Abstract

The role of water in protein folding, specifically its presence or not in the transition-state structure, is an unsolved question. There are two common classes of folding-transition states: diffuse transition states, in which almost all side chains have similar, rather low phi (phi) values, and polarized transition states, which instead display distinct substructures with very high phi-values. Apo-and zinc-forms of Pseudomonas aeruginosa azurin both fold in two-state equilibrium and kinetic reactions; while the apo-form exhibits a polarized transition state, the zinc form entails a diffuse, moving transition state. To examine the presence of water in these two types of folding-transition states, we probed the equilibrium and kinetic consequences of replacing core valines with isosteric threonines at six positions in azurin. In contrast to regular hydrophobic-to-alanine phi-value analysis, valine-to-threonine mutations do not disrupt the core packing but stabilize the unfolded state and can be used to assess the degree of solvation in the folding-transition state upon combination with regular phi-values. We find that the transition state for folding of apo-azurin appears completely dry, while that for zinc-azurin involves partially formed interactions that engage water molecules. This distinct difference between the apo-and holo-folding nuclei can be rationalized in terms of the shape of the free-energy barrier. Study holds ProTherm entries: 21981, 21982, 21983, 21984, 21985, 21986, 21987, 21988, 21989, 21990, 21991, 21992, 21993, 21994, 21995, 21996, 21997, 21998, 21999, 22000, 22001, 22002, 22003, 22004, 22005, 22006, 22007, 22008, 22009, 22010, 22011, 22012, 22013, 22014, 22015, 22016, 22017, 22018, 22019, 22020, 22021, 22022, 22023, 22024, 22025, 22026 Extra Details: protein structure/folding; conformational changes; stability and mutagenesis; circular dichroism; fluorescence; molecular mechanics/dynamics; kinetics

Submission Details

ID: io4yJ3V44

Submitter: Connie Wang

Submission Date: April 24, 2018, 8:53 p.m.

Version: 1

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