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  3. As people age, their cells become less efficient and less able to replace damaged components. At the same time their tissues sti

As people age, their cells become less efficient and less able to replace damaged components. At the same time their tissues sti

admin2010-05-14  31
问题    As people age, their cells become less efficient and less able to replace damaged components. At the same time their tissues stiffen. For example, the lungs and the heart muscle expand less successfully, the blood vessels become increasingly rigid, and the ligaments and tendons tighten.
   Few investigators would attribute such diverse effects to a single cause. Nevertheless, researchers have discovered that a process long known to discolor and toughen foods may also contribute to age-related impairment of both cells and tissues. That process is nonenzymatic glycosylation, whereby glucose becomes attached to proteins without the aid of enzymes. When enzymes attach glucose to proteins (enzymatic glycosylation), they do so at a specific site on a specific protein molecule for a specific purpose. In contrast, the nonenzymatic process adds glucose haphazardly to any of several sites along any available peptide chain within a protein molecule.
   This nonenzymatic glycosylation of certain proteins has been understood by food chemists for decades, al though few biologists recognized until recently that the same steps could take place in the body. Nonenzyrnatic glycosylation begins when an aldehyde group (CHO) of glucose and an amino group (NH2) of a protein are at tracted to each other. The molecules combine, forming what is called a Sehiff base within the protein. This combination is unstable and quickly rearranges itself into a stabler, but still reversible, substance known as an Amadori product.
   If a given protein persists in the body for months or years, some of its Amadori products slowly dehydrate and rearrange themselves yet again, into new glucose-derived structures. These can combine with various kinds of molecules to form irreversible structures named advanced glycosylation end products (AGE’s). Most AGE’s are yellowish brown and fluorescent and have specific spectrographic properties. More important for the body, many are also able to cross-link adjacent proteins, particularly ones that give structure to tissues and organs. Although no one has yet satisfactorily described the origin of all such bridges between proteins, many investigators agree that extensive cross-linking of proteins probably contributes to the stiffening and loss of elasticity characteristic of aging tissues.
   In an attempt to link this process with the development of cataracts (the browning and clouding of the lens of the eye as people age), researchers studied the effect of glucose on solutions of purified crystallin, the major protein in the lens of the eye. Glucose-free solutions remained clear, but solutions with glucose caused the proteins to form clusters, suggesting that the molecules had become cross-linked. The clusters diffracted light, making the solution opaque. The researchers also discovered that the pigmented cross-links in human cataracts have the brownish color and fluorescence characteristic of AGE’s. There data suggest that nonanzymatic glycosylation of lens crystallins may contribute to cataract formation. (454)
According to the passage, which of the following is characteristic of enzymatic glycosylation of proteins?
选项 A、AGE’s are formed after a period of months or years.
B、Proteins affected by the process are made unstable.
C、Glucose attachment impairs and stiffens tissues.
D、Glucose is attached to proteins for specific purposes.
答案D
解析 事实细节题。根据本文第二段第四句When enzymes attach glucose to proteins(enzymatic glycosylation)，they do so at a specific site on a specific protein molecule for a specific purpose．可知D项为正确答案。
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