The enzyme acetolactate decarboxylase () catalyzes the chemical reaction
Hence, this enzyme has one substrate, (S)-2-hydroxy-2-methyl-3-oxobutanoate, and two products, (R)-2-acetoin and CO<sub>2</sub>.
This enzyme belongs to the family of lyases, specifically the carboxy-lyases, which cleave carbon-carbon bonds. The systematic name of this enzyme class is (S)-2-hydroxy-2-methyl-3-oxobutanoate carboxy-lyase [(R)-2-acetoin-forming]. Other names in common use include alpha-acetolactate decarboxylase, and (S)-2-hydroxy-2-methyl-3-oxobutanoate carboxy-lyase. This enzyme participates in butanoate metabolism and c5-branched dibasic acid metabolism. Alpha Acetolactate Decarboxylase (ALDC), sold as Brewzyme-D, is used in brewing beer to limit the formation of 'diacetyl'. Diacetyl causes buttery tastes in beer, considered to be an off-flavor
As of late 2007, only one structure has been solved for this class of enzymes, with the PDB accession code .