron ion undergoes a change in valence here

12 Activation of metabolites Nucleoside phosphates. Group-transfer reaction Phospho transferases (kinases) glucose + ATP glucose-6- phosphate + ADP Nucleotidyl transferases Biosynthesis of glycogen glucose-6-p glucose-1-p + UTP UDP-glucose + diphosphates Biosynthesis of phospholipids choline-p + CTP CDPcholine Ligases (synthetases) glutamate +NH 3 + ATP glutamine + ADP + P

13 Coenzymes transferring groups Coenzyme A Transfer of acyl residues panthoteine = pantoinate + β-alanine+cysteamin (the latter two biogenic amines formed by decarboxylation of aspartate and cystein) Thiamine diphosphate Transfer of hydroxyalkyl residues included in pyruvate dehydrogenase Biotin transfer of CO 2 (carboxylases)

14 Transfer of C1-residues S-Adenosylmethionine, SAM Methylation reaction norepinephrine epinephrine Biosynthesis of creatine Methylation of DNA homocysteine Tetrahydrofolate Transfer of formyl,methylene (-CH 2 -) and methyl Synthesis of purines and pyrimidines sulfonamides

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