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Bsc Chemistry ____________________________________________________________________________________________________ Subject Chemistry Paper No and Title Paper 16, Bioorganic and biophysical chemistry Module No and Title Module 26, Co-enzyme II Module Tag CHE_P16_M26 CHEMISTRY PAPER No. 16 Bio-organic and biophysical chemistry MODULE No.26 Coenzyme- II ____________________________________________________________________________________________________ TABLE OF CONTENTS 1. Learning outcomes 2. Introduction 3. Nicotinamide nucleotides 3.1 Mechanism of oxido-reduction by NAD+/NADP+ 3.1 Examples of NAD+/NADP+ dependent enzymes 4. Co-enzyme FMN and FAD 3.2 Metabolic role as Co-enzyme 3.3 FMN dependent Co-enzyme 5. Lipoic acid 6. Summary CHEMISTRY PAPER No. 16 Bio-organic and biophysical chemistry MODULE No.26 Coenzyme- II ____________________________________________________________________________________________________ 1. Learning Outcomes After studying this module, you shall be able to Know the three important Co-enzymes i.e. Nicotinamide nucleotides, Flavin nucleotides and Lipoic acid. Know the mechanism of oxido-reduction by NAD+/NADP+. Understand NADH and NADPH dependent reaction. Learn about the metabolic role of Co-enzyme. 2. Introduction This group consist of three important coenzymes all of which assist different enzymes in oxidation- reduction reactions. These are a. Nicotinamide nucleotides b. Flavin nucleotides c. Lipoic acid The dehydrogenation of an alcohol to ketone or aldehyde is one of the most frequent biological oxidation- reduction reactions. Although the hydrogen atoms removed from substrate are often indicated simply as 2[H], it was recognized early in the twentieth century that they are actually transferred to hydrogen carrying coenzymes such as NAD+, NADP+,FAD+, FMN, lipoic acid etc. They may be considered as carriers of hydrogen or as carriers of electrons [H = H+ + e-] in metabolic reactions. When NAD+ becomes reduced by dehydrogenation of alcohol, one of the hydrogen atoms removed from alcohol becomes firmly attached to the NAD+, converting it to NADH, the other is released as proton. CHEMISTRY PAPER No. 16 Bio-organic and biophysical chemistry MODULE No.26 Coenzyme- II ____________________________________________________________________________________________________ Why are there four major hydrogen transfer coenzymes, NAD+, NADP+, FAD and riboflavin phosphate [FMN], instead of just one? Part of the answer is that reduced pyridine nucleotides NADPH and NADH are more powerful reducing agents than are reduced flavins conversely, flavin coenzymes are more powerful oxidizing agents than are NAD+ and NADP+. This difference reflects the chemical difference between the vitamins riboflavin and nicotinamide which form the oxidation - reduction centres of coenzymes. Another difference is that NAD+ and NADP+ tend to be present in free forms within cells, diffusing from a site of one enzyme to a site on another. These coenzymes are sometimes tightly bound but flavin coenzymes are usually firmly bound to proteins, fixed and unable to move. Thus, they tend to accepts hydrogen atom from one substrate and to pass them to a second substrate while attached to a single enzyme. 3. Nicotinamide nucleotides These coenzymes are derived from vitamin, niacin [vitamin B3]. Niacin is synonymous with nicotinic acid Fig.1 Nicotinic acid NAD+ and NADP+ are the active form of niacin. They are called nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate respectively. Collectively, they are called pyridine nucleotides CHEMISTRY PAPER No. 16 Bio-organic and biophysical chemistry MODULE No.26 Coenzyme- II ____________________________________________________________________________________________________ Fig.2 Structure of NAD+ and NADP+ NAD+ is a dinucleotide made up to two nucleotides joined by phosphodiester bond. One nucleotide is made up of nicotinamide [N-base] with ribose and phosphate. The second is AMP. In NADP+ the second carbon-hydroxyl group of ribose of AMP is phosphorylated to form NADP -Nicotinamide, nicotinic acid and tryptophan are all the precursors of synthesis of nicotinamide mononucleotide (NMN) which is then converted to NAD+ and NADP+. In the oxidized form the N-atom of nicotinamide is positively charged and written as NAD+ and NADP+. In the reduced form this charge is neutralized. So they are in form of NADH and NADPH. 