<p>Phenylpropanoid compounds are widely available in natural environments, and they can originate from putrefaction of proteins in soil or as breakdown products of several constituents of plants, such as lignin, various oils, and resins. Microbial catabolism of phenylpropanoid compounds plays an important role not only in the natural degradative cycle of these aromatic molecules but also in their industrial applications such as wine making, aging, and storage. In particular, degradation of cinnamic acid (CI), 3-phenylpropionic acid (PP), and their hydroxylated derivatives has been reported in several bacteria, including Pseudomonas species [<cite idref="PUB00053145"/>], Arthrobacter species [<cite idref="PUB00053145"/>], Escherichia coli (strain K12) [<cite idref="PUB00053146"/>], and Rhodococcus globerulus [<cite idref="PUB00053147"/>]. </p><p>In the catabolism of PP, the first step is catalyzed by a PP dioxygenase, which inserts both atoms of molecular oxygen into positions 2 and 3 of the phenyl ring of PP, yielding cis-3-(3-carboxyethyl)-3,5-cyclohexadiene-1,2-diol (PP-dihydrodiol), which is subsequently oxidized by the 3-phenylpropionate-dihydrodiol dehydrogenase to give DHPP [<cite idref="PUB00053146"/>, <cite idref="PUB00053148"/>]. Biochemical studies and the isolation and characterisation of mutants defective in the catabolism of PP and 3HPP revealed that in E. coli the aerobic degradation of these compounds proceeds by two initially separate routes that converge into 3-(2,3-dihydroxyphenyl) propionate (DHPP), which suffers an extradiol ring cleavage and is ultimately degraded to Krebs cycle intermediates [<cite idref="PUB00053149"/>, <cite idref="PUB00053148"/>]. </p><p> PP/CI dioxygenase consists of four proteins: the two subunits of the hydroxylase component (HcaE and HcaF), and a ferredoxin (HcaC) and ferredoxin reductase (HcaD) subunit [<cite idref="PUB00054961"/>, <cite idref="PUB00054929"/>].</p>This entry represents HcaF, the beta subunit of PP/CI dioxygenase. 3-phenylpropionate/cinnamic acid dioxygenase, beta subunit