This view of alcohol dehydrogenase shows its secondary structure of beta-sheets and alpha-helices. The active site is barely visible.

This close-up of the active site shows the Zn²⁺ cation coordinated with two cysteine anion residues (-CH₂-S^-), and a histidine residue (cyclic N-containing group). An acetaldehyde is coordinated to the zinc and the -OH group of a serine residue (not shown in the 3D picture). Held in position, the acetaldehyde is reduced to CH₃CH₂O^- when a hydride (H:^-) is released from the NADH. The reduction is completed when the oxygen is protonated by H⁺ that is ultimately released from the serine hydroxyl group (which in turn acquires a new proton by a series of relays of H⁺ from the enzyme surface).

Here is a Lewis-structure drawing of the molecules in the active site.

Toggle between the molecular view and Lewis structure of the active site