Guardiola J, De Felice M, Klopotowski T, Iaccarino M
J Bacteriol. 1974 Feb;117(2):382-92. doi: 10.1128/jb.117.2.382-392.1974.
The kinetics of isoleucine, leucine, and valine transport in Escherichia coli K-12 has been analyzed as a function of substrate concentration. Such analysis permits an operational definition of several transport systems having different affinities for their substrates. The identification of these transport systems was made possible by experiments on specific mutants whose isolation and characterization is described elsewhere. The transport process with highest affinity was called the "very-high-affinity"process. Isoleucine, leucine, and valine are substrates of this transport process and their apparent K(m) values are either 10(-8), 2 x 10(-8), or 10(-7) M, respectively. Methionine, threonine, and alanine inhibit this transport process, probably because they are also substrates. The very-high-affinity transport process is absent when bacteria are grown in the presence of methionine, and this is due to a specific repression. Methionine and alanine were also found to affect the pool size of isoleucine and valine. Another transport process is the "high-affinity" process. Isoleucine, leucine, and valine are substrates of this transport process, and their apparent K(m) value is 2 x 10(-6) M for all three. Methionine and alanine cause very little or no inhibition, whereas threonine appears to be a weak inhibitor. Several structural analogues of the branched-chain amino acids inhibit the very-high-affinity or the high-affinity transport process in a specific way, and this confirms their existence as two separate entities. Three different "low-affinity" transport processes, each specific for either isoleucine or leucine or valine, show apparent K(m) values of 0.5 x 10(-4) M. These transport processes show a very high substrate specificity since no inhibitor was found among other amino acids or among many branched-chain amino acid precursors or analogues tried. The evolutionary significance of the observed redundancy of transport systems is discussed.
已分析了大肠杆菌K - 12中异亮氨酸、亮氨酸和缬氨酸转运的动力学与底物浓度的函数关系。这种分析允许对几种对其底物具有不同亲和力的转运系统进行操作性定义。通过对特定突变体的实验得以识别这些转运系统，其分离和表征在其他地方有描述。具有最高亲和力的转运过程被称为“极高亲和力”过程。异亮氨酸、亮氨酸和缬氨酸是该转运过程的底物，它们的表观K(m)值分别为10(-8)、2×10(-8)或10(-7)M。蛋氨酸、苏氨酸和丙氨酸抑制该转运过程，可能是因为它们也是底物。当细菌在蛋氨酸存在下生长时，极高亲和力转运过程不存在，这是由于特异性阻遏。还发现蛋氨酸和丙氨酸会影响异亮氨酸和缬氨酸的库大小。另一种转运过程是“高亲和力”过程。异亮氨酸、亮氨酸和缬氨酸是该转运过程的底物，它们对所有三种氨基酸的表观K(m)值均为2×10(-6)M。蛋氨酸和丙氨酸几乎没有或没有抑制作用，而苏氨酸似乎是一种弱抑制剂。支链氨基酸的几种结构类似物以特定方式抑制极高亲和力或高亲和力转运过程，这证实了它们作为两个独立实体的存在。三种不同的“低亲和力”转运过程，每种分别对异亮氨酸或亮氨酸或缬氨酸具有特异性，其表观K(m)值为0.5×10(-4)M。这些转运过程表现出非常高的底物特异性，因为在所尝试的其他氨基酸、许多支链氨基酸前体或类似物中未发现抑制剂。讨论了所观察到的转运系统冗余的进化意义。
