TY - JOUR
T1 - The polyproline II conformation in short alanine peptides is noncooperative
AU - Chen, Kang
AU - Liu, Zhigang
AU - Kallenbach, Neville R.
PY - 2004/10/26
Y1 - 2004/10/26
N2 - The finding that short alanine peptides possess a high fraction of polyproline II (PII) structure (Φ = -75°, Ψ = +145°) at low temperature has broad implications for unfolded states of proteins. An important question concerns whether or not this structure is locally determined or cooperative. We have monitored the conformation of alanine in a series of model peptides AcGG(A)n-GGNH2 (n = 1-3) over a temperature range from -10°C to +80°C. Use of 15N-labeled alanine substitutions makes it possible to measure 3JαN coupling constants accurately over the full temperature range. Based on a 1D next-neighbor model, the cooperative parameter σ of PII nucleation is evaluated from the coupling constant data. The finding that σ is close to unity (1 ± 0.2) indicates a noncooperative role for alanine in PII structure formation, consistent with statistical surveys of the Protein Data Bank that suggest that most PII structure occurs in isolated residues. Lack of cooperativity in these models implies that hydration effects that influence PII conformation in water are highly localized. Using a nuclear Overhauser effect ratio strategy to define the alanine Ψ angle, we estimate that, at 40°C, the time-averaged alanine conformation (Φ = -80°, Ψ = +170°) deviates from canonical PII structure, indicating that PII melts at high temperature. Thus, the high-temperature state of short alanine peptides seems to be an unfolded ensemble with higher distribution in the extended β structure basin, but not a coil.
AB - The finding that short alanine peptides possess a high fraction of polyproline II (PII) structure (Φ = -75°, Ψ = +145°) at low temperature has broad implications for unfolded states of proteins. An important question concerns whether or not this structure is locally determined or cooperative. We have monitored the conformation of alanine in a series of model peptides AcGG(A)n-GGNH2 (n = 1-3) over a temperature range from -10°C to +80°C. Use of 15N-labeled alanine substitutions makes it possible to measure 3JαN coupling constants accurately over the full temperature range. Based on a 1D next-neighbor model, the cooperative parameter σ of PII nucleation is evaluated from the coupling constant data. The finding that σ is close to unity (1 ± 0.2) indicates a noncooperative role for alanine in PII structure formation, consistent with statistical surveys of the Protein Data Bank that suggest that most PII structure occurs in isolated residues. Lack of cooperativity in these models implies that hydration effects that influence PII conformation in water are highly localized. Using a nuclear Overhauser effect ratio strategy to define the alanine Ψ angle, we estimate that, at 40°C, the time-averaged alanine conformation (Φ = -80°, Ψ = +170°) deviates from canonical PII structure, indicating that PII melts at high temperature. Thus, the high-temperature state of short alanine peptides seems to be an unfolded ensemble with higher distribution in the extended β structure basin, but not a coil.
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U2 - 10.1073/pnas.0406657101
DO - 10.1073/pnas.0406657101
M3 - Article
C2 - 15489268
AN - SCOPUS:7444255986
SN - 0027-8424
VL - 101
SP - 15352
EP - 15357
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
IS - 43
ER -