Conformational entropy of alanine versus glycine in protein denatured states

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Members of the homeodomain superfamily are three-helix bundle proteins whose second and third helices form a helix-turn-helix motif (HTH). Their folding mechanism slides from the ultrafast, three-state framework mechanism for the engrailed homeodomain (EnHD), in which the HTH motif is independently stable, to an apparent two-state nucleation-condensation model for family members with an unstable HTH motif. The folding intermediate of EnHD has nearly native HTH structure, but it is not docked with helix1. The determinant of whether two-or three-state folding was hypothesized to be the stability of the HTH substructure. Here, we describe a detailed Φ-value analysis of the folding of the Pit1 homeodomain, which has similar ultrafast kinetics to that of EnHD. Formation of helix1 was strongly coupled with formation of HTH, which was initially surprising because they are uncoupled in the EnHD folding intermediate. However, we found a key difference between Pit1 and EnHD: The isolated peptide corresponding to the HTH motif in Pit1 was not folded in the absence of H1. Independent molecular dynamics simulations of Pit1 unfolding found an intermediate with H1 misfolded onto the HTH motif. The Pit1 folding pathway is the connection between that of EnHD and the slower folding homeodomains and provides a link in the transition of mechanisms from two-to three-state folding in this superfamily. The malleability of folding intermediates can lead to unstable substructures being stabilized by a variety of nonnative interactions, adding to the continuum of folding mechanisms.protein folding | temperature jump | diffusion collision | transition state T he nucleation-condensation mechanism was proposed from a Φ-value analysis of the folding of the protein chymotrypsin inhibitor 2 (CI2) (1, 2). The individual elements of secondary structure in CI2 are not independently stable, leading to highly cooperative formation of structure in the transition state (TS) for folding in which the helix is the best-formed region, stabilized by long-range interactions. No element of secondary structure is fully formed in the TS and its folding is kinetically two-state (3). It was hypothesized that multistate folding requires independently stable elements of structure or an unstable element of structure being stabilized by interacting with other elements. At the other extreme, the three-helix engrailed homeodomain, EnHD, is a small protein that folds ultrafast via a stable intermediate in line with the framework mechanism, whereby secondary structure forms first followed by docking of the helices (4, 5). Its folding intermediate consists of helix1 (H1) being helical but not docked on to the helix2-turn-helix3 (HTH) DNA binding motif (5, 6). The HTH motif is independently stable in the on-pathway intermediate (7). Studies of other members of the homoedomain superfamily show a slide from this multistate framework folding mechanism to two-state folding and nucleation condensation for members with less stable HTH motifs, consistent with our earlier hypoth...

Section: Resultsmentioning

confidence: 99%

Malleability of folding intermediates in the homeodomain superfamily

Banachewicz

Religa

Schaeffer

et al. 2011

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“…Because the purpose of this Φ-value analysis is comparative, the mutations chosen and the conditions used were identical to those used for the wild-type R16 Φ-value analysis. Two mutation types were made: (i) core mutations where a non-disruptive, deletion mutation was made to a core residue to probe tertiary structure formation and core packing at the transition state, and (ii) surface mutations where each position was mutated first to alanine then to glycine and the two compared to probe helix formation (Ala-Gly scanning) (30,31).…”

Section: Resultsmentioning

confidence: 99%

Separating the effects of internal friction and transition state energy to explain the slow, frustrated folding of spectrin domains

