CHAPTER 22: Unit 2. Important Coenzymes in Metabolic Pathways

A coenzyme is a small, organic, non-protein molecule that carries chemical groups between enzymes. In metabolism, coenzymes play a role in group-transfer reactions, such as ATP and coenzyme A, and oxidation-reduction reactions, such as NAD+ and coenzyme Q10. Coenzymes are frequently consumed and recycled.

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Several metabolic reactions that extract energy from food involve both oxidation and reduction reactions.Oxidation involves the loss of hydrogen or electrons by a substance or an increase in the number of bonds to oxygen.Reduction is the gain of hydrogen ions and electrons or a decrease in the number of bonds to oxygen. In both types of the reactions, coenzymes are required to carry the hydrogen ions and electrons from or to the reacting substrate.A coenzyme that gains hydrogen ions and electrons is reduced, whereas a coenzyme that loses hydrogen ions and electrons to a substrate is oxidized.Core Chemistry Skill Identifying Important Coenzymes in Metabolism.

NAD+, (nicotinamide adenine dinucleotide),is an important coenzyme in which the vitamin niacin provides the nicotinamide group, which is bonded to ribose and the nucleotide adenosine diphosphate (ADP) is required in dehydrogenation reactions that produce carbon–oxygen double bonds, such as the oxidation of alcohols to aldehydes and ketones.

The oxidized form of NAD+ undergoes reduction when a carbon atom in the nicotinamide ring reacts with 2H (two hydrogen ions and two electrons), leaving one H+.

Coenzyme NADP+ NADP+, nicotinamide adenine dinucleotide phosphate, is used in anabolic reactions, such as lipid and nucleic acid synthesis.is similar to NAD+ except that a 2′ OH group is replaced by a phosphate group and is reduced to form NADPH.The Structure, Coenzyme FAD, flavin adenine dinucleotide, contains ADP and riboflavin (vitamin B2) undergoes reduction when the two nitrogen atoms in the flavin part of the FAD coenzyme react with two hydrogen atoms (2H e−), reducing it to FADH2. The coenzyme FAD (flavin adenine dinucleotide) made from riboflavin (vitamin B2) and adenosine diphosphate is reduced to FADH2 by adding two hydrogen atoms.

FAD, flavin adenine dinucleotide, participates in reactions that produce a carbon-carbon double bond. An example of a reaction in the citric acid cycle that reduces FAD is the conversion of the carbon-carbon single bond in a succinate to a double bond of fumarate and FADH2 with the aide of an enzyme succinate dehydrogenase.

Structure of Coenzyme A Coenzyme A is derived from a phosphorylated ADP and pantothenic acid bonded by an amide bond to aminoethanethiol, which contains the —SH reactive part of the molecule.

Function, Coenzyme A Important functions of coenzyme A include preparation of small acyl groups (represented by the letter A in the name)  such as acetyl for reactions with enzymes. The reactive feature of coenzyme A is the thiol (-SH), which bonds to a two- carbon acetyl group to production the energy-rich thioester acetyl CoA.

Types of Metabolic ReactionsMany reactions within the cells are similar to the types of reactions in organic chemistry such as hydration, dehydration (condensation), hydrolysis, oxidation and reduction. In the laboratory, organic reactions typically require strong acids (low pH), high temperatures, and/or metallic catalysts. However, metabolic reactions, which take place at body temperature and physiological pH, only requiring enzymes and often a coenzyme as well as their metabolic reactions.

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