Last modified 19th April '95 © Birkbeck College 1995

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Alcohol Dehydrogenase

Horse liver alcohol dehydrogenase exists as a homodimer or heterodimer- there are two different types of monomer, denoted E and S. However these two forms are near-identical (both consisting of 374 amino acids), as there are only six residues which are different. None of these six occur in the interface region between the two. There are three possible combinations of the subunits: EE, SS and ES; these are therefore isozymes (or isoenzymes). When isolated from liver, 40-60% of the enzyme is of the EE form.

Examine the crystal structure of a single subunit of horse liver alcohol dehydrogenase (this is the apo- form, i.e. there is no NAD+ coenzyme).

Click here for a diagram of the dimer,

and here for a space-filling model, where the NAD+ molecules are visible.

Examine the crystal structure .

The domain involved in subunit-subunit interactions includes a six-stranded parallel beta sheet- this is a doubly wound alpha/beta fold . The strands of each subunit are in an antiparallel alignment with respect to each other, giving a twelve-stranded mixed beta sheet.

This is indicated in this diagram of a portion of the interface region. Notice the position of the alpha helix sequentially adjacent to the beta strand making the interfacial contact.

In order to visualize how two monomers pack together to form a dimer, bear in mind the positions of the side chains of His-105 and Tyr-286. The face of the histidine of one subunit stacks against the tyrosine of the other, and vice-versa.

This diagram illustrates this packing.

The following diagram indicates the positions of these side chains in relation to the interfacial beta strands and the monomer as a whole.

The symmetry of the monomer- monomer interaction is shown here.

A larger scale diagram of the dimer is shown.

Consider how precise the interaction surface of the monomer must be in order for this dimerization to occur.

Note that in other organisms the enzyme exists in different multimeric forms. For example, the subunit of yeast alcohol dehydrogenase has a high degree of homology with the horse liver protein, but is tetrameric.

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J. Walshaw