Divalent metal binding by histidine‐rich glycoprotein differentially regulates higher order oligomerisation and proteolytic processing

Abstract

The serum protein histidine-rich glycoprotein (HRG) has been implicated in tissue injury and tumour growth. Several HRG functions are regulated by the divalent metal Zn 2+ , including ligand binding and proteolytic processing that releases active HRG fragments. Although HRG can bind divalent metals other than Zn 2+ , the impact of these divalent metals on the biophysical properties of HRG remains poorly understood. We now show that HRG binds Zn 2+ , Ni 2+ , Cu 2+ and Co 2+ with micromolar affinities, but differing stoichiometries, and regulate the release of specific HRG fragments during proteolysis. Furthermore, HRG binding to Zn 2+ promotes HRG dimer formation in a Zn 2+ -concentration-and pH-dependent manner. Our data highlight the complex divalent metal-dependent regulatory mechanisms that govern HRG function.

Functional Regulation of the Plasma Protein Histidine-Rich Glycoprotein by Zn2+ in Settings of Tissue Injury

“…The number of sites for Zn 2+ was consistent with Kassaar et al [47], although data by Priebatsch et al [48] demonstrated greater affinity for Zn 2+ (K d : 2.85). The discrepancy between the observed affinities was proposed by Priebatsch et al [48] to be potentially due to the non-favorable reaction conditions used in Kassaar et al [47].…”

“…The number of sites for Zn 2+ was consistent with Kassaar et al [47], although data by Priebatsch et al [48] demonstrated greater affinity for Zn 2+ (K d : 2.85). The discrepancy between the observed affinities was proposed by Priebatsch et al [48] to be potentially due to the non-favorable reaction conditions used in Kassaar et al [47]. The comparison between the orders of affinity for rbHRG (Cu 2+ : K d : 0.2 μM > Zn 2+ K d : 1 μM > Ni 2+ K d : 1.3 μM > Co 2+ : K d : 2.1 μM) by Morgan et al [36] and for HRG (Zn 2+ : Kd: 2.85 μM > Ni 2+ : K d : 3.35 μM > Cu 2+ : K d : 3.72 μM > Co 2+ : K d : 9.39 μM) by Priebatsch et al [48], demonstrates considerable differences.…”

“…The discrepancy between the observed affinities was proposed by Priebatsch et al [48] to be potentially due to the non-favorable reaction conditions used in Kassaar et al [47]. The comparison between the orders of affinity for rbHRG (Cu 2+ : K d : 0.2 μM > Zn 2+ K d : 1 μM > Ni 2+ K d : 1.3 μM > Co 2+ : K d : 2.1 μM) by Morgan et al [36] and for HRG (Zn 2+ : Kd: 2.85 μM > Ni 2+ : K d : 3.35 μM > Cu 2+ : K d : 3.72 μM > Co 2+ : K d : 9.39 μM) by Priebatsch et al [48], demonstrates considerable differences. A rationale as to why HRG does not conform to the universal order of affinity of the Irving-William and exhibits the greatest affinity for Zn 2+ , compared to rbHRG exhibiting the greatest affinity for Cu 2+ , may be attributable to the nature of the histidine-rich tandem repeats.…”

“…Recently, the affinities for Zn 2+ (K d : 2.85 μM) > Ni 2+ (K d : 3.35 μM) > Cu 2+ (K d : 3.72 μM) > Co 2+ (K d : 9.39 μM) and stoichiometries for Zn 2+ (9–10 sites), Ni 2+ (13 sites), Cu 2+ (6 sites) Co 2+ (13 sites) of HRG were defined by isothermal titration calorimetry (ITC) [48]. The number of sites for Zn 2+ was consistent with Kassaar et al [47], although data by Priebatsch et al [48] demonstrated greater affinity for Zn 2+ (K d : 2.85).…”

“…Furthermore, the only available ligand donor in the GHHPH-like motif capable of coordinating with Zn 2+ is the nitrogen from histidines, which subsequently also exhibits preference to Zn 2+ ions over other divalent metals and therefore suggests Zn 2+ ions may favor the richest histidine sequences [29,46]. Lastly, the difference in divalent metal affinities and stoichiometries between HRG and rbHRG are also likely to also extend to the differences in the histidine-rich tandem repeats, coordination geometries (Refer to Section 2) or possibly the type of metal-binding experiments undertaken, as Morgan [36] used absorbance titrations and Priebatsch et al [48] used ITC.…”