CHAPTER 23: Unit 4. Co-enzymes in Metabolism

i) Co-Enzyme NAD and NADH

In metabolism, nicotinamide adenine dinucleotide is involved in redox reactions, carrying electrons from one reaction to another. This reaction forms NADH, which can then be used as a reducing agent to donate electrons. These electron transfer reactions are the main function of NAD.NAD+ is simply the oxidized form of NADH, has low energy versus NADH’s high energy profile, and gets destroyed by stomach acid. NADH is produced in the glycolysis and Krebs cycle. It is used in the production of ATP in the electron transport chain.

Because of the positive charge on the nitrogen atom in the nicotinamide ring (upper right), the oxidized forms of these important redox reagents are often depicted as NAD+ and NADP+ respectively.In cells, most oxidations are accomplished by the removal of hydrogen atoms. Both of these coenzymes play crucial roles in this. Each molecule of NAD+ (or NADP+) can acquire two electrons; that is, be reduced by two electrons. However, only one proton accompanies the reduction. The other proton produced as two hydrogen atoms are removed from the molecule being oxidized is liberated into the surrounding medium. For NAD, the reaction is thus:NAD⁺ +2H⟶ NADH + H⁺

ii) NAD and NADP uses

NAD participates in many redox reactions in cells, including those in glycolysis and most of those in the citric acid cycle of cellular respiration. NADP is the reducing agent produced by the light reactions of photosynthesis and is consumed in the Calvin cycle of photosynthesis and used in many other anabolic reactions in both plants and animals. Under the conditions existing in a normal cell, the hydrogen atoms shown in red are dissociated from these acidic substances.

iii) Coenzyme FAD and FADH2

In biochemistry, flavin adenine dinucleotide (FAD) is a redox-active coenzyme associated with various proteins, which is involved with several important enzymatic reactions in metabolism. FAD, in its fully oxidized form, or quinone form, accepts two electrons and two protons to become FADH₂ (hydroquinone form).

CO-ENZYMEFAD / FADH₂Flavin adenine dinucleotide in its oxidized state is called FAD. After being reduced, it is called FADH₂. See the following Figure for a molecular illustration. The vitamin, riboflavin (or B2) is used to derive this compound. Riboflavin provides the ring structures that will directly participate in the transfer of two hydrogen atoms (each with one electron this time). Similar to NAD, FAD works in association with a “dehydrogenase” enzyme. The reaction removes two hydrogen atoms; each a proton with one electron. Both hydrogen atoms bond with FAD. This reaction does not release an H+ into solution like the reduction of NAD does.

Flavin adenine dinucleotide in the oxidized form (FAD) accepts two hydrogen atoms (each with one electron) and becomes FADH₂.As you examine the reactions for metabolism, look for a reaction that yields FADH₂. Similar to NADH, FADH₂ will be important as it will deliver hydrogens and electrons to biochemical processes that can use the electrons and hydrogens to make ATP.

Nucleotides are important constituents not only of RNA and DNA, but also of a number of key biomolecules considered many times in our study of biochemistry. NAD+ and NADP+, coenzymes that function in oxidation-reduction reactions, are metabolites of ATP. The first step in the synthesis of nicotinamide adenine dinucleotide (NAD+) is the formation of nicotinate ribonucleotide from nicotinate and PRPP.

Nicotinate (also called niacin or vitamin B₆) is derived from tryptophan. Human beings can synthesize the required amount of nicotinate if the supply of tryptophan in the diet is adequate. However, nicotinate must be obtained directly if the dietary intake of tryptophan is low. A dietary deficiency of tryptophan and nicotinate can lead to pellagra, a disease characterized by dermatitis, diarrhea, and dementia. An endocrine tumor that consumes large amounts of tryptophan in synthesizing the hormone and neurotransmitter serotonin (5-hydroxytryptamine) can lead to pellagra-like symptoms.An AMP moiety is transferred from ATP to nicotinate ribonucleotide to form desamido-NAD⁺. The final step is the transfer of the ammonia generated from the amide group of glutamine to the nicotinate carboxyl group to form NAD⁺.

NADP+ is derived from NAD+ by phosphorylation of the 2′-hydroxyl group of the adenine ribose moiety. This transfer of a phosphoryl group from ATP is catalyzed by NAD+ kinase.
Flavin adenine dinucleotide (FAD) is synthesized from riboflavin and two molecules of ATP. Riboflavin is phosphorylated by ATP to give riboflavin 5′-phosphate (also called flavin mononucleotide, FMN). FAD is then formed from FMN by the transfer of an AMP moiety from a second molecule of ATP.

The AMP moiety of coenzyme A also comes from ATP. A common feature of the biosyntheses of NAD+, FAD, and CoA is the transfer of the AMP moiety of ATP to the phosphate group of a phosphorylated intermediate. The pyrophosphate formed in these condensations is then hydrolyzed to orthophosphate. As in many other biosyntheses, much of the thermodynamic driving force comes from the hydrolysis of the released pyrophosphate.

iv) Coenzyme A

Coenzyme A (CoA, SHCoA, CoASH) is a coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle. All genomes sequenced to date encode enzymes that use coenzyme A as a substrate, and around 4% of cellular enzymes use it (or a thioester) as a substrate. In humans, CoA biosynthesis requires cysteine, pantothenate (vitamin B₅), and adenosine triphosphate (ATP).

Figure above: Structure of coenzyme A: 1: 3′-phosphoadenosine. 2: diphosphate, organophosphate anhydride. 3: pantoic acid. 4: β-alanine. 5: cysteamine.Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized for energy production.Coenzyme A (CoASH or CoA) itself is a complex and highly polar molecule, consisting of adenosine 3′,5’‑diphosphate linked to 4‑phosphopantetheinic acid (vitamin B₅) and thence to β‑mercaptoethylamine, which is directly involved in acyl transfer reactions. The adenosine 3’,5’‑diphosphate moiety functions as a recognition site, increasing the affinity of CoA binding to enzymes. In mitochondria and peroxisomes, the concentrations of CoA are reported to lie in the range 2-5 mM and 0.7 mM, respectively, while that in the cytosol is much lower (0.05 to 0.14 mM). Although acyl-dephospho-CoA esters lacking the 3’‑phosphate group on the ribose moiety have been detected in tissues, their function is unknown.

The genes encoding the enzymes for CoA biosynthesis have been identified and the structures of many proteins in the pathway have been determined. Although there are sequence differences between prokaryotes and eukaryotes, coenzyme A is assembled in five steps from pantothenic acid, cysteine and ATP in essentially the same way in both groups. However, pantothenic acid per se can only be synthesised by microorganisms (including gut microflora) and plants, and animals must acquired it largely from the diet. In animals, CoA biosynthesis is believed to occur entirely in the cytosol of cells, and the first and rate-limiting step involves the enzyme pantothenate kinase, several isoforms of which are known.

CHEM 1152

CHAPTER 23: Unit 4. Co-enzymes in Metabolism