Introduction

Xyloglucans are plant cell wall polysaccharides, which belong to the hemicellulose class; i.e., they can bind to cellulose. They are the most abundant hemicellulose in the walls of many nongraminaceous species. Xyloglucans can function both as a structural and as a reserve polysaccharide. In the primary cell wall, xyloglucans can crosslink cellulose fibers, yielding a network that determines to a large extent the strength of the wall. This complex usually constitutes approximately half of the amount of wall polysaccharides, the cellulose (30%) being a bit more abundant than the xyloglucan (20%). Plants possess many enzymes to remodel this network. Certain degradation products of xyloglucan may serve as signaling molecules, the activity of which can be regulated by various glycosidases. Structural xyloglucans are also important from a food industry point of view. They are an important target for fungal enzymes in applications, which aim at a complete degradation of the plant cell wall, such as the "liquefaction process'' for fruit juice manufacturing. Further, xyloglucans might play a role in determining the shelf life of fruits and vegetables.

Xyloglucans can also be deposited in the secondary cell walls of seeds of several species, such as Tamarindus indica, Tropaeolum majus, Hymenaea courbaril, and Copaifera langsdorffii. The seeds can contain > 50% xyloglucan. Upon germination of these seeds, a battery of enzymes is induced to mobilize the xyloglucan food reserves of the hypocotyl. The storage xyloglucans, in particular, are used as food ingredients or in pharmaceutical applications. The decoration (or side-chain configuration) of these xyloglucans determines to a large extent their functional properties. Depending on the application, the decoration may be adjusted by using enzymes of either plant or fungal origin. With the discovery of the first enzymes in xyloglucan biosynthesis, it may be possible in the future to realize these structural modifications in the plant itself. Biosynthetic enzymes using nucleoside-diphosphate sugars will not be discussed in this chapter.

In the paragraphs below, the structural variations of xyloglucans will be reviewed first. As we will see later, substitution determines the rate at which xylo-glucan molecules are degraded. Subsequently, the anchoring of xyloglucan in the plant cell wall will be discussed. Some background information on this is provided, because certain enzyme purification procedures and enzyme assays make use of these principles. The following sections deal with (plant and microbial) enzymes involved in degradation or modification of xyloglucan. After this, a number of methods for activity measurements are summarized. These methods deal predominantly with the determination of transferase activity, because these assays follow different procedures from those used for degradative enzymes. Finally, a few applications in the food industry will be described.

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