Exchange of Trace Elements

Major exchange of trace elements (Table 2) occurs from many carrier or buffer proteins. For example, iron is carried in the blood stream of higher animals by the protein transferrin in the form of Fe3 +, in co-association with the carbonate anion, CO2". The uptake into the cell involves the transfer of the whole protein to a vesicle, the lysozyme, which is much more acidic than blood. There the protein loses both iron and CO3", which are exchanged for bound protons. The iron is processed further into the cell as Fe2 + , only to be reoxidized in a precipitate inside a protein matrix, ferritin, where it is stored as Fe(OH)3. From this small particle, iron exchanges into the cytoplasm, to give a great variety of compounds (see Figure 1) for the case of bacteria.

A second example of cytoplasmic ion exchange of trace elements involves both copper and zinc bound to sulfur-containing proteins. The proteins release copper or zinc (M) to the cytoplasm in equilibrium with free hydrogen ions:

where Pr is a protein such as metallothionein. This equilibration ensures constant levels of Zn2 + and Cu + in the cytoplasm.

Trace anions are also carried in the blood stream by special proteins and are then transported into cells. The proteins which transfer SO4", SeO2" and MoO2~ are known to be similar to transferrin.

Table 2 Trace cations and anions of cells

Cation Anion

Mn2+ vesicular SeO4" cytoplasmic

Fe2 + /Fe3+ cytoplasmic (precipitate) MoO2" cytoplasmic

Co2+ cytoplasmic I" cytoplasmic

Ni2+ vesicular NO3" cytoplasmic Cu2+ rejected Zn2+ cytoplasmic

Many of the ions are held in complexes with organic agents.

External Fe

Figure 1 An outlined scheme of the variety of ion exchange reactions in the uptake of iron. Iron is captured as Fe3+ outside the cell by a small organic molecule, ferroxamine (X). FeX is passed through membranes (TON B) using energy, and is then exchanged in a variety of paths. Some involve proteins, P; some enzymes catalyse for example the citrate cell. Some iron is exchanged into new small organic molecules, porphyrins, to make new enzymes. Finally, much is stored in a bound small particle of Fe(OH)3, called ferritin.

External Fe

Figure 1 An outlined scheme of the variety of ion exchange reactions in the uptake of iron. Iron is captured as Fe3+ outside the cell by a small organic molecule, ferroxamine (X). FeX is passed through membranes (TON B) using energy, and is then exchanged in a variety of paths. Some involve proteins, P; some enzymes catalyse for example the citrate cell. Some iron is exchanged into new small organic molecules, porphyrins, to make new enzymes. Finally, much is stored in a bound small particle of Fe(OH)3, called ferritin.

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