Active Site Environment of Heme-Bound Amyloid β Peptide Associated with Alzheimer’s Disease

Abstract

Recent reports show that there is a large increase in heme in the temporal brain of Alzheimer's disease (AD) patients, as heme, biosynthesized in brain cells, binds to amyloid β (Aβ), forming heme-Aβ complexes. This leads to the development of symptoms that are characteristic pathological features of AD, e.g., abnormal iron homeostasis, decay of iron regulatory proteins, dysfunction in mitochondrial complex IV, oxidative stress, etc. However, the active site resulting from heme binding to Aβ is not well characterized. For example, the coordinating residue, relevant second-sphere residues, and spin state of the Fe center are not known. In this study we have used wild-type and mutated Aβ peptides and investigated their interaction with naturally occurring heme. Our results show that, out of several possible binding sites, His(13) and His(14) residues can both bind heme under physiological conditions, resulting in an axial high-spin active site with a trans axial water-derived ligand. Peroxidase assays of these heme-peptide complexes along with pH perturbations indicate that Arg(5) is a key second-sphere residue that H-bonds to the trans axial ligand and is responsible for the peroxidase activity of the heme-Aβ complexes. The His(13) and Arg(5) residues identified in this study are both absent in rodents, which do not show AD, implicating the significance of these residues as well as heme in the pathology of AD.

Key roles of Arg5, Tyr10 and His residues in Aβ–heme peroxidase: Relevance to Alzheimer’s disease

“…1B). The result further indicated that the hydrophilic N-terminal of Ab was involved in heme binding [16][17][18], and played an important role in promoting the peroxidase activity of heme. This peroxidase activity of Ab-heme complexes (with k obs of 4.2 Â 10 À3 min À1 ) was $3 times faster compared to that of free heme (with k obs of 1.5 Â 10 À3 min À1 ), similar to a previous study [16].…”

“…His residues were essential in peroxidase activity of Ab-heme complexes It is well-known that heme binds to Ab peptides in the hydrophilic portion (residues 1-16) and His residues are found to be the possible binding ligands [16][17][18]. Thus, Ab 1À40 , Ab 1À16 and Ab 17À40 fragments were used to investigate the possible relationship between histidine-bound complex and peroxidase-like active site.…”

“…Recent studies have shown that His 13 and His 14 residues in the N-terminal hydrophilic region of Ab are the heme-binding ligands, and His 13 binds to the iron center of heme preferentially when both residues are present [16][17][18]. Due to the similar active site environments of the heme-bound complexes in Ab 1À40 and Ab 1À16 , Ab 1À16 and site-directed mutants of Ab 1À16 including single mutants (His 13 Ala, His 14 Ala), and double mutant (His 13 Gly, His 14 Gly) were thus used to observe the effect of His residues on peroxidase activity of Ab-heme complexes.…”

“…Furthermore, these amino acids Arg, Tyr, and His are found to participate in heme-binding in heme-proteins and peroxidases [12][13][14][15], which drive us to propose that Ab-heme peroxidase is a key molecular link between these residues present in human Ab and the increased human susceptibility to AD. Recent results showed that Arg 5 residue was required for Ab-heme peroxidase activity and His 13 and His 14 residues were the heme-binding ligands [16,17].…”

Curcumin inhibits appoptosin-induced apoptosis via upregulating heme oxygenase-1 expression in SH-SY5Y cells

“…Recent evidence shows that Aβ can induce intracellular heme synthesis and iron uptake in vitro [9,32] . Heme can bind to Aβ to form heme-Aβ, promoting [24,26,33,34] .…”