1. The dependence of V and V/K(m) for threonine transport into Trypanosoma brucei upon the external concentration of H(+) was studied. 2. Two ionizing groups, the alpha-amino group of the substrate and a group at the substrate-binding site of the carrier, were found to influence the observed kinetic behaviour of transport. 3. The pK of the group at the substrate-binding site on the free carrier was found to be 6.95 at 30 degrees C and to be temperature-dependent; its heat of ionization was -63.8kJ, which is outside the range for most proton dissociations and suggests a significant contribution from some other source, possibly the remainder of the carrier or the membrane environment. 4. Binding of substrate caused the pK of its alpha-amino group to shift to a higher value, whereas that of the carrier group shifted to a lower value (6.65 at 30 degrees C). 5. The ionic interaction between substrate and carrier appeared to be involved in the stabilizing of the protonated substrate and the species of the carrier-substrate complex required for the membrane-translocation step. 6. The same ionic species of carrier-substrate complex is required for both substrate dissociation and translocation of the substrate through the membrane. 7. H(+) symport or antiport did not occur during threonine uptake.

译文

1。研究了苏氨酸向布氏锥虫转运的V和V/K(m) 对外部H () 浓度的依赖性。2.发现两个电离基团,即底物的 α-氨基基团和载体的底物结合位点的基团,会影响观察到的运输动力学行为。3.发现自由载体上的底物结合位点的基团的pK在30 ℃ 下是6.95的,并且是温度依赖性的; 其电离热为-63.8kj,这超出了大多数质子解离的范围,并暗示了其他来源的重大贡献,可能是载体的其余部分或膜环境。4.底物的结合使其 α-氨基的pK转变为较高的值,而载体基团的pK转变为较低的值 (在30 ℃ 下6.65)。5.底物与载体之间的离子相互作用似乎参与质子化底物的稳定以及膜转运步骤所需的载体-底物复合物的种类。6.底物解离和底物通过膜的转运都需要相同离子种类的载体-底物复合物。7.苏氨酸摄取过程中未发生H(+) 共生或反端口。

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