In this review we examine the fascinating array of microbial and enzymatic transformations of ferulic acid. Ferulic acid is an extremely abundant, preformed phenolic aromatic chemical found widely in nature. Ferulic acid is viewed as a commodity scale, renewable chemical feedstock for biocatalytic conversion to other useful aromatic chemicals. Most attention is focused on bioconversions of ferulic acid itself. Topics covered include cinnamoyl side-chain cleavage; nonoxidative decarboxylation; mechanistic details of styrene formation; purification and characterization of ferulic acid decarboxylase; conversion of ferulic acid to vanillin; O-demethylation; and reduction reactions. Biotransformations of vinylguaiacol are discussed, and selected biotransformations of vanillic acid including oxidative and nonoxidative decarboxylation are surveyed. Finally, enzymatic oxidative dimerization and polymerization reactions are reviewed.

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Flavonoids are among the most ubiquitous phenolic compounds found in nature. These compounds have diverse physiological and pharmacological activities such as estrogenic, antitumor, antimicrobial, antiallergic, and anti-inflammatory effects. They are well-known antioxidants and metal ion-chelators. In the present review, biotransformations of numerous flavonoids catalyzed mainly by microbes and few plant enzymes are described in four different flavonoid classes, viz., chalcones, isoflavones, catechins, and flavones. Both phase I (oxidative) and phase II (conjugative) biotransformations representing a variety of reactions including condensation, cyclization, hydroxylation, dehydroxylation, alkylation, O-dealkylation, halogenation, dehydrogenation, double-bond reduction, carbonyl reduction, glycosylation, sulfation, dimerization, or different types of ring degradations are elaborated here. In some cases, the observed microbial transformations mimic mammalian and/or plant metabolism. This review recognizes Norman Farnsworth, who through his fascination and hard work in pharmacognosy has fostered the excitement of discovery by numerous students and faculty far and beyond the halls of the University of Illinois at Chicago. It is with grateful thanks for these efforts that we dedicate this review to him.

We have cloned, sequenced, and expressed the gene for a unique ATP-and NADPH-dependent carboxylic acid reductase (CAR) from a Nocardia species that reduces carboxylic acids to their corresponding aldehydes. Recombinant CAR containing an N-terminal histidine affinity tag had K m values for benzoate, ATP, and NADPH that were similar to those for natural CAR, and recombinant CAR reduced benzoic, vanillic, and ferulic acids to their corresponding aldehydes. car is the first example of a new gene family encoding oxidoreductases with remote acyl adenylation and reductase sites.Aromatic, aliphatic, and alicyclic aldehydes and alcohols are useful intermediates in the chemical, pharmaceutical, and food industries. Chemical methods for carboxylic acid reductions are limited, and they usually require prior derivatization and product deblocking with reactants containing competing functional groups. Biocatalytic reductions of carboxylic acids are attractive because the substrates are water soluble, blocking chemistry is not necessary, reductions are enantiospecific, and the scope of the reaction is very broad (24, 32).Although microbial reductions of carboxylic acids, usually producing the acids' corresponding aldehydes or alcohols, have been observed with whole-cell reactions of bacteria and fungi (3,4,6,8,20,22,24,25,30,(36)(37)(38)40), enzymatic reductions of carboxylic acids are relatively new and unexploited biocatalytic reactions of great potential value in organic synthesis (12).Aldehyde oxidoreductases, also known as carboxylic acid reductases (CAR), require ATP, Mg 2ϩ , and NADPH as cofactors (16,17,18,21,24). The reduction is a stepwise process involving initial binding of both ATP and the carboxylic acid to the enzyme in order to form mixed 5Ј-adenylic acid-carbonyl anhydride intermediates (9,15,25,27,39) that are subsequently reduced by hydride delivery from NADPH to form aldehyde products (16,25) (Fig. 1). Aromatic CARs have been purified to homogeneity only from Neurospora (17) and Nocardia (21, 24) species. Although N-terminal and internal amino acid sequences were reported for our Nocardia enzyme (24), sequences have never been determined for any gene coding for CARs. CAR from Nocardia sp. strain NRRL 5646 has an extremely wide substrate range, and it enantiospecifically reduces carboxylic acids (8,24,32). We report here the cloning and expression of the first CAR gene, car, and the use of cloned enzyme in vitro and in vivo to reduce carboxylic acid substrates.Strains, plasmids, media, and growth. Bacteria and plasmids used in this study are listed in Table 1. Nocardia sp. strain NRRL 5646 (14) was grown at 30°C in Luria-Bertani (LB) medium containing 0.05% Tween 80 (vol/vol [liquid medium only]). With Escherichia coli as the recombinant host for pHAT-based vectors, cells were grown at 37°C on solid or in liquid LB medium. Ampicillin (100 g/ml) was incorporated into LB medium to select for recombinants, and isopropyl-␤-D-thiogalactopyranoside (IPTG) (1 mM) and/or 5-bromo-4-chloro-3-indolyl-␤-D-galactopyranosi...