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The Cost of Long Catalytic Loops in Folding and Stability of the ALS-Associated Protein SOD1
journal contribution
posted on 2018-10-25, 00:00 authored by Fan Yang, Huabing Wang, Derek T. Logan, Xin Mu, Jens Danielsson, Mikael OlivebergA conspicuous feature
of the amyotrophic lateral sclerosis (ALS)-associated
protein SOD1 is that its maturation into a functional enzyme relies
on local folding of two disordered loops into a catalytic subdomain.
To drive the disorder-to-order transition, the protein employs a single
Zn2+ ion. The question is then if the entropic penalty
of maintaining such disordered loops in the immature apoSOD1 monomer
is large enough to explain its unusually low stability, slow folding,
and pathological aggregation in ALS. To find out, we determined the
effects of systematically altering the SOD1-loop lengths by protein
redesign. The results show that the loops destabilize the apoSOD1
monomer by ∼3 kcal/mol, rendering the protein marginally stable
and accounting for its aggregation behavior. Yet the effect on the
global folding kinetics remains much smaller with a transition-state
destabilization of <1 kcal/mol. Notably, this 1/3 transition-state
to folded-state stability ratio provides a clear-cut example of the
enigmatic disagreement between the Leffler α value from loop-length
alterations (typically 1/3) and the “standard” reaction
coordinates based on solvent perturbations (typically >2/3). Reconciling
the issue, we demonstrate that the disagreement disappears when accounting
for the progressive loop shortening that occurs along the folding
pathway. The approach assumes a consistent Flory loop entropy scaling
factor of c = 1.48 for both equilibrium and kinetic
data and has the added benefit of verifying the tertiary interactions
of the folding nucleus as determined by phi-value analysis. Thus,
SOD1 not only represents a case where evolution of key catalytic function
has come with the drawback of a destabilized apo state but also stands
out as a well-suited model system for exploring the physicochemical
details of protein self-organization.
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aggregation behaviorresults showprotein self-organizationdestabilized apo statephi-value analysisFlory loop entropyapoSOD 1 monomertransition-state destabilizationdisorder-to-order transitionSOD 1SOD 1-loop lengthsentropic penaltyALS-Associated Protein SOD 1accountingLong Catalytic Loopsphysicochemical detailsmodel systemprotein redesign
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