The enzyme aconitate decarboxylase (EC 4.1.1.6) (i.e., ACOD1, also termed cis-aconitate decarboxylase, immune-responsive gene 1, immune response gene 1, and IRK1^[1][2]) is a protein enzyme that in humans is encoded by the decarboxylase 1 aconitate decarboxylase 1 gene located at position 22.3 on the long arm (i.e., p-arm) of chromosome 13.^[3] ACOD1 catalyzes the following reversible (i.e., runs in both directions, as indicated by ${\displaystyle \rightleftharpoons }$) decarboxylation chemical reaction:^[4][5][6]

cis-aconitate ${\displaystyle \rightleftharpoons }$ itaconate + CO₂

Hence, ACOD1 converts cis-aconitate into two products, itaconate and CO₂ or itaconate and CO₂ into one product, aconitate.

ACOD1 belongs to the family of lyases, specifically the carboxy-lyases, which cleave carbon-carbon bonds. The systematic name of this enzyme class is cis-aconitate carboxy-lyase (itaconate-forming). Other names once in common use for this enzyme class include CAD and cis-aconitate carboxy-lyase. ACOD1 participates in c5-branched dibasic acid metabolism.^[7]

Ustilago maydis (a species of Ustilago fungi) converts cis-aconitate to its thermodynamically favored product, trans-aconitate, by the enzyme aconitate delta-isomerase (i.e., Adi1). The trans-aconitate product is decarboxylated to itaconate by trans-aconitate decarboxylase (i.e., Tad1).^[8] This Adi followed by Tad 1 enzymatic metabolic pathway is:

cis-aconitate ${\displaystyle \rightleftharpoons }$ trans-itaconate → itaconate + CO₂

Trans-aconitate decarboxylase does not metabolize cis-aconitate to itaconate.^[8] (The genes for aconitate delta-isomerase and trans-aconitate decarboxylase have been reported in several types of fungi hut not in other organisms, including humand, and are classified as provisional, i.e., accepted provisional to further studies.)

• BENTLEY R, THIESSEN CP (1957). "Biosynthesis of itaconic acid in Aspergillus terreus. III. The properties and reaction mechanism of cis-aconitic acid decarboxylase". J. Biol. Chem. 226 (2): 703–20. doi:10.1016/S0021-9258(18)70852-1. PMID 13438855.