XYLOGLUCAN ENDOTRANSGLYCOSYLASE, A NEW WALL-LOOSENING ENZYME-ACTIVITY FROM PLANTS
XYLOGLUCAN ENDOTRANSGLYCOSYLASE, A NEW WALL-LOOSENING ENZYME-ACTIVITY FROM PLANTS
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DOI：
10.1042/bj2820821
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发表时间：
1992-03-15
影响因子：
4.1
通讯作者：
MATTHEWS, KJ
中科院分区：
文献类型：
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作者：
FRY, SC;SMITH, RC;MATTHEWS, KJ
1.所有供试植物的无细胞提取物都含有一种新的酶活性(木葡聚糖内糖基转移酶，XET)，能够将高M(R)部分从供体木聚糖转移到合适的受体，如木葡聚糖衍生的非天冬氨酸糖(Glc4Xyl3GalFuc；XG9)。2.以[H-3]XG9为底物，根据[H-3]多糖产物与滤纸结合的能力，建立了一种测定该酶的简便方法。3.该酶对作为糖基供体的木葡聚糖具有很高的专一性，而对包括CM-纤维素在内的其他多糖的转糖基化作用很小。4.XG9的K(M)为50-mU-M，某些H-3标记的低聚木糖也作为受体，某些非放射性低聚木糖与[H-3]XG9竞争，最小受体结构为：[图形]5。5.5，在pH为7.0时酶的活性不到其一半。Ca~(2+)、Mg~(2+)、Mn~(2+)、亚精胺、抗坏血酸和2-巯基乙醇对酶有轻微激活作用，而Ag~(2+)、Hg2+、Zn~(2+)和La~(3+)对酶有抑制作用。6.XET活性基本被低离子强度的水溶液萃取，Triton X-100、Ca~(2+)、La~(3+)、Li~(3+)对萃取率无明显促进作用。在无法提取的(富含细胞壁的)残留物中，活性可以忽略不计。7.该酶与豌豆主要纤维素酶(EC 3.2.1.4)的不同之处在于：(A)对纤维低聚糖抑制的敏感性，(B)多糖底物的专一性，(C)生长素的诱导性，(D)对提取缓冲液中盐的要求，(E)2-巯基乙醇的激活。因此，XET被认为是一种新的酶活性(木葡聚糖：木葡聚糖木糖转移酶；EC 2.4.1.-)。8.在双子叶植物、单子叶植物(禾本科和百合科)和苔藓植物的生长部分提取液中检测到XET。9.豌豆茎不同部位的酶活性与生长速率呈正相关。10.我们认为XET负责切断和重新连接微纤维间的木聚糖链，从而导致植物细胞扩张所需的壁松动。
1. Cell-free extracts of all plants tested contained a novel enzyme activity (xyloglucan endotransglycosylase, XET) able to transfer a high-M(r) 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 [H-3]XG9 and based on the ability of the [H-3]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 K(m) for XG9 was 50-mu-M; certain other H-3-labelled xyloglucan oligosaccharides also acted as acceptors, and certain non-radioactive xyloglucan oligosaccharides competed with [H-3]XG9 as acceptor; the minimum acceptor structure was deduced to be:[GRAPHICS]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.