Arabinoxylans ferulic acid

A spedal feature is the presence of ferulic acid (its cis- and trans isomers in a ratio of about 1 1), which is 0.1-0.2% by weight of arabinoxylans. Ferulic acid is bound by an ester linkage at C-5 of the arabinose residue, which is connected to the C-3 of the xylose residue. In the secondary cell wads, ferulic add forms ethers, C-C dimers and addition products with thiol residues of proteins, which play an important role in the development of cross-Hnks between arabinoxylan molecules and other components of cell walls (Figure 4.18). Ferulic acid in Ugnified cells is bound to Hgnin. Proteins are also present (0.9-3.9% by weight) as in a number of other polysaccharides. 

FAE was specific for esterified xylan-oligomers, but did not show selectivity towards a specific ester. This enzyme could release ferulic acid as well as acetyl groups from esterified arabinoxylans in the presence of an endoxylanase. 

Ferulic acid is the most abundant hydroxycinnamic acid in cereal grains. The content in wheat grain is approximately 800-2000 mg/100 g DW. It is found chiefly in the outer part of the grain, in the trans form, which is transformed into arabinoxylans and... 

The presence of feruloyl and p-coumaroyl acids linked via L-arabinose residues has been verified in several studies of xylan (6-8). The amounts of these components, however, are rather small. Every 15th arabinose unit in barley straw arabinoxylan is estimated to be esterified with ferulic acid, and every 31st with p-coumaric acid (7). These hydroxycinnamic acids are bound to C-5 of the arabinose residue (7, 8). It has been suggested that oxidative dimerization of ferulic acid residues crosslinks the arabinoxylan chains and renders them insoluble as a result of the diferuloyl bridges (9). 

In addition to enzymes hydrolyzing the glycosidic linkages in xylans, the requirement of esterases to remove esterified acids from xylans has recently been discovered. Acetyl xylan esterases remove O-acetyl groups from the C-2 and C-3 positions of xylose residues in both xylan and g looligomers. Feruioyl esterases liberate ferulic acid from arabinoxylans of monocotyledons. 

Feruloyl esterase activity was first detected in culture filtrates of Strepto-myces olivochromogenes (49), and has thereafter also been reported for some hemicellulolytic fungi (Table III). A partially purified feruloyl esterase from S. commune liberated hardly any ferulic acid without the presence of xylanase (65). Very recently a feruloyl esterase was purified from Aspergillus oryzae (Tenkanen, M. Schuseil, J. Puls, J. Poutanen, K., /. Biotechnol, in press). The enzyme is an acidic monomeric protein having an isoelectric point of 3.6 and a molecular weight of 30 kDa. It has wide substrate specificity, liberating ferulic, p-coumaric, and acetic acids from steam-extracted wheat straw arabinoxylan. 

It therefore seems probable that, in the cell walls of the growing plant, dimerization of p-coumaric and ferulic acid units linked to arabinoxylan occurs under the influence of sunlight. Earlier work (34) showed that trans-p-coumaric acid converts readily in sunlight to 4,4 -dihydroxytruxillic acid. It has recently been shown (40) that FA-FA and PCA-FA type cyclodimers can be obtained in low yield (less than 10%) by irradiating mixtures of the... 

Phenolic acids, and especially ferulic acid, which is abundantly present in cereals, is found esterified to the polysaccharides present in primary and secondary cell walls of plants. Ferulic acid is the major phenolic acid occurring in the cell walls of monocotyledons and appears as cis and the more abundant trans isomers (reviewed in [Klepacka and Forna, 2006]). Ferulic acid is found in wheat, maize, rye, barley [Sun et al., 2001], oats, spinach, sugar beet, and water chesnuts [Clifford, 1999], generally esterified, and rarely as free form, such as in barley [Yu et al., 2001]. It is esterified in primary cell walls to arabinoxylans (Fig. 2.4) in the aleurone layer and pericarp [Clifford, 1999], as in spinach [Fry, 1982] or in wheat bran [Smith and Hartley, 1983], Ferulic acid can also be found esterified to other hydroxycinnamic acids such as in Mongolian medicinal plants where it is found as feruloylpodospermic acid, which is... 

Piber, M., Koehler, P. 2005. Identification of dehydro-ferulic acid-tyrosine in rye and wheat Evidence for a covalent cross-link between arabinoxylans and proteins. J Agric Food Chem 53 5276-5284. 

Esterases. Acetyl esterase (EC 3.1.1.6) removes acetyl esters from acetylated xylose and short-chain xylo-oligomers. It s polymeracting counterpart, acetyl xylan esterase (EC 3.1.1.72), has a similar activity, but prefers polymeric xylan.244 In addition to acetate-specific enzyme detection kits, HPLC or GC analysis of acetate release from native extracted xylan and chemically acetylated xylan, colorimetric substrates, such as p-nitrophenol acetate and /3-napthyl acetate, or the fluorometric substrate, 4-methylumbelliferyl acetate are also used to assay acetyl esterases.244,253 The third esterase, ferulic acid esterase (EC 3.1.1.73), hydrolyzes the ester bond between ferulic acid or coumaric acid and the arabinose side chain of arabinoxylan. Assays for this activity are usually carried out using starch-free wheat bran or cellulase-treated gramineous biomass as a substrate and monitoring ferulic or coumaric acid released by HPLC or TLC. When preparing enzyme-treated substrates, care must be taken to employ phenolic-acid-esterase-free cellulases.244 Other substrates include methyl and ethyl esters of the phenolic acids, as well as finely ground plant biomass.240,254,255... 

