问题
单项选择题
酶的必需基团是指______
A．酶的辅酶部分
B．与酶催化功能有关的基团
C．一些金属离子
D．酶蛋白的肽键
E．酶蛋白的表面电荷
答案
参考答案：B
解析：一般将与酶活性有关的基团称为酶的必需基团，有活性中心内和活性中心外。活性中心内的必需基团有催化基团和结合基团。活性中心外的必需基团为维持酶活性中心的空间构象所必需。
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酶的必需基团是指______
A．酶的辅酶部分
B．与酶催化功能有关的基团
C．一些金属离子
D．酶蛋白的肽键
E．酶蛋白的表面电荷
参考答案：B
解析：一般将与酶活性有关的基团称为酶的必需基团，有活性中心内和活性中心外。活性中心内的必需基团有催化基团和结合基团。活性中心外的必需基团为维持酶活性中心的空间构象所必需。
| Modern industrial society grants little status to old people. In fact, such a society has a system of built-in obsolescence. There is no formal system for continuing our education throughout our life in order to keep up with rapidly changing knowledge. When our education and job skills have grown obsolete, we are treated exactly like those who have never gained an education or job skills and are not encouraged or given the opportunity to begin anew. As a society becomes more highly developed, the overall status of older people diminishes. Improved health technology creates a large pool of old people, who compete for jobs with the young. However, economic technology lowers the demand for workers and creates new jobs for which the skills of the aged are obsolete, forcing older people into retirement. At the same time, young people are being educated in the new technology and are keeping pace with rapid changes in knowledge. Finally, urbanization creates age-segregated neighborhoods. Because the old live on fixed incomes, they must often live in inferior housing. All these factors -- retirement, obsolete knowledge and skills, inferior standards of living -- lower the status of the aged in society. A century ago, when one could expect to live only to 50 or so, the life span more or less coincided with the occupation and family cycle. But today the average life span allows for fifteen to twenty years of life after these cycles. It appears that our life span is outpacing our usefulness in society. |
The sentence "our life span outpaces our usefulness" means we can live longer______.
A．and make progress
B．and do more work
C．but move slowly
D．but become useless