Protein aggregation

Protein aggregation is a biological phenomenon in which mis-folded proteins aggregate (i.e., accumulate and clump together) either intra- or extracellularly. These protein aggregates are often correlated with diseases. In fact, protein aggregates have been implicated in a wide variety of disease known as amyloidoses, including ALS, Alzheimer's, Parkinson's and prion disease.

After synthesis, proteins typically fold into a particular three-dimensional conformation that is the most thermodynamically favorable: their native state. This folding process is driven by the hydrophobic effect: a tendency for hydrophobic (water-fearing) portions of the protein to shield itself from the hydrophilic (water-loving) environment of the cell by burying into the interior of the protein. Thus, the exterior of a protein is typically hydrophilic, whereas the interior is typically hydrophobic.

Protein structures are stabilized by non-covalent interactions and disulfide bonds between two cysteine residues. The non-covalent interactions include ionic interactions and weak van der waals interactions. Ionic interactions form between an anion and a cation and form salt bridges that help stabilize the protein. Van der waals interactions include nonpolar interactions (i.e. London dispersion forces) and polar interactions (i.e. hydrogen bonds, dipole-dipole bond). These play an important role in a protein's secondary structure, such as forming an alpha helix or a beta sheet, and tertiary structure. Interactions between amino acid residues in a specific protein are very important in that protein's final structure.

When there are changes in the non-covalent interactions, as may happen with a change in the amino acid sequence, the protein is susceptible to misfolding or unfolding. In these cases, if the cell does not assist the protein in re-folding, or degrade the unfolded protein, the unfolded/misfolded protein may aggregate, in which the exposed hydrophobic portions of the protein may interact with the exposed hydrophobic patches of other proteins. There are three main types of protein aggregates that may form: amorphous aggregates, oligomers, and amyloid fibrils.

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