Abstract
Under physiological conditions, a protein undergoes a spontaneous disorder º order transition called “folding.” The protein polymer
is highly flexible when unfolded but adopts its unique native, three-dimensional structure when folded. Current experimental knowledge
comes primarily from thermodynamic measurements in solution or the structures of individual molecules, elucidated by either
x-ray crystallography or NMR spectroscopy. From the former, we know the enthalpy, entropy, and free energy differences between
the folded and unfolded forms of hundreds of proteins under a variety of solvent cosolvent conditions. From the latter, we know
the structures of 35,000 proteins, which are built on scaffolds of hydrogen-bonded structural elements, -helix and -sheet. Anfinsen
showed that the amino acid sequence alone is sufficient to determine a protein’s structure, but the molecular mechanism
responsible for self-assembly remains an open question, probably the most fundamental open question in biochemistry. This perspective
is a hybrid: partly review, partly proposal. First, we summarize key ideas regarding protein folding developed over the past
half-century and culminating in the current mindset. In this view, the energetics of side-chain interactions dominate the folding
process, driving the chain to self-organize under folding conditions. Next, having taken stock, we propose an alternative model that
inverts the prevailing side-chainbackbone paradigm. Here, the energetics of backbone hydrogen bonds dominate the folding process,
with preorganization in the unfolded state. Then, under folding conditions, the resultant fold is selected from a limited repertoire
of structural possibilities, each corresponding to a distinct hydrogen-bonded arrangement of alpha-helices andor strands of beta-sheet.
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