ENZYMES Proteins  Elmhurst College
Lock and Key Theory Coenzyme - NAD+ Enzyme Regulation  Chemistry Department
Carboxypeptidase Enzyme Inhibitors  Virtual ChemBook


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Coenzymes

Coenzymes:

A nonprotein component of enzymes is called the cofactor. If the cofactor is organic, then it is called a coenzyme. Coenzymes are relatively small molecules compared to the protein part of the enzyme. Many of the coenzymes are derived from vitamins.

The coenzymes make up a part of the active site, since without the coenzyme, the enzyme will not function.

In the graphic on the left is the structure for the coenzyme, NAD+, Nicotinamide Adenine Dinucleotide. 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.

 Vitamin

 Coenzyme

 Function

 niacin

 nicotinamide adenine dinucleotide (NAD+)

 oxidation or hydrogen transfer

 riboflavin

 flavin adenine dinucleotide (FAD)

 oxidation or hydrogen transfer

 pantothenic acid

 coenzyme A (CoA)

 Acetyl group carrier

 vitamin B-12
 coenzyme B-12

 Methyl group transfer

 thiamin (B-1)

 thiaminpyrophosphate (TPP)

 Aldehyde group transfer


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Alcohol Dehydrogenase

Active Site:

The NAD+ is represented as cyan in the graphic on the left. 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.

Reaction: CH3CH2OH + NAD+ ---> CH3CH=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.

Alcohol Dehydrogenase - Chime in new window