Amino acid transporters evolution

With few exceptions, metallothioneins consist of relatively simple amino acids, aromatic amino acids and histidine only being found in a small number of species [329]. This amino acid composition suggests that metallothioneins evolved early in the evolution of life, probably even before the oxygenation of the atmosphere. A further clue is one of their functions. As metal-transport and storage proteins, thioneins are capable of binding metal ions but release them relatively easily as well. Metallothioneins can therefore be considered a transition from non-metal to metalloproteins. It is improbable, however, that the known copper proteins evolved from copper metallothioneins as there are no homologies between them and other copper proteins or enzymes. 

Hsu et ah [6] developed an ass for the high-affinity Mn-binding site in PS II. In this assay /xM concentrations of Mn competitively inhibit 1,5-diphenylcarbazide (DPC) supported 2,6-dichlorophenol indophenol (DCIP) electron transport. Using this assay on PS II membranes modified with proteases and amino acid chemical modifiers, we have identified four components in the high-affinity Mn-binding site, corresponding to the four Mn required for functional Oj evolution. 

In closely related species, the primary structures of common proteins are similar. Counting the number of differences in amino acid sequences among these proteins gives some idea of how far various species have diverged in the course of evolution. For example, the protein cytochrome c is an excellent protein for evolutionary comparisons because it is found in the respiratory electron transport system, which is present in all aerobic organisms (Figure 27.8). 

Crystallographic studies have revealed stractural similarities between the kinesin and myosin protein families, and there are short amino acid stretches displaying sequence conservation (75). These findings have led to the proposal that kinesins and myosins have a common ancestor. This precursor protein apparently also led to the G protein superfamily, which has a phosphate sensor domain that is very similar to the ATP binding sites of kinesins and myosins. As proposed for evolution of the bacterial flagellar motor, transport and locomotion functions appear to be evolutionarily connected. 

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