Kanazawa University NanoLSI Podcast: Researchers fix the chirality of helical proteins

Researchers fix the chirality of helical proteins

Hello and welcome to the NanoLSI podcast. Thank you for joining us today. In this episode we feature the latest research by Naoki Ousaka, Mark J. MacLachlan and Shigehisa Akine at the Kanazawa University NanoLSI.

The research described in this podcast was published in Nature Communications in October 2023

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Researchers fix the chirality of helical proteins

Researchers at Kanazawa University report in Nature Communications how they can control chirality inversion in α helical peptides.

The function of a protein is determined by its structure – prompting great interest in how to manipulate these structures. The structure is defined not just by the sequence of amino acids that make it, but the shape these acids make – the secondary structure – as well as how that shape is then folded. The most common secondary protein structure is the α-helix, which can coil to the right or left. This coiling direction in turn determines how it engages with other chiral structures, which may be the form of a light beam or another molecule. Although molecular components and environmental factors can favor a particular coiling direction over the other, helical molecules tend to flip between the two coil directions. Now Naoki Ousaka, Mark J. MacLachlan and Shigehisa Akine at Kanazawa University in Japan have shown how they can control and fix the coil direction.

Helical proteins are chiral molecules, which means that the molecule’s shape cannot be fitted into its mirror image. In nature helical proteins often have other chiral components, such as sugars or amino acids, and these will determine which way the protein coils. However, there is a lot of interest in synthesizing artificial helical proteins that have different chemical components and hence functions not found in nature, and these may not have other chiral components. Nonetheless having both types or “enantiomers” of the chiral molecule can be hazardous because of the significant differences in behavior between the two chiral forms, one of which may be benign or even therapeutic while the other is toxic. Hence, there is demand for other ways of selecting and fixing the chirality.

So how did they go about this?

Ousaka, MacLachlan and Akine synthesized α helical molecules solely from achiral components. They included bulky segments so that the molecule tended towards the larger rings of the α helical structure, as well as side chains of piperidine – molecular components that are common in pharmaceuticals. These side chains can be cross linked to “staple” the molecule into either the righthanded or lefthanded coil, inhibiting flipping between the two – chiral inversion. Finally they added another molecular component, known as an ester  – the L-Val-OH residue. This would switch the direction of the coil in response to acidic or basic environments due to preferences in the interaction between oxygen atoms in the ester and the amino acid backbone.

The researchers used a range of chiral characterization methods including circular dichroism, nuclear magnetic resonance and liquid chromatography. They found that with the molecule stapled just once, it would slow down the flipping between enantiomers by a factor of 106, although this still occurred over minutes. Changing the solution to acid or alkali also successfully determined which enantiomer was favoured. However, stapling the molecule twice slowed down the chirality inversion by a factor of 1012, so that the molecular chirality was stable for years. This increased energy barrier to chirality inversion could then be overcome by heating the sample to very high temperatures to switch bet

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