Question

What is an amyloid?

Are all amyloid structurally the same? Provide examples to support the case.

Please be detailed in response

Answer 1

What is an amyloid?

. Amyloid is an abnormal protein that is produced in your bone marrow and can be deposited in any tissue or organ. Amyloid is a general term used to describe insoluble aggregates of various types of protein, all of which have a beta-pleated sheet structure that enables them to stick together and form fibrils. Secondary amyloid consists of the protein AA (amyloid A) an 8.5 kDa protein that is heterogeneous (at the amino-terminal end). It is putatively derived by proteolysis from the normal circulating protein apoSAA (serum amyloid A related protein) which has a molecular weight of 12 kDa. apoSAA is an apolipoprotein associated with high-density lipoprotein (HDL) especially HDL₃ and acts as a very sensitive acute-phase protein.

Are all amyloid structurally the same?

The structural information indicates that different proteins polymerize into filamentous structures that are rather broadly considered amyloid but based on structurally different arrangements. In some cases, there is the stacking of peptide strands in parallel and antiparallel sheets. In these cases, each contributing polypeptide is part of the greater fold of the fibril but does not necessarily compose a stably folded subunit in itself. In other cases, each subunit stacks to form the fibril, but the subunits are themselves in conformations that are recognizable as distinct protein folds. So, all the amyloid are not the same

Examples of structurally characterized amyloid-forming proteins

aa, amino acids.

Protein/peptide	Pathology/function linked to amyloid	Structural information
α-Synuclein (aa 1–140)	Parkinson disease	Five β-strands within fibril core comprising residues 35–96 (92); parallel in-register β-sheet core region from residues 36–98 (19)
Aβ1–40	Alzheimer disease	Parallel in-register β-sheet (16); two fibril morphologies (twisted and striated ribbons) with 2- and 3-fold symmetry; both are parallel β-sheets, using almost same β-strand segments (8, 24)
Aβ1–40 seeded with a diseased brain	Alzheimer disease	Parallel in-register β-sheet (75)