Structural and kinetic characterization of early folding events in β-lactoglobulin

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Abstract
We have defined the structural and dynamic properties of an early folding intermediate of β-lactoglobulin known to contain non-native α-helical structure. The folding of β-lactoglobulin was monitored over the 100 μs–10 s time range using ultrarapid mixing techniques in conjunction with fluorescence detection and hydrogen exchange labeling probed by heteronuclear NMR. An initial increase in Trp fluorescence with a time constant of 140 μs is attributed to formation of a partially helical compact state. Within 2 ms of refolding, well protected amide protons indicative of stable hydrogen bonded structure were found only in a domain comprising β-strands F, G and H, and the main α-helix, which was thus identified as the folding core of β-lactoglobulin. At the same time, weak protection (up to ∼ 10-fold) of amide protons in a segment spanning residues 12–21 is consistent with formation of marginally stable non-native α-helices near the N-terminus. Our results indicate that efficient folding, despite some local non-native structural preferences, is insured by the rapid formation of a native-like α/β core domain.
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