Effects of temperature on active amino acid transport in Escherichia coli strain 7

The effects of temperature on the apparent Michaelis constant (Km) and the apparent maximum velocity (Vmax) for L-glutamine transport as well as for L-proline transport in Escherichia coli strain 7 have been investigated. The Lineweaver-Burk plot for L-glutamine transport was biphasic, yielding a high affinity Km (usually less than 0.1 x 10(-6) M) at glutamine concentrations below 1.25 x 10(-6) M and a low affinity Km (1 x 10(-6) M) above 1.25 x 10(-6) M glutamine. For both transport systems, the high affinity Km increased with temperature unlike that with E. coli K 1060. The Km at room temperature (21 degrees C) was 7.5 x 10(-6) M for L-glutamine, and 0.7 x 10(-6) M for L-proline transport. Arrhenius plots of the Vmax displayed breaks (Tb) at 17 degrees C for L-glutamine and at 13 degrees C for L-proline transport. Tb reasonably correlates with the midpoint (15 degrees C) of the broad gel-to-liquid crystalline phase transition of E. coli strain 7 membrane lipids. These findings, especially the changes in Km with temperature, emphasize the need to investigate Vmax rather than initial rates during temperature dependence studies of membrane transport.