tailieunhanh - Biochemistry, 4th Edition P24

Biochemistry, 4th Edition P24. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | What Is the Structure and Chemistry of Oligosaccharides 193 A DEEPER LOOK Trehalose A Natural Protectant for Bugs Insects use an open circulatory system to circulate hemolymph insect blood . The blood sugar is not glucose but rather trehalose an unusual nonreducing disaccharide see figure . Trehalose is found typically in organisms that are naturally subject to temperature variations and other environmental stresses bacterial spores fungi yeast and many insects. Interestingly honeybees do not have trehalose in their hemolymph perhaps because they practice a colonial rather than solitary lifestyle. Bee colonies maintain a rather constant temperature of 18 C protecting the residents from large temperature changes. What might explain this correlation between trehalose utilization and environmentally stressful lifestyles Konrad Bloch suggests that trehalose may act as a natural cryoprotectant. Freezing Bloch K. 1994. Blondes in Venetian Paintings the Nine-Banded Armadillo and Other Essays in Biochemistry. New Haven Yale University Press. tAttfield P. V. 1987. Trehalose accumulates in Saccharomyces cerevisiae during exposure to agents that induce heat shock responses. FEBS Letters 225 259. and thawing of biological tissues frequently causes irreversible structural changes destroying biological activity. High concentrations of polyhydroxy compounds such as sucrose and glycerol can protect biological materials from such damage. Trehalose is particularly well suited for this purpose and has been shown to be superior to other polyhydroxy compounds especially at low concentrations. Support for this novel idea comes from studies by Paul Allfield 1 which show that trehalose levels in the yeast Saccharomyces cerevisiae increase significantly during exposure to high salt and high growth temperatures the same conditions that elicit the production of heat shock proteins P-D-Lactose O-j8-D-galactopyranosyl- 1 4 -D-glucopyranose Figure is the principal carbohydrate in milk