Effect of zinc, copper, and calcium on the structure and stability of serum amyloid A.
Serum amyloid A (SAA) is a highly conserved acute phase reactant protein, and its concentration in serum can increase up to approximately 1000 times after an inflammatory stimuli. SAA is mainly associated with high-density lipoproteins in serum, and its main function appears to involve cholesterol transport and lipid metabolism. However, SAA has also been associated with many other functions and a number of diseases, although these potential links remain poorly understood. The three-dimensional structure of SAA is not known, but we have shown that murine SAA2.2 can exist in solution as a marginally stable hexamer, which at 37 degrees C dissociates to a monomeric species that misfolds irreversibly and self-assembles into amyloid fibrils. Thus, the structure and function of SAA in vivo appear to be modulated when it binds to other proteins or small ligands. Herein, the effect of copper (Cu2+), zinc (Zn2+), and calcium (Ca2+) on the structure and stability of SAA2.2 in aqueous solution was examined using various probes of quaternary, tertiary, and secondary structure. At different concentrations of metals, including those found in the serum, the results show that the structure and stability of SAA2.2 are differently affected depending on the metal type and concentration. Copper (10-100 microM) was found to shift the equilibrium from hexamer to monomer without affecting significantly the stability of the tertiary and secondary structure of SAA2.2. In contrast, zinc (1-10 microM) bound to SAA2.2 and stabilized its quaternary, tertiary, and secondary structure. Calcium (1-10 mM) destabilized all elements of SAA2.2 structure and induced its aggregation at 10 mM. Complete aggregation of SAA2.2 was also observed when it was incubated with 1 mM Cu2+ or Zn2+, further demonstrating the tenuous structure and stability of SAA2.2. Thus, these results suggest that the many functional and pathological roles attributed to SAA may rely on its precarious structure, modulated by its interaction with ligands under homeostasis conditions and during the acute phase response.
Wang
Limin
L
Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180, USA.
Colón
Wilfredo
W
D000210:Acute-Phase Reaction / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism; D003300:Copper / Q000737:chemistry* / Q000378:metabolism; D006706:Homeostasis; D008024:Ligands; D051379:Mice; D011485:Protein Binding; D011487:Protein Conformation; D011489:Protein Denaturation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D000685:Serum Amyloid A Protein / Q000737:chemistry* / Q000378:metabolism* / Q000502:physiology; D013329:Structure-Activity Relationship; D015032:Zinc / Q000737:chemistry* / Q000378:metabolism
D008024:Ligands; C489995:Saa2 protein, mouse; D000685:Serum Amyloid A Protein; D003300:Copper; D015032:Zinc; D002118:Calcium