Transporters of trace elements can be classified into four distinct groups [2,13,167,180,182,307-323]:
• Electrochemical potential-driven transporters utilize a carrier-mediated process to catalyze: transport of a single species by mediated diffusion or in a membrane potential-dependent manner (uniport); two or more species in opposite directions in a tightly coupled process without utilizing chemical free energy (antiport); or two or more species in the same direction in a coupled process (symport), again without using any form of energy other than the electrochemical potential gradient.
• Primary active (P-P bond hydrolysis driven) transporters utilize the free energy of the P-P bond hydrolysis to drive the movement of ions against their chemical or electrochemical potential gradient. The transport protein may be transiently phosphorylated during the transport cycle but the substrate is not phosphorylated. These transporters occur universally in all domains of life.
• Transporters whose mode of transport or energy coupling is unknown are awaiting their final placement after their transport mode and energy coupling have been resolved. These families include at least one member for whom a transport function has been described, but the mode of transport or the energy coupling is not known.
• Auxiliary transport proteins in some way facilitate transport across one or more biological membranes, but they do not participate directly in the transmembrane translocation of a substrate. They may provide a function connected with energy coupling to transport; play a structural role in complex formation; serve a biogenic or stability function; or function in regulation.
Transporter families of the four groups involved in trace metal metabolism are summarized in Table 11.2.
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