Arabinose

Abstract: Glycation and oxidation reactions contribute to protein modification in aging and diabetes. Formation of dicarbonyl sugars during autoxidation of glucose is the hypothetical first step in the autoxidative glycosylation and subsequent browning of proteins by glucose [Wolff, S. P., & Dean, R. T. (1987) Biochem. J. 245, 243-250]. In order to identify the dicarbonyl sugar(s) formed during autoxidation of glucose under physiological conditions, glucose was incubated in phosphate buffer (pH 7.4) at 37 degrees C under air (oxidative conditions) or nitrogen with transition metal chelators (antioxidative conditions). Dicarbonyl compounds were analyzed spectrophotometrically and by HPLC after reaction with Girard-T reagent. Carbohydrates were analyzed by gas chromatography-mass spectrometry. Both dicarbonyl sugar and arabinose concentrations increased with time and glucose concentration in incubations conducted under oxidative conditions; only trace amounts of these products were detected in glucose incubated under antioxidative conditions. HPLC analysis of adducts formed with Girard-T reagent indicated that glyoxal was the only alpha-dicarbonyl sugar formed on autoxidation of glucose. Glyoxal and arabinose accounted for > or = 50% of the glucose lost during a 21 day incubation. Neither glucosone nor its degradation product, ribulose, was detectable. Reaction of glyoxal with RNase yielded the glycoxidation product, N epsilon-(carboxymethyl)lysine, while arabinose is a source of pentosidine. Our results implicate glyoxal and arabinose as intermediates in the browning and crosslinking of proteins by glucose under oxidative conditions. They also provide a mechanism by which antioxidants and dicarbonyl trapping reagents, such as aminoguanidine, limit glycoxidation reactions and support further evaluation of these types of compounds for inhibition of chemical modification and crosslinking of proteins during aging and diabetes.

Abstract: Experimental trials of the dilute acid hydrolysis of bagasse hemicellulose to produce xylose, arabinose, glucose, acid-soluble lignin (ASL) and furfural were conducted using a temperature-controlled digester. The reaction conditions varied were; temperature (80–200°C), mass ratio of solid to liquid (1:5–1:20), type of bagasse material (i.e. bagasse or bagacillo), concentration of acid (0.25–8 wt% of liquid), type of acid (hydrochloric or sulphuric) and reaction time (10–2000 min). Kinetic modelling of the global rates of formation of products was performed. The most accurate kinetic model of the global reaction for the decomposition of xylan was a simple series hydrolysis of xylan to xylose followed by xylose decomposition. Similar schemes were used to model the production of arabinose, glucose and furfural from the hemicellulose. The production of ASL was modelled by a first-order decomposition of lignin to ASL followed by a reversible decomposition of ASL. Yields of up to 220 mg xylose/g solid were achieved, i.e. about 80% of the theoretical xylose available from the bagasse. The bagasse particle size was found to negligibly affect the rate of hydrolysis. Hydrochloric acid was found to be less active for the degradation of xylose compared to sulphuric acid.

Abstract: Gene expression from plasmids containing the araBAD promoter can be regulated by the concentration of arabinose in the growth medium. Guzman et al. (Guzman, L.-M., Belin, D., Carson, M. J. & Beckwith, J. (1995) J. Bacteriol. 177, 4121-4130) showed that expression of a cloned gene could be modulated over several orders of magnitude in cultures grown in the presence of subsaturating concentrations of arabinose. We constructed plasmids expressing a fast-folding mutant Aequorea victoria green fluorescent protein from the araBAD promoter to examine the distribution of expressed gene products in individ- ual cells at intermediate induction levels. Microscopic examina- tion of cells grown at low arabinose concentrations shows mix- tures of brightly fluorescent and dark cells, suggesting that intermediate expression levels in cultures reflect a population average of induced and uninduced cells. The kinetics of green fluorescent protein induction suggest that this reflects an ''au- tocatalytic'' induction mechanism due to accumulation of the inducer by active transport. This mechanism, which is analogous to the induction of the lac operon at subsaturating inducer concentrations in lacY 1 cells, was described 40 years ago by Novick and Weiner (Novick, A. & Weiner, M. (1957) Proc. Natl. Acad. Sci. USA 43, 553-566).

Abstract: The substrate fermentation range of the ethanologenic bacterium Zymomonas mobilis was expanded to include the pentose sugar, L-arabinose, which is commonly found in agricultural residues and other lignocellulosic biomass. Five genes, encoding L-arabinose isomerase (araA), L-ribulokinase (araB), L-ribulose-5-phosphate-4-epimerase (araD), transaldolase (talB), and transketolase (tktA), were isolated from Escherichia coli and introduced into Z. mobilis under the control of constitutive promoters that permitted their expression even in the presence of glucose. The engineered strain grew on and produced ethanol from L-arabinose as a sole C source at 98% of the maximum theoretical ethanol yield, based on the amount of consumed sugar. This indicates that arabinose was metabolized almost exclusively to ethanol as the sole fermentation product, with little by-product formation. Although no diauxic growth pattern was evident, the microorganism preferentially utilized glucose before arabinose, apparently reflecting the specificity of the indigenous facilitated diffusion transport system. This microorganism may be useful, along with the previously developed xylose-fermenting Z. mobilis (M. Zhang, C. Eddy, K. Deanda, M. Finkelstein, and S. Picataggio, Science 267:240-243, 1995), in a mixed culture for efficient fermentation of the predominant hexose and pentose sugars in agricultural residues and other lignocellulosic feedstocks to ethanol.