Metabolism of Uronic Acids in Plant Tissues

Abstract

A new enzyme, named uronic acid oxidase, was extracted and Free uronic acids generated within the plant by hydrolysis of uronic acid-rich polysaccharides or by hydrolysis of nucleotides are quickly metabolized; they are reduced, oxidized, decarboxylated, or converted to cell wall polysaccharides (10). Tracer studies have demonstrated that plants are capable of oxidizing D-galacturonic acid and D-glucuronic acid to the corresponding hexaric acids, galactaric acid, and D-glucaric acid (9. 11), but the enzymes involved have not yet been investigated. The enzymic conversion of hexuronic acids to hexaric acids has been studied in animals (13, 18) and especially in bacteria (3, 5, 23). Bacteria contain an enzyme, hexuronic acid dehydrogenase, which catalyzes the NAD-linked oxidation of hexuronic acids to hexaric acids. This reaction is the first step in the catabolism of uronic acids in bacteria (2, 4, 6).Recently, in studying the role of polygalacturonase in abscission of citrus leaf explants (17), we observed that the enzymic preparations used contained an enzyme which oxidized the free galacturonic acid released by the polygalacturonase, as well as the free reducing groups of the substrate, sodium polypectate. Based on preliminary studies, the enzyme was named uronic acid oxidase (17). The present work was designed to isolate and characterize the enzyme which catalyzes the oxidation of uronic acids in citrus leaves.

Identification and Characterization of Citrus Peel Uronic Acid Oxidase

“…Citrus peel UAO was partially purified by cation exchange chromatography, using a procedure modified from previous reports (see details in the Supporting Information). SDS‐PAGE analysis of the partially purified UAO sample showed three prominent bands with molecular weight of approximately 60 kDa (Figure A), close to the reported molecular weight of C. sinensis UAO as analyzed by gel filtration chromatography (62 kDa) . Mass spectrometric analysis of this region yielded a C. sinensis BBE enzyme (UniProt accession A0A067GII5, calculated molecular weight 61 kDa) as the top hit, with 21 % sequence coverage (Figure B).…”

“…Uronic acid oxidase (UAO) activity in extracts of ethylene‐treated Shamouti orange ( Citrus sinensis L. Osbeck) leaves was reported by Riov in 1975 . The enzyme catalyzed O 2 ‐dependent oxidation at the C1 position of d ‐galacturonic acid, d ‐glucuronic acid, and polygalacturonic acid, coupled to the production of H 2 O 2 (Scheme ).…”

Production and applications of carbohydrate-derived sugar acids as generic biobased chemicals

“…These oxidases probably contain flavin prosthetic groups (Riov, 1975). Microbial synthesis of D-glucaric acid has been demonstrated in E. coli using the synthetic pathway in which glucose is converted to D-glucuronate via inositol-1-P and myo-inositol and D-glucuronate to D-glucaric acid via an uronate dehydrogenase (Figure 2).…”

“…Uronate oxidases, which oxidize D-glucuronic and D-galacturonic acids to the corresponding aldaric acids using oxygen as the electron acceptor and generating H 2 O 2 , have been described in plants (Cantu et al, 2006;Riov, 1975). These oxidases probably contain flavin prosthetic groups (Riov, 1975).…”

Enzymes Suitable for Biorefinery to Coproduce Hexaric Acids and Electricity from Hexuronic Acids Derived from Biomass

“…[12] To construct enzymatic biofuel cells to produce hexaric acids and electricity simultaneously,i ti sn ecessary to identify appropriate correspondinge nzymes.Although some enzymes have been reported to catalyze the productiono fh exaric acids,m ost are not suitablef or use in biofuelc ells because they requiret he addition of expensive cofactors [20c, 23] or donateelectrons to oxygen, reducingthe electric current. [24] To obtain the intended chemicals,itisp ossible for the conversions to occur through one or more chemical reactions that are not seen in nature.E nzymesc an be used as catalysts to achieve such "unnatural" reactions in environmentally friendly ways.A lthough enzymes are traditionally considered to be very specific catalysts, many enzymes are also able to catalyze reactions that are not physiologically specialized. [21c, 25] Through enzymep romiscuity,b road specificity,o r substratea mbiguity,ab road range of chemical conversions has been achieved.…”