310 helix

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Side view of a 310-helix of alanine residues in atomic detail. Two hydrogen bonds to the same peptide group are highlighted in magenta; the oxygen-hydrogen distance is 1.83 Å (183 pm). The protein chain runs upwards, i.e., its N-terminus is at the bottom and its C-terminus at the top of the figure. Note that the sidechains point slightly downwards, i.e., towards the N-terminus.

A 310 helix is a type of

secondary structure found in proteins and polypeptides. Of the numerous protein secondary structures present, the 310-helix is the fourth most common type observed; following α-helices, β-sheets and reverse turns
. 310-helices constitute nearly 10–15% of all helices in protein secondary structures, and are typically observed as extensions of α-helices found at either their N- or C- termini. Because of the α-helices tendency to consistently fold and unfold, it has been proposed that the 310-helix serves as an intermediary conformation of sorts, and provides insight into the initiation of α-helix folding.

carbonyl groups are pointing upwards towards the viewer, spaced roughly 120° apart on the circle, corresponding to 3.0 amino-acid
residues per turn of the helix.

Discovery

DNA double helix.[5] Pauling was highly critical of the helical structures proposed by Bragg, Kendrew, and Perutz, taking a triumphal tone in declaring them all implausible.[1][3] Perutz describes in his book I wish I'd made you angry sooner[6]
the experience of reading Pauling's paper one Saturday morning:

I was thunderstruck by Pauling and Corey's paper. In contrast to Kendrew's and my helices, theirs was free of strain; all of the amide groups were planar and every carbonyl group formed a perfect hydrogen bond with an amino group four residues further along the chain. The structure looked dead right. How could I have missed it?

— Max Perutz, 1998, pp.173-175.[6]

Later that day, an idea for an experiment to confirm Pauling's model occurred to Perutz, and he rushed to the lab to carry it out. Within a few hours, he had the evidence to confirm the alpha helix, which he showed to Bragg first thing on Monday.[1] Perutz' confirmation of the alpha helix structure was published in Nature shortly afterwards.[7] The principles applied in the 1950 paper to theoretical polypeptide structures, true of the 310 helix, included:[2]

The 310 helix was eventually confirmed by Kendrew in his 1958 structure of

haemoglobin[9][10][11] and in subsequent work on both its deoxygenated[12][13] and oxygenated forms.[14][15]

The 310 helix is now known to be the third principal structure to occur in globular proteins, after the α-helix and β-sheet.[16] They are almost always short sections, with nearly 96% containing four or fewer amino acid residues,[17]: 44  appearing in places such as the "corners" where α-helices change direction in the myoglobin structure, for example.[8] Longer sections, in the range of seven to eleven residues, have been observed in the voltage sensor segment of voltage-gated potassium channels in the transmembrane domain of certain helical proteins.[18]

Structure

The amino acids in a 310-helix are arranged in a right-handed

C=O group of the amino acid three residues earlier; this repeated i + 3 → i hydrogen bonding defines a 310-helix. Similar structures include the α-helix (i + 4 → i hydrogen bonding) and the π-helix i + 5 → i hydrogen bonding.[17]: 44–45 [19]

Residues in long 310-helices adopt (φψ) dihedral angles near (−49°, −26°). Many 310-helices in proteins are short, so deviate from these values. More generally, residues in long 310-helices adopt dihedral angles such that the ψ dihedral angle of one residue and the φ dihedral angle of the next residue sum to roughly −75°. For comparison, the sum of the dihedral angles for an α-helix is roughly −105°, whereas that for a π-helix is roughly −125°.[17]: 45 

The general formula for the rotation angle Ω per residue of any polypeptide helix with trans isomers is given by the equation:[17]: 40 

and since Ω = 120° for an ideal 310 helix, it follows that φ and ψ should be related by:

consistent with the observed value of φ + ψ near −75°.[17]: 44 

The dihedral angles in the 310 helix, relative to those of the α helix, could be attributed to the short lengths of these helices – anywhere from 3 to 5 residues long, compared with the 10 to 12 residue lengths of their α-helix contemporaries. 310-helices often arise in transitions, leading to typically short residue lengths that result in deviations in their main-chain torsion angle distributions and thus irregularities. Their hydrogen bond networks are distorted when compared with α-helices, contributing to their instability, though the frequent appearance of the 310-helix in natural proteins demonstrate their importance in transitional structures.[19][20]

Stability

Through research carried out by Mary Karpen, Pieter De Haseth and Kenneth Neet,[21] factors in the partial stability in 310-helices were uncovered. The helices are most noticeably stabilized by an aspartate residue at the nonpolar N-terminus that interacts with the amide group at the helical N-cap. This electrostatic interaction stabilizes the peptide dipoles in a parallel orientation. Much like the contiguous helical hydrogen bonds that stabilize α-helices, high levels of aspartate are just as equally important in the survival of the 310-helix. High frequency of aspartate in both 310-helix and α-helices is indicative of its helix initiation, but at the same time suggests that it favors stabilization of the 310-helix by inhibiting the propagation of α-helices.[21]

See also

References

Other readings