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The genome of the bacterium E. coli has at least 427 genes coding for
membrane transport proteins. This amounts to a startling 10% of all E. coli
genes, and makes this the mostabundant type of gene in the genome. By way of
comparison, the second most abundant type is biosynthetic genes, at 8% of the
total. The large number of transport genes reflects the omnivorous appetite of E.
coli for the wide variety of organic substrates it encounters in the complex
biochemical stew of the mammalian gut.
Another reason there are so many transport genes in E. coli is that often there
are 2 or more redundant transport mechanisms for the same solute. A typical
example is transport of the pentose, arabinose, which is transported by the ATPdependent ara F,G,H multi-component system and, independently, by the araE
system which couples arabinose transport to proton transport.
The equation for electrochemical potential, and a basic understanding of
biochemical thermodynamics, properly applied, allow you to explain the
existence of the two redundant arabinose transport systems.
We notice that the two arabinose transport systems are redundant in the sense
that they both transport arabinose, but they are not identical. The ara F, G, H
multi-component system couples arabinose transport to ATP hydrolysis. The ara
E system couples transport to proton transport (i.e. proton symport). Both
systems use an exogenous source of free energy to drive endergonic arabinose
uptake, therefore both are examples of active transport.
Additionally, the binding constants (Km) of the two systems differ by several
orders of magnitude. The ATP-dependent system exhibits high solute affinity,
and the proton symport system relatively low affinity for arabinose. (The Km
values are given in the diagram above.)
Your task is to estimate (by calculation) the free energy change (in kJ per mole)
for arabinose uptake by these two systems. Assume the intracellular arabinose
concentration of 10 mM, and extracellular concentrations equal to the respective
Km values of the two systems.
NAME/S:
Free energy required for arabinose uptake by the ATP-dependent system:
__________kJ/mole
Free energy required for arabinose uptake by the proton symport system:
__________kJ/mole