Liebl W, Feil R, Gabelsberger J, Kellermann J, Schleifer K H
Institute for Microbiology, Technical University Munich, Federal Republic of Germany.
Eur J Biochem. 1992 Jul 1;207(1):81-8. doi: 10.1111/j.1432-1033.1992.tb17023.x.
Maltodextrin glycosyltransferase (4-alpha-glucanotransferase) of the extremely thermophilic ancestral bacterium Thermotoga maritima has been purified from an Escherichia coli clone expressing the corresponding T. maritima MSB8 chromosomal gene. T. maritima 4-alpha-glucanotransferase, an approximately 53-kDa monomeric enzyme, is the most thermophilic glycosyltransferase described to date. It retained more than 90% of its maximum activity at temperatures from 55 degrees C up to 80 degrees C. The proposed action modus is the transfer of 1,4-alpha-glucanosyl chains, thus resulting in the disproportionation of 1,4-alpha-glucans. It converted soluble starch, amylopectin, and amylose, thereby changing the iodine staining properties of these substrates. The addition of low-molecular-mass malto-oligosaccharides, which act as glucanosyl acceptor molecules, enhanced the reaction and resulted in the formation of a series of linear maltohomologues from two to more than nine glucose units in size. Use of either of the malto-oligosaccharides maltotetraose, maltopentaose, maltohexaose, or maltoheptaose as sole substrate also yielded linear maltohomologues. On the other hand, maltose and maltotriose were not disproportionated by 4-alpha-glucanotransferase, although both were good acceptors for glucanosyl transfer. Glucose did not function as an acceptor in transfer reactions. Glucose also never appeared as a reaction product. The chain length of glucanosyl segments transferred ranged from two to probably far more than six glucose residues. Comparison of the N-terminal amino acid sequence of 4-alpha-glucanotransferase with other published protein sequences revealed significant similarity to sequences near the N-termini of various eucaryotic maltases and bacterial cyclodextrin glycosyltransferases, suggesting its relatedness on the molecular level with other starch- and maltodextrin-converting enzymes.
嗜热栖热袍菌这种嗜热祖先细菌的麦芽糊精糖基转移酶（4-α-葡聚糖转移酶）已从表达相应嗜热栖热袍菌MSB8染色体基因的大肠杆菌克隆中纯化出来。嗜热栖热袍菌4-α-葡聚糖转移酶是一种约53 kDa的单体酶，是迄今为止所描述的最嗜热的糖基转移酶。在55℃至80℃的温度范围内，它保留了超过90%的最大活性。推测的作用方式是1,4-α-葡聚糖链的转移，从而导致1,4-α-葡聚糖的歧化反应。它能转化可溶性淀粉、支链淀粉和直链淀粉，从而改变这些底物的碘染色特性。添加作为葡聚糖受体分子的低分子量麦芽寡糖可增强反应，并导致形成一系列大小从两个到九个以上葡萄糖单位的线性麦芽同系物。使用麦芽四糖、麦芽五糖、麦芽六糖或麦芽七糖中的任何一种作为唯一底物也能产生线性麦芽同系物。另一方面，麦芽糖和麦芽三糖不会被4-α-葡聚糖转移酶歧化，尽管它们都是葡聚糖转移的良好受体。葡萄糖在转移反应中不起受体作用。葡萄糖也从未作为反应产物出现。转移的葡聚糖片段的链长范围从两个到可能远超过六个葡萄糖残基。将4-α-葡聚糖转移酶的N端氨基酸序列与其他已发表的蛋白质序列进行比较，发现与各种真核麦芽糖酶和细菌环糊精糖基转移酶N端附近的序列有显著相似性，这表明它在分子水平上与其他淀粉和麦芽糊精转化酶有关。
