Membrane transport proteins substrate design

In order for a metal ion to reach its intracellular protein target, a number of complex barriers must be crossed. First, the metal existing in the extracellular environment must traverse the plasma membrane of the cell. The lipid bilayers of cellular membranes are generally impermeable to metals and cellular uptake of the ion requires the action of metal transport proteins. A host of membrane transporters reside at the cell surface, some of which are specific for certain ions (e.g. only copper or only zinc), while others are more promiscuous in their choice of metal ion substrate (e.g. can transport both copper and zinc). But all are designed to ensure that cells acquire proper levels of the essential heavy metal ions such as copper, zinc, iron, and manganese. 

Figure 3. Hierarchical levels of metabolic control. Sites of metabolic control are designated as (1) plasma membrane level active transport systems, hormone receptors (2) cytoplasmic level hormone binding protein complex, signal molecule generation (3) enzymatic level steady-state enzymatic pathway, servomechanisms, enzyme degradation (4) ribosomal level protein biosynthesis (5) nuclear level hormonal control of gene action, operon control of gene action (substrate induction, product repression). The symbol,, indicates inhibition of a reaction.

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