Scientists uncover molecular secrets of how we see colour
A global team has cracked a decades-old mystery, revealing the atomic structures of the molecules in our eyes that allow us to see colours.
“To understand how we detect light and perceive colours, we need to know the exact structure of light-sensitive molecules in our eyes,” said The Australian National University (ANU) researcher Emeritus Professor Trevor Lamb.
“Our perception of colour is mainly determined by the relative excitation of red, green, and blue-sensitive cone photoreceptor cells found inside our retinas, that contain these molecules.”
Published in Science, the discovery was made by a team of scientists from research institutions in China, Germany and Australia.
Within the John Curtin School of Medical Research at ANU, Professor Lamb was central to interpreting the important role these molecules play within the eye.
There are three versions of the molecules, called cone opsins, with each converting red, green or blue light into chemical signals.
“Revealing the atomic structures for each of the molecules in their light-activated state shows how they work inside cone cells to trigger signals that are ultimately sent to the brain,” said Professor Lamb.
“Our results reveal fundamental differences between the cone opsins when they enter their active state after being hit with light.”
Like how a high shutter speed lets a camera capture sharper images, having colour-detecting molecules in our eyes that turn on and off quickly is thought to allow us to see sharp detail and colour in motion accurately in daylight.
All three cone opsins contain the same light-sensitive vitamin A-derived molecule, with red, green and blue opsins binding to this molecule, called retinaldehyde, differently.
“Our study provides a molecular understanding of how each cone opsin interacts with retinaldehyde to tune it to different wavelengths of light,” said Professor Lamb.
“The red and green opsins appear to use very different placement of chemical electronic charges around the retinaldehyde. We suspect this difference explains how they shut off faster than the blue opsin, and much faster than the rod pigment.”
The structure of the rod pigment molecule, which we use for low-light vision, was solved decades ago. But it’s only now, with access to new microscopy techniques, that the researchers have been able to solve the structure of the colour detecting molecules.
“It’s taken so long because it hasn’t been possible to make crystals of cone opsins,” said Professor Lamb. “Instead, our work used flash-frozen samples of each opsin, which are then examined by electron microscopy.”
In the long term, this development could help scientists discover better treatments for some vision disorders, such as cone dystrophies and altered colour vision.
“In many cases, cone vision disorders result from problems with the cone opsins,” said Professor Lamb.
Understanding the structure of cone opsins is important because it helps explain exactly how these disorders arise at a molecular level.
The study, titled 'Cryo-electron microscopy structures of human cone visual pigments', is published in Science.
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