Crosstalk between zinc and free fatty acids in plasma

Abstract

In mammalian blood plasma, serum albumin acts as a transport protein for free fatty acids, other lipids and hydrophobic molecules including neurodegenerative peptides, and essential metal ions such as zinc to allow their systemic distribution. Importantly, binding of these chemically extremely diverse entities is not independent, but linked allosterically. One particularly intriguing allosteric link exists between free fatty acid and zinc binding. Albumin thus mediates crosstalk between energy status/metabolism and organismal zinc handling. In recognition of the fact that even small changes in extracellular zinc concentration and speciation modulate the function of many cell types, the albumin-mediated impact of free fatty acid concentration on zinc distribution may be significant for both normal physiological processes including energy metabolism, insulin activity, heparin neutralisation, blood coagulation, and zinc signalling, and a range of disease states, including metabolic syndrome, cardiovascular disease, myocardial ischemia, diabetes, and thrombosis.

Keywords: Albumin; Non-esterified fatty acids; Plasma; Serum; Zinc.

Graphical abstract

Fig. 1

Serum albumin can bind free fatty acids and zinc ions. (a) High-affinity (red) and low-affinity (blue) FA binding sites 1–7 of the human serum albumin-palmitate complex (PDB: 1E7H) [57]. The roman numerals and letters denote the three domains and each subdomain of albumin. Each homologous domain (labelled I-III) of HSA is divided into two subdomains. Figure adapted from Fujiwara & Amisaki, 2013 [58]. (b) Location of the interdomain zinc (purple) binding site A (PDB: 5IJH) [33]. (c) Tetrahedral coordination of His67, His247 and Asp249 to Zn²⁺ in site A (PDB: 5IJH) [33]. The fourth ligand, a water molecule, is not shown. (d) ¹¹¹Cd-NMR has aided in identification of zinc binding site A on human serum albumin. One equivalent of zinc is sufficient to displace Cd from site A. Mutation of His67 shows this amino acid to be essential to Cd²⁺ binding, and addition of fatty acids interferes with metal binding to site A [48].

Fig. 2

Effect of FFA-binding to site FA2 on zinc site A. The image has been generated by structural alignment of domain II of HSA in its Zn-bound form (PDB: 5IJH [33]; coloured in light yellow and light green, with zinc-binding residues in grey) with domain II of myristate-bound HSA (PDB: 1BJ5 [61]; coloured in dark yellow and dark green, with zinc-binding residues in orange and green). Myristate in site FA2 (light pink) leads to a relative movement of Nε2 of His67 by 7.5 Å (red arrow).

Fig. 3

Isothermal titration calorimetry (ITC) data for the modulation of Zn²⁺ binding to albumins. (a) Relative to native HSA, mutation of H67A disrupts albumin's ability to bind Zn²⁺ by a factor of approximately 5. (b) Increasing equivalents (0–5 mol. equiv.) of myristate (C14:0 FFA) reduce the ability of HSA to bind Zn²⁺. (c) The presence of 1 mol. equiv. of bound Zn²⁺ alters the energetics but not stoichiometry of the binding reaction of myristate (Myr) to BSA. The decrease in exothermicity can be explained by the energetic cost of first breaking the Zn²⁺-albumin bonds before Myr can bind to FA2. Data in (a) and (b) were acquired at near-physiological pH and ionic strength (50 mM Tris, 140 mM NaCl, pH 7.4) [33,56]; experiments in (c) were carried out in pure water as this caused fewer problems regarding fatty acid solubility [55].

Fig. 4

Evaluation of ITC data and plasma zinc speciation modelling for a system consisting of 620 μM HSA sites A and B, the abundant zinc-binding protein HRG at 1–2 μM, 15 μM Zn²⁺, and varying amounts of fatty acids (0–3.1 mM). The Figure is based on data published in reference [56].

Fig. 5

Zinc-α2-glycoprotein can bind FFAs and Zn²⁺ ions. X-ray crystallographic structure of zinc-α2-glycoprotein (ZAG) showing hydrophobic sidechains Tyr14, Arg73, Ile76, Phe101, Trp115, Tyr117, Trp148, Tyr161 (red) involved in the postulated inter-domain FA binding site between α1 (yellow) and α2 (green) helices (PDB: 1T7V) [123,126,127]. The high affinity zinc-binding site shown (blue) involving Asp90, His95 and Asp120 has been predicted using molecular modelling, and is located nearby to the FA binding site [127].