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A survey of 322 proteins showed that the short polar (SP) side chains of four residues, Thr, Ser, Asp, and Asn, have a very strong tendency to form hydrogen bonds with neighboring backbone amides. Specifically, 32% of Thr, 29% of Ser, 26% of Asp, and 19% of Asn engage in such hydrogen bonds. When an SP residue caps the N terminal of a helix, the contribution to helix stability by a hydrogen bond with the amide of the N3 or N2 residue is well established. When an SP residue is in the middle of a helix, the side chain is unlikely to form hydrogen bonds with neighboring backbone amides for steric and geometric reasons. In essence the SP side chain competes with the backbone carbonyl for the same hydrogen-bonding partner (i.e., the backbone amide) and thus SP residues tend to break backbone carbonyl-amide hydrogen bonds. The proposition that this is the origin for the low propensities of SP residues in the middle of alpha helices (relative to those of nonpolar residues) was tested. The combined effects of restricting side-chain rotamer conformations (documented by Creamer and Rose, Proc Acad Sci USA, 1992;89:5937-5941; Proteins, 1994;19:85-97) and excluding side- chain to backbone hydrogen bonds by the helix were quantitatively analyzed. These were found to correlate strongly with four experimentally determined scales of helix-forming propensities. The correlation coefficients ranged from 0.72 to 0.87, which are comparable to those found for nonpolar residues (for which only the loss of side-chain conformational entropy needs to be considered).

To determine the role of primary structure in specifying turns, random sequences (guests) were substituted for the native turn sequences in a series of proteins (hosts) of differing thermodynamic stabilities.The fraction of inserts that result in active proteins is measured as a function of the stability of the host and temperature. With a highly stable host, more than half of the inserts give functional proteins. However, a smaller fraction of sequences supports folding as the stability of the host decreases, and the temperature increases. The sequences of many of the selected inserts resemble the wild-type turn, and those that diverge match other established turn preferences. Thermodynamic measurements show that turn sequences selected under stringent conditions result in the most stable proteins. Thus, beta-turns appear to be under evolutionary pressure favouring thermodynamically stable structures.

A significant fraction of the amino acids in proteins are alpha helical in conformation. Alpha helices in globular proteins are short, with an average length of about twelve residues, so that residues at the ends of helices make up an important fraction of all helical residues. In the middle of a helix, H-bonds connect the NH and CO groups of each residue to partners four residues along the chain. At the ends of a helix, the H-bond potential of the main chain remains unfulfilled, and helix capping interactions involving bonds from polar side chains to the NH or CO of the backbone have been proposed and detected. In a study of synthetic helical peptides, we have found that the sequence Ser-Glu-Asp-Glu stabilizes the alpha helix in a series of helical peptides with consensus sequences. Following the report by Harper and Rose, which identifies SerXaaXaaGlu as a member of a class of common motifs at the N termini of alpha helices in proteins that they refer to as "capping boxes," we have reexamined the side chain-main chain interactions in a variant sequence using 1H NMR, and find that the postulated reciprocal side chain-backbone bonding between the first Ser and last Glu side chains and their peptide NH partners can be resolved. Deletion of two residues N terminal to the Ser-Glu-Asp-Glu sequence in these peptides has no effect on the initiation of helical structure, as defined by two-dimensional (2D) NMR experiments on this variant.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of an amino acid on the stability of alpha-helical structure depends on the position of the residue in the helix with respect to the ends. Short alpha helices in proteins are stabilized both by H-bonding of the main-chain NH and CO groups and by capping interactions between side chains and unfulfilled peptide groups at the N and C termini. Peptide models based on consensus position-dependent helix sequences allow one to model capping effects in isolated helices and to establish a base line for these interactions in proteins. We report here an extended series of substitutions in the cap positions of our peptide models and the solution structure of peptide S3, with serine at the N-cap position defined as the N-terminal residue with partly helix and partly coil conformation. The resulting model, determined by 2D 1H NMR, is consistent with a structure at the N-cap involving H-bonding between the serine gamma oxygen and the peptide NH of the glutamic acid residue three amino acids toward the C terminus. A bifurcated H-bond of Ser O gamma with the NH of Asp5 is possible also, since this group is within interacting distance. This provides direct evidence that specific side-chain interactions with the main chain stabilize isolated alpha-helical structure.