Enzymes are biocatalysts, which are responsible for converting a substrate into a product, without themselves taking part in the reaction. All enzymes contain an active site, which is a substrate specific.
The coenzymes are defined as cofactors, which are either metal ions or small organic molecules derived from water soluble vitamins. These coenzymes are bound by ionic bonds, and other non-covalent bonding interactions. They may bind by covalent interactions as well. Enzymes can be used in organic synthesis.
The dehydrogenation of a substrate is catalysed by the aldehyde dehydrogenase in the presence of a cofactor NADH (reduced nicotinamide adenine dinucleotide). The NADH binds towards the active site of enzyme and acts as a reducing agent during the catalysis of the reaction.
The NADH is primarily used to generate ATP (adenosine triphosphate). NADH is then oxidized to NAD+ (oxidized nicotinamide adenine dinucleotide). If ethanol is added to the reaction, then NAD+ undergoes reduction in the process. The reaction is catalyzed by aldehyde dehydrogenase.