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

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

admin2011-01-02  79
问题    As people age, their cells become less efficient and less able 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, although few biologists recognized until recently that the same steps could take place in the body. Nonenzymatic glycosylation begins when an aldehyde group (CHO) of glucose and an amino group (HN2) of a protein are attracted to each other. The molecules combine, forming what is called a Schiff 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. These data suggest that nonenzymatic glycosylation of lens crystallins may contribute to cataract formation.
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 该题问：根据文章，下列哪一个是酶催化糖基化的特点？D项意为“葡萄糖为特定目的依附于蛋白质”。见第二段第四句：“当酶把葡萄糖依附于蛋白质(即有酶催化糖基化)时，它们这样做是为了特定的目的，在特定的部位上发生在特定的蛋白分子上”。A项意为“高级糖基化最终产品是在几个月或几年后形成”。文章第四段讲了“某一特定的蛋白质在人体内持续存在几个月或几年，它的阿马多利产品渐渐脱水，再次重新组合自己，成为新的导源于葡萄糖结构。这些结构可以和各种类型的分子组成不可逆转的结构，称之为高级糖基化最终产品”。本题问的是酶催化基化的特点，而不是高级糖基化最终产品，所以只能选D。B项意为“受这过程影响蛋白质变得不稳定”，及C项意为“葡萄糖依附会损害和僵化组织”也是不对的。
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