Recent Advances in Metalloproteomics

Abstract

Interactions between proteins and metal ions and their complexes are important in many areas of the life sciences, including physiology, medicine, and toxicology. Despite the involvement of essential elements in all major processes necessary for sustaining life, metalloproteomes remain ill-defined. This is not only owing to the complexity of metalloproteomes, but also to the non-covalent character of the complexes that most essential metals form, which complicates analysis. Similar issues may also be encountered for some toxic metals. The review discusses recently developed approaches and current challenges for the study of interactions involving entire (sub-)proteomes with such labile metal ions. In the second part, transition metals from the fourth and fifth periods are examined, most of which are xenobiotic and also tend to form more stable and/or inert complexes. A large research area in this respect concerns metallodrug-protein interactions. Particular attention is paid to separation approaches, as these need to be adapted to the reactivity of the metal under consideration.

Keywords: essential metals; ligand exchange kinetics; metallodrugs; metalloproteome; xenobiotic metals.

Figure 1

Metal ions may be classified into labile and inert according to the water exchange rates of their aqua complexes [34]. Water exchange rate constants at 298 K (k(H₂O)) vary over more than 18 orders of magnitude. Labile metal ions (k(H₂O) = 10¹⁰–10⁴ s⁻¹) are shown in blue, inert metal ions (k(H₂O) = 10⁻³–10⁻¹⁰ s⁻¹) in red, and intermediate metal ions in purple. Hatched boxes indicate faster exchange rates for M²⁺ and slower exchange rates for M³⁺ ions. Grey boxes are shown for non-metals, metalloids, and metals for which oxidation states between I and III are rare or for which the respective data were not available.

Figure 2

Typical workflows for metalloproteomics and associated techniques. Metal–protein associations need to be preserved during sample extraction and preparation as well as during each separation step.

Figure 3

Zinc and cadmium distribution in S. pneumoniae proteomes. (a) Zn(II)-bound proteins in untreated bacteria. (b) Zn(II)-bound proteins in Cd(II)-treated bacteria. (c) Cd(II)-bound proteins in Cd²⁺-bound bacteria. Reproduced from reference [88] with permission.

Figure 4

Results obtained from a 1D metalloproteomic study evidencing Zn(II) redistribution as a consequence of fatty-acid loading of albumin [102]. The black lines refer to absorbance at 280 nm; the yellow bars and points refer to Zn levels in the absence of myristate; the blue bars and points refer to Zn levels in the presence of 3 mM myristate, expected to be fully bound to albumin (ca. 600 μM). In the absence of other proteins, Zn(II) transfers from BSA to the column material (left panel). When 3 mM myristate is added to serum (middle panel) or plasma (right panel), Zn(II) displaced from albumin transfers to other proteins with higher molecular weight (lower elution volume).

Figure 5

Observation of metallated peptides in digests of BSA (a) before and after reaction with metallodrugs (b) trans,trans,trans-[Pt(N₃)₂(OH)₂(py)₂], (c) [(η⁵-Cp*)Ir(2-(R’-phenyl)-R-pyridine)Cl], and (d) [(η⁶-bip)Os(en)Cl]⁺ (where bip = biphenyl, and en = ethylenediamine). Reproduced from reference [162] with permission.