Substrate specificity studies on the malonyl-CoA-dependent chain elongation of all-cis polyunsaturated fatty acids by rat liver microsomes.
Substrate specificity studies on the malonyl-CoA-dependent chain elongation of all-cis polyunsaturated fatty acids by rat liver microsomes.
复制标题
大鼠肝微粒体对全顺式多不饱和脂肪酸丙二酰辅酶A依赖性链延长的底物特异性研究。
DOI：
10.1016/0003-9861(79)90253-4
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发表时间：
1979
影响因子：
3.9
通讯作者：
H. Sprecher
中科院分区：
文献类型：
--
作者：
S. Ludwig;H. Sprecher
观察到八种十八碳二烯异构体的缩合速率和总链长之间的密切一致，其中双键从4，7-位移动到11，14-位。结果表明，7，10-和6，9-十八碳二烯酸的总延长链比活性分别为5.2 0和2.89nmol产物min−1 mg−1，而其他6种异构体的比活性均低于0.84。使用延长5，8-，7，10-和8，11-十八碳二烯酸链所需的适当底物，测量了β-羟基酰辅酶A脱水酶和2-反式-烯基-辅酶A还原酶反应的比活力。尽管这些速率没有受到结构修饰的明显影响，但它们都比初始反应大得多，因此意味着缩合是速率限制。尽管6，9-和7，10-十八碳二烯酸都很容易与丙二酰辅酶A缩合，但7-和9-十八碳二烯酸对于整个链的延长来说都是很差的底物。当引发剂的链长从14-碳链延长到18-碳链时，7，10-不饱和酸的总伸长率增加，而7，10-二十碳二烯酸几乎不起作用。当用其中双键从4，7，10-转移到9，12，15-位的六个十八碳三烯酸异构体系列测量总的链长时，只有5，8，11-，6，9，12-和7，10，13-异构体容易链长。同样，与十八碳二烯酸异构体一样，最好的底物在第7位有第一个双键。同样，由于5，8，11-和7，10，13-二十碳三烯酸的链长比它们的18碳类似物的链长慢，所以链的伸长速度与链长有关。当根据共同的末端结构将底物分组时，没有单一特征可识别，该单一特征决定了是否会容易地延长引物的链长。因此，我们的发现最符合底物高度专一性的缩合反应，这涉及羧基识别，但也由链长、双键位置和不饱和度决定。
Close agreement between rates of condensation and overall chain elongation have been observed with eight octadecadienoic isomers in which the double bonds were moved from the 4,7- to the 11,14-positions. The specific activities for overall chain elongation of 7,10-and 6,9-octadecadienoic acids were, respectively, 5.20 and 2.89 nmol product min−1mg−1rat liver microsomal protein, while the specific activities for the other six isomers were all below 0.84. The specific activities for both the β-hydroxyacyl-CoA dehydrase and 2-trans-enoyl-CoA reductase reactions were measured using the appropriate substrates required in chain elongating 5,8-, 7,10-, and 8,11-octadecadienoic acids. Although these rates were not as markedly influenced by structural modification, they were all much greater than the initial reaction thus implicating condensation as rate limiting. Both 7- and 9-octadecenoic acids were poor substrates for overall chain elongation even though both 6,9- and 7,10-octadecadienoic acids readily condensed with malonyl-CoA. The rate of overall elongation increased for 7,10-unsaturated acids as the chain length of the primer was extended from 14- to 18-carbons, however, 7,10-eicosadienoic acid was virtually inactive. When rates of overall chain elongation were measured with an isomeric series of six octadecatrienoic acids in which the double bonds were shifted from the 4,7,10- to the 9,12,15-positions, only the 5,8,11-, 6,9,12-, and 7,10,13-isomers were readily chain elongated. Again, as with the octadecadienoic acid isomers the best substrate had the first double bond at position 7. Again the rate of chain elongation was chain length dependent since both 5,8,11- and 7,10,13-eicosatrienoic acid were chain elongated at lower rates than were their 18 carbon analogs. When the substrates were grouped according to common terminal structures no single feature was identifiable which dictated whether a primer would readily be chain elongated. Our findings are thus most consistent with a high degree of substrate specificity for condensation which involves carboxyl recognition but is also dictated both by chain length, double-bond positions, and degree of unsaturation.