High-affinity transport system

The fact that Mn2+ may also be used105 as a probe for Ca2+ serves to emphasize those problems, as Ca2+ and Mg2+ tend to be mutually inhibitory. It appears that Mn2+ resembles Ca2+ in terms of movement in and out of certain cells. However, Mn2+ is taken up by some microbes through low affinity magnesium transport systems. These organisms usually have specific high affinity transport systems for Mn2+ as well.106... 

Demarest KT, Moore KE (1979a) Lack of a high affinity transport system for dopamine in the median eminence and posterior pituitary. Brain Res 777 545-551. 

Virtually all microorganisms—with the exception of certain lactobacilli— require iron as cofactor of many metabolic enzymes and regulatory proteins because of its ability to exist in two stable oxidation states. Although iron is one of the most abundant elements in the environment, it is often a limiting factor for bacterial growth. This is so because of the formation of insoluble ferric hydroxide complexes under aerobic conditions at neutral pH, which impose severe restrictions on the availability of the element. Consequently, bacteria have evolved specialized high-affinity transport systems in order to acquire sufficient amounts of this essential element. 

Phosphate uptake was studied in phosphate-limited chemostat cultures of Methanobacterium thermoautotrophicum. In cells adapted to nmolar phosphate concentrations the presence of a high-affinity transport system (A m = 25 nM Fmax = 60nmol/min x mg) was demonstrated [273]. 

Uptake systems appear to be simplest for dissolved Mn(ii), which is taken up in phytoplankton by a single high-affinity transport system that is under negative feedback regulation. In this negative feedback, as the concentration of dissolved Mn(ii) decreases, the Vmax of the transport system is increased... 

Eukaryotic phytoplankton do not appear to produce siderophores and there is little evidence for direct cellular uptake of ferric siderophore chelates. Instead there is mounting evidence for the utilization of a high-affinity transport system that accesses ferric complexes via their reduction at the cell surface and subsequent dissociation of the resulting ferrous-ligand complexes. The released ferrous ions bind to iron(ii) receptors on iron transport proteins located on the outer cell membrane, which transport the iron into the cell. This intracellular transport involves the reoxidation of bound iron(ii) to iron(iii) by a copper protein, and thus copper is required for cellular iron uptake. The availability of iron to this transport... 

Fig. 1.9. Evolution of glucose transport system activity of S. cerevisiae fermenting a medium model (Salmon et al, 1993). LF = Length of the fermentation as a decimal of total time GP = Glucose penetration speed (mmol/h/g of dry weight) 0 = Low affinity transport system activity = High affinity transport system activity...

---