Xiangyang Liang, Dominic Campopiano and Peter Sadler at the University of Edinburgh, UK, examine how and why metals cross membranes.

Cell membranes are natural barriers, surrounding the cytoplasm and cell compartments and separating them from the external environment. They comprise a lipid bilayer with transmembrane proteins and proteins attached to the membrane surface. These membrane proteins perform various functions, serving as channels, pumps, transporters, enzymes and receptors (sensory proteins). Metal ions are essential in many of these biological processes. 

Membrane proteins can be involved in transporting metal ions (and sometimes their associated ligands) through membranes. Ion channels are transmembrane proteins that form pores that allow ions to cross into or out of a cell. Ions crossing ion channels always flow in the same direction as diffusion: from a more to a less concentrated solution, or from positive to negative potential. The channel pores are gated. For ligand-gated channels, the gates open or close in response to a ligand such as Ca(II), a guanine nucleotide binding protein (G-protein), or glutamate; for voltage-gated channels, the gates respond to a change in membrane potential. These channels are highly selective and recognise only certain ions and allow them to pass through.

Transmembrane proteins allow metal ions and complexes to cross cell membranes

A metal ion's passage across a membrane can be passive, without energy requirement, or active, with energy supplied from adenosine triphosphate (ATP) hydrolysis. Ion pumps and transporters transfer metal ions against the direction of diffusion using this energy of hydrolysis. These include P-type ATPase pumps, enzymes that catalyse the conversion of ATP to adenosine diphosphate (ADP). The released phosphate is transferred to an aspartate residue to form a phosphorylated (P) intermediate, hence the term P-type. Ion transporters are a very large and diverse family of membrane proteins, including ATP-binding cassette transporters, the Zip family of zinc transporters, the cation diffusion facilitator family, the copper transporters Ctr and COPT and iron-regulated transporters, which also actively transport various metal ions, as is apparent from their names.

Receptors are also transmembrane proteins. Binding of signalling molecules to a receptor on one side of the membrane initiates a response on the other side. These proteins play important roles in cellular communication and signal transduction, the transfer of signals from outside to inside the cell. G-protein-coupled-receptors (GPCRs) with 7 transmembrane helices are bound to G-proteins on the inner side of the cell membrane. Over 800 genes are known to encode such proteins and members of this superfamily include receptors for many hormones, neurotransmitters, chemokines (small proteins involved in cell migration) and calcium ions, as well as sensory receptors for various odorants, bitter and sweet taste, and even photons. GPCRs are thought to be the protein targets in around 40% of all therapeutic interventions.

There is a variety of other enzymes in membranes, including receptor-like kinases and respiratory enzymes. Kinases are a class of enzymes that phosphorylate substrates by transferring phosphate onto them. Metal ions such as Mg²⁺ or Mn²⁺ are essential for the phosphorylation process. Receptor-like kinases play dual roles, acting as both receptors and kinases. Respiratory enzymes are mainly metalloproteins, containing metals such as iron and copper. In the inner membrane of cell mitochondria, electrons are passed along a series of respiratory enzyme complexes. These electrons are generated from NADH (reduced nicotinamide adenine dinucleotide), produced by oxidation of nutrients such as glucose, and are ultimately transferred to molecular oxygen. The passage of electrons between the complexes releases energy that is stored in the form of a proton gradient across the membrane and is then used by ATP synthase to make ATP from ADP and phosphate.

With their many roles in metal transport, cell signalling and energy release, membrane proteins can be drug targets. Membranes can also play important roles in the action of therapeutic and diagnostic metal complexes. For example, the thermodynamics and kinetics of metal reactions such as ligand substitution and redox reactions, are highly dependent on the medium. Reactions of metal complexes within membranes can differ significantly from those in media with higher dielectric constants, such as more aqueous extracellular fluids. Targeting metals to membrane sites and controlling their reactivity in these sites therefore presents considerable challenges.

Metal ion affinity for ligands in a lipid environment can differ from that in aqueous media (extra- and intracellular environments). The thermodynamically-preferred binding sites for metal ions in membranes cannot easily be predicted given current knowledge of metal-ligand stabilities, since most have been determined for aqueous solutions only. Seemingly poor ligands could bind tightly to metal ions in protein cavities with low dielectric constants. Understanding the interactions of metals with transmembrane proteins will aid the design of more effective metallodrugs.

For example, the long-established use of organomercurials as diuretics has now been correlated with the compounds' ability to bind to aquaporins (water channels) and inhibit water transport through them. The new anti-leukaemia drug arsenic trioxide, which is largely the neutral molecule As(OH)₃ at physiological pH, enters cells through aquaglyceroporin, which facilitates transport of glycerol. High affinity copper transport proteins containing methionine-rich sequences have recently been associated with the uptake of the platinum drug cisplatin; Pt(II) is known to have a high affinity for the sulfur atom of methionine.

GPCRs have a negatively-charged electrostatic surface directed towards the cell membrane exterior and are strong potential targets for metal ions. The affinity of the antiviral drug AMD3100 for its target, the chemokine receptor CXCR4, is enhanced by binding to Cu(II), Ni(II) or Zn(II). In models, metal ions bound in the macrocyclic rings of AMD3100 can coordinate to specific aspartic and glutamic acid carboxylate groups in the extracellular loops of CXCR4 and amine groups in the macrocycles can form hydrogen bonds to CXCR4 side-chains. Also, hydrophobic interactions between the indole rings of tryptophan residues in CXCR4 and the carbon backbone of the bicyclam are possible. It should be possible to design new generations of metal complexes that will bind specifically to different GPCRs based on such interactions.

These examples provide a stimulus for further exploration of the chemistry of metal ions in membranes and offer promise for the discovery of drugs with novel modes of action.

Read Xiangyang Liang, Dominic Campopiano and Peter Sadler's critical review: 'Metals in membranes' in issue 6 of Chemical Society Reviews.