Final Answer:
The formation of amyloid fibers is primarily driven by the β-sheet stabilization of abnormally folded proteins, as indicated by option (d).
Step-by-step explanation:
Amyloid fibers are insoluble protein aggregates associated with several neurodegenerative diseases, including Alzheimer's and Parkinson's. The driving force behind the formation of these fibers is the aberrant folding of proteins into β-sheet structures. The β-sheet conformation allows proteins to align and aggregate, forming stable and resistant structures known as amyloid fibers.
When proteins misfold, adopting β-sheet structures, they become prone to aggregation. These aggregated proteins can then further extend into long β strands, promoting the assembly of insoluble amyloid fibers. This process is often associated with the pathogenesis of diseases where abnormal protein aggregation plays a central role.
Option (a), denaturation of proteins containing β sheets, is not the main driver of amyloid fiber formation. Denaturation involves the unfolding of proteins, and while it may expose β-sheet structures, it is not the primary mechanism leading to amyloid aggregation.
Options (c) and (b), involving the formation of biofilms by infectious bacteria and the extension of β sheets into longer β strands, respectively, are unrelated to the process of amyloid fiber formation in the context of protein misfolding.
In conclusion, the stabilization of β-sheet structures in abnormally folded proteins is the key factor in driving the formation of amyloid fibers implicated in various neurodegenerative disorders.