Burmester's team used a number of techniques to characterize globin E and found that it is responsible for storing and delivering oxygen to the retina.
The finding is intriguing for a number of reasons.
Firstly, it helps explain how birds evolved to have such large eyes, relative to their body mass, without a dense network of ocular capillaries for blood delivery. (Some owls, for instance, have bigger eyes than humans.)
"The exact origin of globin E is still somewhat a mystery," Burmester said. "It clearly evolved from some type of globin, but it has no obvious relative outside the birds."
The globins are all thought to share a common ancestor, and the most well-known members of the family are myoglobin and hemoglobin. Myoglobin is responsible for oxygen storage and release in heart and skeletal muscle fibers. Hemoglobin, meanwhile, transports oxygen from the lungs to other parts of the body in red blood cells.
Burmester explains: "Bird eyes have evolved to have a system not unlike those in our heart, which uses myoglobin to store and release oxygen to maintain respiration and energy-consumption during muscle contraction. In eyes, oxygen and energy are needed to generate neuronal signals."
Secondly, the finding puts to rest an earlier hypothesis that another molecule, neuroglobin, might be the oxygen-delivery vehicle for the avian eye. Neuroglobin is known to deliver oxygen to brain tissue, so it was only natural to suspect it. But it turns out that the messenger RNA fingerprint of globin E was 100-fold more prevalent than that of neuroglobin in Burmester's chicken retina samples, indicating that neuroglobin probably has another, yet-to-be defined function in the avian eye.
Lastly, globin E is another interesting illustration of the convergent evolution of "myoglobin-like" molecules. Among the org
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American Society for Biochemistry and Molecular Biology