The World of Functional Amyloid - The Functional Fold: Amyloid Structures in Nature - page 4

5-7

Recent efforts have yielded both NMR- and X-ray diffraction-

8

based high-resolution structures of peptide-derived amyloid

fibres, revealing paired

β β

-strands form in-register
intermolecular interactions with identical

-strands. These

-strands via hydrogen
bonding and side-chain interactions (Fig. 1.1c). While parallel,
in-register cross-

β

β

assemblies seem to predominate based on the
limited structural information available, both parallel and anti-
parallel

β

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Amyloid formation can be assessed using a variety of techniques
in vitro

-sheet orientations have been observed.

. The fibrillar morphology of amyloid can be demonstrated
using atomic force microscopy (AFM) or electron microscopy
(EM). Far ultraviolet circular dichroism (CD) spectroscopy and
Fourier-transform infrared spectroscopy (FTIR) have been
used to demonstrate the -sheet-rich quaternary structure of

amyloid. Amyloid has distinctive reflections at 4.7 and 10 Å in its
X-ray diffraction pattern because of its repetitive cross-

β

β

-sheet
structure (Fig. 1.1b). The amyloidophilic, environment-dependent
fluorophores Congo red and thioflavin S/T

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can be used to reveal
amyloid deposits in tissue using optical microscopy as well as to
follow amyloid formation in vitro using fluorescence spectroscopy.

1. Amyloid Formation Mechanisms

Extensive biophysical studies of the mechanism of amyloid formation
(amyloidogenesis)

have revealed that amyloid is formed by the
self-assembly of protein monomers that can occur by many different
pathways. A nucleated polymerization mechanism has been invoked
to explain the unusual lag in amyloid appearance observed for some
amyloidogenesis reactions. In a nucleated polymerization, the lag
phase corresponds to the slow, energetically unfavourable formation
of the highest energy oligomeric species on the aggregation pathway,
the nucleus (Fig. 1.2a). Fibre extension after nucleus formation is
generally rapid and thermodynamically favourable. Another feature
of a nucleated polymerization is that the monomer concentration
must exceed a critical concentration below which amyloidogenesis
will not occur. The nucleated polymerization mechanism is not the
only mechanism of amyloidogenesis, as amyloid formation reactions
that lack a lag phase have also been observed. These reactions
are thought to proceed via a downhill polymerization in which all

in vitro