The ability of anaerobic and facultative aerobic bacteria to O-demethylate free ferulic acid and arabinoxylan ester-linked ferulic acid into caffeic acid was investigated. Clostridium methoxybenzovorans (strain SR3) was selected for free ferulic acid consumption and caffeic acid production criteria. To perform the reaction on ferulic acid esterified onto arabinoxylans, sonicated cellular extract of SR3 has been prepared and tested on free and arabinoxylan ester-linked ferulic... 

Demethylation of Free and Ester-linked Ferulic Acid by Sonicated Cellular Extracts of SR3. The O-demethylase activity of the SR3 strain was not excreted in the culture medium. To demethylate ferulic acid esterified to WEAX, which cannot penetrate the cells, a cell extract of SR3 strain was prepared by anaerobic sonication (MSON 05 Bio block, 20 Hz, 3 cycles of sonication of 2 min 2 s pulses separated by 2 s lag phase). The sonicated cellular extract of SR3 was tested in anaerobiose on free ferulic acid and esterified ferulic acid at 37 °C with a bacteria protein/ferulic acid ratio of 100 mg/ mol. Composition of reaction medium has been chosen in agreement with previous works on the demethylation of methoxylated phenolic compounds by other strains.15 20-22 In order to limit the viscosity of the medium, a 0.5% arabinoxylan concentration was used, corresponding to a 50 / M ferulic acid concentration. 

In order to O-demethylate ferulic acid linked to arabinoxylans, the cellular extract of SR 3 was sonicated to release its demethylase activity. The consumption of free ferulic acid and the production of caffeic add by the sonicated SR3 cellular extract are presented in Table 2. After 18 h of reaction, the totality of ferulic acid has been used. Caffeic acid production was proportional to the consumption of ferulic acid during the first 5 h then caffeic acid concentration remained constant while ferulic add was still consumed. Caffeic acid was probably degraded by oxidation phenomena. Indeed, the use of a reducing agent (cysteine) can reduce the caffeic acid loss from 40% to 15%. With cysteine (ferulic acid/cysteine = 0.01), the final amount of caffeic acid yielded 84% of the initial free ferulic acid content after 18 h of reaction. 

The sonicated SR3 cellular extract (100 mg of protein//imol of ferulic acid) has been applied to a 0.5% arabinoxylan solution corresponding to a 50 jtM ferulic... 

Lempereur, L, Rouau, X., and Abecassis, J., Genetic and agronomic variation in arabinoxylan and ferulic acid contents of durum wheat (Triticum durum L.) grain and its milling fractions, J. Cereal Sci., 25, 103,... 

Hydroxycinnamic acids are partially recovered in beer (Fig. 75.4). Most of them are in combined forms in the raw materials, either with quinic acid, glucose, or cell-wall constituents [8, 11-14, 70-78]. hi malt, p-coumaric and ferulic acids are esterified with arabinoxylans [79]. They can be both water extracted and enzymatically solubilized by cinnamoyl esterases [80]. After mashing, an additional release of ferulic acid may occur during fermentation due to yeast cinnamoyl esterases [81]. 

Beech TMP fibres - - ferulic acid-arabinoxylan dimer Paper with higher wet strength and decrease in bulk thickness [23-26]... 

In contrast to the previous examples, oxidative treatments may promote positive consequences in some other cases, as they decrease concentrations in phenolic acids (and other phenolics) and limit subsequent oxidations. Therefore, hyperoxidation of musts during wine processing has sometimes been proposed, and this process particularly affected HCA derivatives [27]. In addition, a peroxidasic treatment allowed the dimerization of ferulic acid in wheat bran tissues and consequently decreased arabinoxylan solubility and increased mechanical strength of the tissue [58]. 

Figure 4.18 Reactions of ferulic acid residues In arabinoxylans. 

Ferulic acid is ubiquitously present in plants, mainly as part of the cell walls esterified to polysaccharides such as arabinoxylans. It is also an intermediate in the secondary metabolism of phenylalanine and tyrosine. It can be conveniently isolated from corn hulls, rice bran, and sugar beet. Biocatalytic processes using cinnamoyl esterases and glycosyl hydrolases have been developed for this purpose [32]. Another potentially interesting source is lignin, since ferulic acid is one of the prominent byproducts of its degradation. 

Kern et al. (2003) measured the urinary excretion and plasma concentration of ferulic acid metabolites after ingestion of breakfast cereals by humans. They deduced from the kinetic data that absorption of ferulic acid from cereals mainly took place in the small intestine, from the soluble fraction present in cereals. Only a minor absorption of feruhc acid linked to arabinoxylans was absorbed after hydrolysis in the large intestine. 

Arabinoxylans (Figure 3B) have a backbone composed of 1,4-linked P-D-Xylp residues—some of the Xyl/ residues may be 0-acetylated [7]. Ara/ residues are linked to position C-2 and/or C-3 of the backbone. In primary walls P-d-GIc/jA or 4-0-Me p-D-Glc A residues are also attached to the xylan backbone and these polysaccharides are called glucuronarabinoxylans. The Ara/ side chains of arabi-noxylan and glucuronarabinoxylan may also contain ester-linked phenolic acids such as ferulic acid [9] that are potential sites for cross-linking by oxidative coupling (Figure 3B and C). 

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