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Xyloglucan endotransglycosylase: evidence for the existence of a relatively stable glycosyl-enzyme intermediate.

ABSTRACT: Xyloglucan endotransglycosylases (XETs) catalyse the breakdown of xyloglucan molecules predominantly by transglycosylation. In this process, fragments of cleaved polysaccharide are preferentially transferred to other xyloglucan molecules or their oligosaccharide subunits, with overall retention of the anomeric configuration of the glycosidic bond. In accordance with the theory, we propose that the cleavage and re-formation of the glycosidic bond in xyloglucan involves the formation of a glycosyl-enzyme intermediate which decomposes by transfer of the glycosyl moiety to a suitable carbohydrate acceptor. XETs from nasturtium seed cotyledons, mung bean hypocotyls and cauliflower florets interacted with xyloglucan to form complexes of high Mr as judged by gel-permeation chromatography. The nasturtium enzyme also showed evidence of XET-xyloglucan complex-formation according to anion-exchange chromatography and adsorption of the complex to filter paper on the basis of affinity of its xyloglucan moiety for cellulose. The XET-xyloglucan complex was stable in water, 6 M urea and acidic and alkaline buffers (pH 2.5-9.5), but readily decomposed by transferring its glycosyl moiety to xyloglucan-derived oligosaccharides or by incubation with the strong nucleophile imidazole at pH 3.8-9.6. These results strongly support the assumption that XET forms a relatively stable covalently linked glycosyl-enzyme intermediate.

SUBMITTER: Sulova Z

PROVIDER: S-EPMC1219298 | biostudies-other | 1998 Mar

REPOSITORIES: biostudies-other

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Project description:1. Cell-free extracts of all plants tested contained a novel enzyme activity (xyloglucan endotransglycosylase, XET) able to transfer a high-Mr portion from a donor xyloglucan to a suitable acceptor such as a xyloglucan-derived nonasaccharide (Glc4Xyl3GalFuc; XG9). 2. A simple assay for the enzyme, using [3H]XG9 and based on the ability of the [3H]polysaccharide product to bind to filter paper, is described. 3. The enzyme was highly specific for xyloglucan as the glycosyl donor, and showed negligible transglycosylation of other polysaccharides, including CM-cellulose. 4. The Km for XG9 was 50 microM; certain other 3H-labelled xyloglucan oligosaccharides also acted as acceptors, and certain non-radioactive xyloglucan oligosaccharides competed with [3H]XG9 as acceptor; the minimum acceptor structure was deduced to be: [formula: see text] 5. The pH optimum was approx. 5.5 and the enzyme was less than half as active at pH 7.0. The enzyme was slightly activated by Ca2+, Mg2+, Mn2+, spermidine, ascorbate and 2-mercaptoethanol, and inhibited by Ag+, Hg2+, Zn2+ and La3+. 6. XET activity was essentially completely extracted by aqueous solutions of low ionic strength; Triton X-100, Ca2+, La3+, and Li+ did not enhance extraction. Negligible activity was left in the unextractable (cell-wall-rich) residue. 7. The enzyme differed from the major cellulases (EC 3.2.1.4) of pea in: (a) susceptibility to inhibition by cello-oligosaccharides, (b) polysaccharide substrate specificity, (c) inducibility by auxin, (d) requirement for salt in the extraction buffer and (e) activation by 2-mercaptoethanol. XET is therefore concluded to be a new enzyme activity (xyloglucan: xyloglucan xyloglucanotransferase; EC 2.4.1.-). 8. XET was detected in extracts of the growing portions of dicotyledons, monocotyledons (graminaceous and liliaceous) and bryophytes. 9. The activity was positively correlated with growth rate in different zones of the pea stem. 10. We propose that XET is responsible for cutting and rejoining intermicrofibrillar xyloglucan chains and that it thus causes the wall-loosening required for plant cell expansion.

Project description:BACKGROUND: Idiopathic pulmonary fibrosis (IPF) is a progressive, chronic interstitial lung disease that is unresponsive to current therapy and often leads to death. However, the rate of disease progression differs among patients. We hypothesized that comparing the gene expression profiles between patients with stable disease and those in which the disease progressed rapidly will lead to biomarker discovery and contribute to the understanding of disease pathogenesis. METHODOLOGY AND PRINCIPAL FINDINGS: To begin to address this hypothesis, we applied Serial Analysis of Gene Expression (SAGE) to generate lung expression profiles from diagnostic surgical lung biopsies in 6 individuals with relatively stable (or slowly progressive) IPF and 6 individuals with progressive IPF (based on changes in DLCO and FVC over 12 months). Our results indicate that this comprehensive lung IPF SAGE transcriptome is distinct from normal lung tissue and other chronic lung diseases. To identify candidate markers of disease progression, we compared the IPF SAGE profiles in stable and progressive disease, and identified a set of 102 transcripts that were at least 5-fold up regulated and a set of 89 transcripts that were at least 5-fold down regulated in the progressive group (P-value</=0.05). The over expressed genes included surfactant protein A1, two members of the MAPK-EGR-1-HSP70 pathway that regulate cigarette-smoke induced inflammation, and Plunc (palate, lung and nasal epithelium associated), a gene not previously implicated in IPF. Interestingly, 26 of the up regulated genes are also increased in lung adenocarcinomas and have low or no expression in normal lung tissue. More importantly, we defined a SAGE molecular expression signature of 134 transcripts that sufficiently distinguished relatively stable from progressive IPF. CONCLUSIONS: These findings indicate that molecular signatures from lung parenchyma at the time of diagnosis could prove helpful in predicting the likelihood of disease progression or possibly understanding the biological activity of IPF.

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Xyloglucan endotransglycosylase: evidence for the existence of a relatively stable glycosyl-enzyme intermediate.

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