Trehalose ( a,a- 1,1 configuration)
Fig. 14.1 Chemical structures of sucrose and trehalose
In the vast majority of plants, trehalose is only present in trace amounts.
The different chemistries of trehalose and sucrose dictate their biology, the functions they perform, and the mechanisms that determine the concentration of these compounds in vivo. Several arguments can be put forward to explain the prevalence of sucrose as translocated sugar in plants:
1. Sucrose is more soluble than trehalose, particularly at low temperatures, and hence may be more suited as a transport sugar in plant phloem at concentrations as high as 1 M.
2. Sucrose can be cleaved by invertase into glucose and fructose, and by sucrose synthase into uridine diphosphoglucose (UDPG) and fructose, preserving energy as UDPG.
3. Cell wall polysaccharides are synthesized from UDPG; thus, the ability to liberate UDPG directly from sucrose to synthesize cell wall polysaccharides may be the main reason that sucrose dominates in plants.
The importance of trehalose in stress conditions compared with other sugars can be explained by several unique physical properties, which include high hydrophilicity, chemical stability, and the absence of internal hydrogen bond formation that account for the principal ability of trehalose for protein stabilization. It has been proposed that in the absence of water, trehalose preserves membrane or protein structures by forming an amorphous glass structure and interacting through hydrogen bonds with polar phos-pholipids head groups or with amino acids (Crowe et al. 1984). Thereby trehalose is helping the protein to keep in shape and concentrate the remaining water next to the protein (Schiraldi et al. 2002). Trehalose is among the most chemically unreactive sugars and its strong stability is result of the very low energy (1 kcal mol-1) of the glycoside oxygen bond joining the two hexose rings. In comparison, sucrose has an energy bond of 27 kcal mol-1 (Paiva and Panek 1996). Therefore, trehalose does not dissociate into two reducing monosaccharidic constituents unless exposed to extreme hydrolytic conditions or to the action of trehalose.
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