图1. XLFG型和XXGG型木葡聚糖(XyG)的生物合成与降解示意图
木葡聚糖生物合成酶有FUT (XyG特异岩藻糖基转移酶)、GS (β-1,4-葡聚糖合成酶)、MUR/XLT (XyG特异半乳糖基转移酶)、AXY4L/TBL22和AXY4/TBL27 (XyG特异乙酰基转移酶)、XXT (XyG特异木糖基转移酶)以及XYBAT (XyG葡聚糖骨架乙酰转移酶)。XyG降解酶类有AE (乙酰酯酶)、BGAL (β-半乳糖苷酶)、FUC (α-岩藻糖苷酶)、XTH (XyG内转糖苷酶/水解酶)和XYL (α-木糖苷酶)。图下方为根据XyG命名法获得的相应单字母代码。G: XyG主链葡萄糖残基未连接其它任何糖残基或非糖基团; G: XyG主链葡萄糖残基发生乙酰化; X: XyG主链葡萄糖残基O-6位连接木糖基团; L: X型基团的木糖残基O-2位连接半乳糖基团; F: L型基团的半乳糖残基O-2位连接岩藻糖基团, 且半乳糖残基发生乙酰化。

Figure 1. Schematic of the biosynthesis and degradation of XLFG-type and XXGG-type xyloglucan (XyG)
Enzymes involved in XyG biosynthesis are FUT (XyG: fucosyltransferase), GS (β-1,4-glucan synthase), MUR/XLT (XyG: galactosyltransferase), AXY4L/TBL22 and AXY4/TBL27 (XyG: acetyltransferases), XXT (XyG: xylosyltransferase), and XYBAT (XyG backbone acetyltransferase). Enzymes involved in XyG degradation are AE (acetylesterases), BGAL (β-galactosidase), FUC (α-fucosidase), XTH (XyG endotransglycosylase/hydrolases), and XYL (α-xylosidase). The corresponding one-letter codes from the XyG nomenclature have shown below the pictograms. G: Unsubstituted glucosyl residue of the backbone glucose of XyG; G: Backbone glucose of XyG carries an O-acetyl substituent; X: Xylosyl residue is attached to the glucan backbone of XyG at O-6; L: Galactosyl residue is attached to the xylosyl residue of X group at O-2; F: Fucosyl residue is attached to the galactosyl residue of L group at O-2, and galactosyl residue carries an O-acetyl substituent.