This image shows a secondary structural representation of the cofactor binding domain of liver alcohol dehydrogenase, an enzyme in the liver that converts ethyl alcohol into acetaldehyde. The catalyzed reaction involves the coenzyme NAD; this receives a hydrogen ion removed from an alcohol molecule bound elsewhere in the protein. This representation shows regions of alpha helix as regular spirals, and stretches of beta strand as arrows. These structural 'motifs' are joined by stretches of randomly coiled polypeptide chain. The image shows two examples of a structural pattern denoted a 'supersecondary' structure. Each consists of consecutive beta-alpha- beta-alpha-beta units; this is known as the 'Rossmann-fold', and such a structure is often found making up the nucleotide binding sites of proteins. The cofactor NAD itself is shown in place in the cofactor binding site, in a 'stick' representation, coloured magenta. A version of this appeared in Scientific American recently.