Investigating Cu(I) binding to model peptides of N-terminal Aß isoforms.

Amyloid beta (Aß) peptides and copper (Cu) ions are each involved in critical biological processes including antimicrobial activity, regulation of synaptic function, angiogenesis, and others. Aß binds to Cu and may play a role in Cu trafficking. Aß peptides exist in isoforms that vary at their C-and N-termini; variation at the N-terminal sequence affects Cu binding affinity, structure, and redox activity by providing different sets of coordinating groups to the metal ion. Several N-terminal isoforms have been detected in human brain tissues including Aß1-40/42, Aß3-42, pEAß3-42, Aß4-42, Aß11-40 and pEAß11-40 (where pE denotes an N-terminal pyroglutamic acid). Several previous works have individually investigated the affinity and structure of Cu(I) bound to some of these isoforms' metal binding domains. However, the disparately reported values are apparent constants collected under different sets of conditions, and thus an integrated comparison cannot be made. The work presented here provides the Cu(I) coordination structure and binding affinities of these six biologically relevant Aß isoforms determined in parallel using model peptides of the Aß metal binding domains (Aß1-16, Aß3-16, pEAß3-16, Aß4-16, Aß11-16 and pEAß11-16). The binding affinities of Cu(I)-Aß complexes were measured using solution competition with ferrozine (Fz) and bicinchoninic acid (BCA), two colorimetric Cu(I) indicators in common use. The Cu(I) coordination structures were characterized by X-ray absorption spectroscopy. The data presented here facilitate comparison of the isoforms' Cu-binding interactions and contribute to our understanding of the role of Aß peptides as copper chelators in healthy and diseased brains.

Exploration of the Potential Role of Serum Albumin in the Delivery of Cu(I) to Ctr1.

Human serum albumin (HSA) is the major copper (Cu) carrier in blood. The majority of previous studies that have investigated Cu interactions with HSA have focused primarily on the Cu(II) oxidation state. Yet, cellular Cu uptake by the human copper transport protein (Ctr1), a plasma membrane-embedded protein responsible for Cu uptake into cells, requires Cu(I). Recent in vitro work has determined that reducing agents, such as the ascorbate present in blood, are sufficient to reduce the Cu(II)HSA complex to form Cu(I)HSA and that Cu(I) is bound to HSA with pM affinity. The biological accessibility of Cu(I)HSA suggests that HSA-bound Cu(I) may be an unappreciated form of Cu cargo and a key player in extracellular Cu trafficking. To better understand Cu trafficking by HSA, we sought to investigate the exchange of Cu(I) from HSA to a model peptide of the Cu-binding ectodomain of Ctr1. In this study, we used X-ray absorption near-edge spectroscopy to show that Cu(I) becomes more highly coordinated as increasing amounts of the Ctr1-14 model peptide are added to a solution of Cu(I)HSA. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to further characterize the interaction of Cu(I)HSA with Ctr1-14 by determining the ligands coordinating Cu(I) and their bond lengths. The EXAFS data support that some Cu(I) likely undergoes complete transfer from HSA to Ctr1-14. This finding of HSA interacting with and releasing Cu(I) to an ectodomain model peptide of Ctr1 suggests a mechanism by which HSA delivers Cu(I) to cells under physiological conditions.