Mechanism of alcohol dehydrogenase

General Information on ADH

• Alcohol Dehydrogenase belongs to the oxidoreductase family of enzymes.

• ADH is found in high concentrations within the human liver and kidney.

General Information on ADH

• The primary and most common role of ADH in humans is to detoxify incoming ethanol by converting it into acetaldehyde.

• The resulting acetaldehyde, a more toxic molecule than ethanol, is quickly converted into acetate and other molecules easily utilized by the cell

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

Role Of NAD

• One hydrogen is removed from alcohol with 2 electrons as a hydride ion (H-) while the other is removed as the positive ion (H+).

Alcohol Dehydrogenase(Oxidation-Reduction Reaction)

Only one of the protons of EtOH is abstracted and is

added to a specific side of NAD+.

H

C

C

H

O

H

H

H

H

N

R

C NH2

O

N

R

C NH2

OH H

C

CH3

OH

++

Ethanol

++

AlcoholDehydrogenase

H+

NADHNAD+

H+

H:–

• ADH is a zinc metalloenzyme 0f broad specificity.

• It oxidizes a wide range of aliphatic, aromatic alcohols to their corresponding aldehydes.

• The molecule is a symmetrical dimer, composed of two identical chains of 40,000 KDa each.

• Each chain has one binding site for NAD+ but two sites for Zn 2+.

• Only one Zn ion is directly concerned with catalysis.

• The nicotinamide ring of the NAD is bound close to the Zinc ion at the bottom of the pocket.

• The 2ÓH group of ribose ring is located between the Ser-48 OH group and imidazole group of His-51.

Active Site Characteristics of ADH

• As mentioned earlier, each subunit of one monomer contains one binding site for NAD+ and two binding sites for Zn2+

• Each Zinc ion is ligated directly between the side chains of Cys-46, His-67, Cys-174 and a water molecule which is hydrogen bonded to Ser-48.

• Between the two binding sites where the zinc is located, there are two clefts. One which binds NAD+ and one which binds the substrate (ethanol)

• Zinc bound to Cys-46, His-67, Cys-174, and Ser-48 (Blue) and the coenzyme NAD+ (purple) attached to His-51 (yellow) The eight zinc molecules are in red. The four zincs seen easily are not directly involved in the proton transfer chain.

But we Must understand More!

• The two active sites are in clefts between the coenzyme binding core and the catalytic domains

• Ethanol binds to the hydrophobic core lined by nine amino acids, which surround the substrate

• After binding NAD+, the 100 rotation makes the protein go from its apo "open" form to the halo "closed". This narrows the cleft, brings the substrate binding site closer and excludes water from the active site which is vital for the activity of ADH

• The hydrophobic pocket:- Leu-57, Phe-93, Leu-116, Phe-110, Phe-140, Leu-141, Val-294, Pro-295 and Ile-318 (red). Zinc (orange), Cys-174 (purple), Cys-46 (yellow) and His-67 (green) Cxf (in this case) in blue and oxygen involved in the dehydrogenation reaction shown in white

Conformation Change at the Active Site

• Binding of NAD induce the release of Zn2+ water.• Ionized alcohol displaced the Zn bound water.• Ser 48 helps the release of hdrogen proton from alcohol to

NAD+