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Nicotinamide Adenine Dinucleotide

In the graphic below, the structure for the coenzyme, NAD+, Nicotinamide Adenine Dinucleotide is shown. Nicotinamide is from the niacin vitamin. The NAD+ coenzyme is involved with many types of oxidation reactions where alcohols are converted to ketones or aldehydes. It is also involved in the first enzyme complex 1 of the electron transport chain.

Role of NAD+

One role of \(NAD^+\) is to initiate the electron transport chain by the reaction with an organic metabolite (intermediate in metabolic reactions). This is an oxidation reaction where 2 hydrogen atoms (or 2 hydrogen ions and 2 electrons) are removed from the organic metabolite. (The organic metabolites are usually from the citric acid cycle and the oxidation of fatty acids--details in following pages.) The reaction can be represented simply where M = any metabolite.

\[ MH_2 + NAD^+ \rightarrow NADH + H^+ + M: + \text{energy}\]

One hydrogen is removed with 2 electrons as a hydride ion (\(H^-\)) while the other is removed as the positive ion (\(H+\)). Usually the metabolite is some type of alcohol which is oxidized to a ketone.

594NAD.gif    571NAD.gif

Alcohol Dehydrogenase

The NAD+ is represented as cyan in below the graphic. The alcohol is represented by the space filling red, gray, and white atoms. The reaction is to convert the alcohol, ethanol, into ethanal, an aldehyde.

\[ CH_3CH_2OH + NAD^+ \rightarrow CH_3CH=O + NADH + H^+ \]

This is an oxidation reaction and results in the removal of two hydrogen ions and two electrons which are added to the NAD+, converting it to NADH and H+. This is the first reaction in the metabolism of alcohol. The active site of ADH has two binding regions. The coenzyme binding site, where NAD+ binds, and the substrate binding site, where the alcohol binds. Most of the binding site for the NAD+ is hydrophobic as represented in green. Three key amino acids involved in the catalytic oxidation of alcohols to aldehydes and ketones. They are ser-48, phe 140, and phe 93.