Department of Biotechnology and Food Engineering, Technion-Israel Institute of Technology, Haifa, Israel.
FEBS J. 2013 Feb;280(3):950-64. doi: 10.1111/febs.12089. Epub 2013 Jan 7.
Type I galactan is a pectic polysaccharide composed of β-1,4 linked units of d-galactose and is part of the main plant cell wall polysaccharides, which are the most abundant sources of renewable carbon in the biosphere. The thermophilic bacterium Geobacillus stearothermophilus T-6 possesses an extensive system for the utilization of plant cell wall polysaccharides, including a 9.4-kb gene cluster, ganREFGBA, which encodes galactan-utilization elements. Based on enzyme activity assays, the ganEFGBA genes, which probably constitute an operon, are induced by short galactosaccharides but not by galactose. GanA is a glycoside hydrolase family 53 β-1,4-galactanase, active on high molecular weight galactan, producing galactotetraose as the main product. Homology modelling of the active site residues suggests that the enzyme can accommodate at least eight galactose molecules (at subsites -4 to +4) in the active site. GanB is a glycoside hydrolase family 42 β-galactosidase capable of hydrolyzing short β-1,4 galactosaccharides into galactose. Applying both GanA and GanB on galactan resulted in the full degradation of the polymer into galactose. The ganEFG genes encode an ATP-binding cassette sugar transport system whose sugar-binding lipoprotein, GanE, was shown to bind galacto-oligosaccharides. The utilization of galactan by G. stearothermophilus involves the extracellular galactanase GanA cleaving galactan into galacto-oligosaccharides that enter the cell via a specific transport system GanEFG. The galacto-oligosaccharides are further degraded by the intracellular β-galactosidase GanB into galactose, which is then metabolized into UDP-glucose via the Leloir pathway by the galKET gene products.
Nucleotide sequence data have been deposited in the GenBank database under the accession number JF327803.
I 型半乳糖是一种由β-1,4 键连接的半乳糖单元组成的果胶多糖，是植物细胞壁多糖的一部分，而植物细胞壁多糖是生物圈中最丰富的可再生碳源。嗜热细菌 Geobacillus stearothermophilus T-6 拥有广泛的植物细胞壁多糖利用系统，包括一个 9.4kb 的基因簇 ganREFGBA，该基因簇编码半乳糖利用元件。根据酶活性测定，ganEFGBA 基因（可能构成一个操纵子）被短半乳糖苷诱导，但不受半乳糖诱导。GanA 是糖苷水解酶家族 53 的β-1,4-半乳糖苷酶，对半乳糖聚糖具有活性，主要产物是半乳糖四糖。活性部位残基的同源建模表明，该酶可以在活性部位容纳至少 8 个半乳糖分子（在亚位点-4 到+4）。GanB 是糖苷水解酶家族 42 的β-半乳糖苷酶，能够将短的β-1,4 半乳糖苷水解成半乳糖。在半乳糖聚糖上应用 GanA 和 GanB 可将聚合物完全降解成半乳糖。ganEFG 基因编码一个 ATP 结合盒糖转运系统，其糖结合脂蛋白 GanE 被证明能结合半乳糖低聚糖。嗜热脂肪芽孢杆菌对半乳糖聚糖的利用涉及细胞外半乳糖苷酶 GanA 将半乳糖聚糖切割成半乳糖低聚糖，这些低聚糖通过特定的转运系统 GanEFG 进入细胞。半乳糖低聚糖进一步被细胞内β-半乳糖苷酶 GanB 降解成半乳糖，然后通过 Leloir 途径由 galKET 基因产物代谢成 UDP-葡萄糖。
核苷酸序列数据已在 GenBank 数据库中以 JF327803 登录号进行了存储。
