310 helix
A 310 helix is a type of
Discovery
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 chains are held together by hydrogen bonding between the hydrogen and oxygen atoms of different by nearby amide (peptide) links formed as the amino acids condenseto form the polypeptide chain. These form helical arrangements that cannot be uncoiled without breaking the hydrogen bonds.
- Those structures in which all available NH and CO groups are hydrogen bonded are inherently more probable, because their free energy is presumably lower.
The 310 helix was eventually confirmed by Kendrew in his 1958 structure of
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
Residues in long 310-helices adopt (φ, ψ) 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
- alpha helix
- pi helix
- secondary structure
- beta turn
- beta bend ribbon
References
- ^ PMID 12966187.
- ^ .
- ^ PMID 14816373.
- PMID 14834147.
- PMID 13054692.
- ^ ISBN 9780879696740.
- S2CID 4186097.
- ^ S2CID 4162786.
- S2CID 4208282.
- PMID 14224496.
- ISBN 9789810230579.
- PMID 6051747.
- PMID 5646648.
- S2CID 1383359.
- S2CID 1383359.
- PMID 1949158.
- ^ ISBN 9781439882047.
- PMID 21115694.
- ^ PMID 12761385.
- PMID 8844857.
- ^ PMID 1303752.
Other readings
- A 310 Helix Is a Type of Protein Secondary." Biochemistries. N.p., 20 Oct. 2013. Web. 06 Dec. 2015. <http://biochemistri.es/the-3-10-helix[permanent dead link]>.