Wensley

Kwa

Shammas

et al. 2012

The elongated three-helix bundle domains spectrin R16 and R17 fold some two to three orders of magnitude more slowly than their homologue R15. We have shown that this slow folding is due, at least in part, to roughness in the free-energy landscape of R16 and R17. We have proposed that this roughness is due to a frustrated search for the correct docking of partly preformed helices. However, this accounts for only a small part of the slowing of folding and unfolding. Five residues on the A helix of R15, when inserted together into R16 or R17, increase the folding rate constants, reduce landscape roughness, and alter the folding mechanism to one resembling R15. The effect of each of these mutations individually is investigated here. No one mutation causes the behavior seen for the five in combination. However, two mutations, E18F and K25V, significantly increase the folding and unfolding rates of both R16 and R17 but without a concomitant loss in landscape roughness. E18F has the greatest effect on the kinetics, and a Φ-value analysis of the C helix reveals that the folding mechanism is unchanged. For both E18F and K25V the removal of the charge and resultant transition state stabilization is the main origin of the faster folding. Consequently, the major cause of the unusually slow folding of R16 and R17 is the non-native burial of the two charged residues in the transition state. The slowing due to landscape roughness is only about fivefold.free energy landscape | frustration | phi value | protein folding T he 15th, 16th, and 17th domains of chicken brain α-spectrin (R15, R16, and R17) have very similar structures, stabilities, and β-Tanford (β T ) values (which reflect the compactness of the transition state for folding and unfolding) (1-6). However, the folding of R15 differs from that of R16 and R17 in a number of respects. R15 folds and unfolds two orders of magnitude faster than R16 and three orders of magnitude faster than R17. R16 and R17 have two sequential transition states, and for both domains the first of these (TS1) shows significant landscape roughness (or "internal friction") (7). This has not been seen for any other domain of comparable size and folding kinetics, although theory has long predicted the possibility of such landscape roughness (8-15). R15 has a broad transition state (characterized by "rollover" in the unfolding limb for some mutants and for wild type under some conditions), but due to the speed of folding and unfolding it is not known whether it has two sequential transition states (16). However, the early transition state of R15 (which corresponds to TS1 of R16 and R17 and will be referred to as such) has a smoother, less frustrated landscape (7).Φ-value analysis shows that for all three domains the A and C helices are partially structured at TS1, whereas the B helix is relatively unstructured (16-18). R16 and R17 fold via a framework-like mechanism, with some tertiary contacts formed but more extensive secondary structure that extends throughout the A and C helices. In contrast, R1...

“…The lowest was 13% for Pro, followed by Ϸ30% for Asp and Ile. For comparison, the values for Ala and Gly at 498 K are 68 and 82%, respectively (22).…”

Section: Amino Acid Conformations In Different Structuralmentioning

confidence: 99%

The intrinsic conformational propensities of the 20 naturally occurring amino acids and reflection of these propensities in proteins

Beck

Alonso

Inoyama

et al. 2008

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127

183

Here, we compare the distributions of main chain (⌽,⌿) angles (i.e., Ramachandran maps) of the 20 naturally occurring amino acids in three contexts: (i) molecular dynamics (MD) simulations of GlyGly-X-Gly-Gly pentapeptides in water at 298 K with exhaustive sampling, where X ‫؍‬ the amino acid in question; (ii) 188 independent protein simulations in water at 298 K from our Dynameomics Project; and (iii) static crystal and NMR structures from the Protein Data Bank. The GGXGG peptide series is often used as a model of the unstructured denatured state of proteins. The sampling in the peptide MD simulations is neither random nor uniform. Instead, individual amino acids show preferences for particular conformations, but the peptide is dynamic, and interconversion between conformers is facile. For a given amino acid, the (⌽,⌿) distributions in the protein simulations and the Protein Data Bank are very similar and often distinct from those in the peptide simulations. Comparison between the peptide and protein simulations shows that packing constraints, solvation, and the tendency for particular amino acids to be used for specific structural motifs can overwhelm the ''intrinsic propensities'' of amino acids for particular (⌽,⌿) conformations. We also compare our helical propensities with experimental consensus values using the host-guest method, which appear to be determined largely by context and not necessarily the intrinsic conformational propensities of the guest residues. These simulations represent an improved coil library free from contextual effects to better model intrinsic conformational propensities and provide a detailed view of conformations making up the ''random coil'' state.coil library ͉ Dynameomics ͉ molecular dynamics ͉ protein folding ͉ host-guest P rotein secondary structure was predicted before the atomic structures of protein were determined (1-3). Conformational preferences of the amino acids were also estimated very early on, beginning with Ramachandran's ''map'' in 1963, ''based solely on repulsive van der Waals'' forces in dipeptides (4, 5). Remarkably, these predictions regarding structure and conformational preferences were later largely validated in protein crystal structures (6-8).In the protein folding field, these preferences are seen as both means of excluding regions of conformational space and as driving forces for the formation of secondary structure, both of which limit and bias the necessary search of conformational space required during protein folding.(⌽,⌿) dihedral angle distributions are increasingly used to check the validity of structures. Although there can be no doubt about the general tendency of amino acids in globular proteins to populate some regions of (⌽,⌿) space relative to others, the use of such distributions to judge and refine structures leads to dangerous circular reasoning. That is, (⌽,⌿) preferences are used as tests of crystal structures, and those very crystal structures are then used to define and support the Ramachandran (⌽,⌿) angle distributions.Many exper...