3.1 Mechanism of oxido-reduction by NAD+ / NADP+ Two hydrogen atom are transferred from the substrate of NAD+ or NADP+. One H-atom is transferred as hydride ion (H-) to pyridine ring to nicotinamide whose positive charge in neutralized. The other proton goes free in medium as a byproduct. So only one hydrogen atom is attached to pyridine ring. That is why reduced NAD+ is always written as NADH + H+ but not as NADH2. CHEMISTRY PAPER No. 16 Bio-organic and biophysical chemistry MODULE No.26 Coenzyme- II ____________________________________________________________________________________________________ Why are these two pyridine nucleotide, NAD+ or NADP+, differing only in presence or absence of extra phosphate group? One important answer is that they are members of two different oxidation reduction systems, both based on nicotinamide but functionally independent. The experimentally measured ratio of [NAD+]/[NADH] is much higher than the ratio [NADP+] / [NADPH]. Thus, these two coenzymes systems can operate within a cell at different redox potentials. A related generalization that holds much of time is that NAD+ is usually involved in pathways of catabolism, where is functions as an oxidant, while NADPH often used as reducing agent in biosynthetic processes. Typically NADPH is said to participate in reductive biosynthesis. 3.2 Examples of NAD+ / NADP+ dependent enzymes Table1. Examples of NAD+ / NADP+ dependent enzymes CHEMISTRY PAPER No. 16 Bio-organic and biophysical chemistry MODULE No.26 Coenzyme- II ____________________________________________________________________________________________________ Some examples of NAD+ / NADH and NADP+ / NADPH + H+ dependent reactions 1. 2. 3.2.1 NADH dependent reaction NADPH dependent reactions are usually reductions in biosynthetic reaction. Hence these are called as reductive biosynthetic reactions. 3.2.2 NADPH-Dependent Reactions B-keto acyl - CoA dehydrogenase in fatty acid biosynthesis. HMG-CoA reductase in cholesterol synthesis. CHEMISTRY PAPER No. 16 Bio-organic and biophysical chemistry MODULE No.26 Coenzyme- II ____________________________________________________________________________________________________ Phenylalanine hydroxylase for conversion of phenylalanine to tyrosine. Dihydrofolate reductase for synthesis of tetrahydrofolate from folic acid. 4. Coenzyme FMN and FAD Flavin nucleotides are derived from vitamin B2 [riboflavin] and are actively involved in hydrogen transfer reactions. Fig.4 Structure of FMN and FAD Riboflavin is 6, 7 dimethyl-iso-alloxan bound to ribitol [alcohol of ribose]. The intense yellow color is due to iso-alloxan ring. Active co-enzyme forms are FMN and FAD FMN [flavin mono-nucleotide] and FAD [flavin adenine dinucleotide] are the active or coenzyme form of riboflavin. CHEMISTRY PAPER No. 16 Bio-organic and biophysical chemistry MODULE No.26 Coenzyme- II ____________________________________________________________________________________________________ FMN is formed by phosphorylation of riboflavin by kinase using ATP in intestine 4.1 Metabolic role as Co-enzyme Riboflavin containing proteins are called flavoproteins. Many oxido-reductase enzymes are flavoproteins containing FMN and FAD as prosthetic group. Example L-amino oxidase [FMN dependent], succinate dehydrogenase in citric acid cycle is FAD+ dependent. These enzymes participate in reversible oxidation reduction reactions. N-atoms at position 1 and 10 in isoalloxan ring of riboflavin undergo oxidation reduction CHEMISTRY PAPER No. 16 Bio-organic and biophysical chemistry MODULE No.26 Coenzyme- II ____________________________________________________________________________________________________ Example of enzymes requiring FMN or FAD as a coenzyme and reaction where they are involved. Table.2 role of different flavoprotien enzymes in different reactions 4.2 FMN dependent enzymes 4.2.1 L- amino acid oxidases They catalyze oxidative deamination of amino acid producing H2O2 which is split by catalase. CHEMISTRY PAPER No. 16 Bio-organic and biophysical chemistry MODULE No.26 Coenzyme- II ____________________________________________________________________________________________________ 4.2.2 NADH dehydrogenase complex This enzyme is a part of electron transport chain and contains FMN. The electrons are transferred from NADH to FMN and then to CoQ in the electron transport chain Reactions of FAD dependent enzymes: 1. 2. 3. CHEMISTRY PAPER No. 16 Bio-organic and biophysical chemistry MODULE No.26 Coenzyme- II ____________________________________________________________________________________________________ 4. 5. Lipoic acid Lipoic acid acts in the transfer of hydrogen during oxidative decarboxylation reactions. Fig.6 Lipoic acid It occurs both in oxidized and reduced forms as shown in fig.7. CHEMISTRY PAPER No. 16 Bio-organic and biophysical chemistry MODULE No.26 Coenzyme- II ____________________________________________________________________________________________________ Fig.7 Oxidized and reduced form of lipoic acid Also fig.8 shows the structure of lipoamide, where lipoic acid is bound in the amide linkage to the ε amino
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