Biological complexity: how hemoglobin appeared (Introduction)

by David Turell , Wednesday, May 20, 2020, 19:47 (1648 days ago) @ David Turell

Without hemoglobin present in red blood cells, we would not be here:

https://phys.org/news/2020-05-reveal-complex-hemoglobin-resurrecting-ancient.html

The group identified the evolutionary "missing link" through which hemoglobin—the essential four-part protein complex that transports oxygen in the blood of virtually all vertebrate animals—evolved from simple precursors. And they found that it took just two mutations more than 400 million years ago to trigger the emergence of modern hemoglobin's structure and function.

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Each hemoglobin molecule is a four-part protein complex made up of two copies each of two different proteins, but the proteins to which they are most closely related do not form complexes at all. The team's strategy, pioneered in Thornton's lab over the last two decades, was a kind of molecular time travel: use statistical and biochemical methods to reconstruct and experimentally characterize ancient proteins before, during and after the evolution of the earliest forms of hemoglobin. This allowed them to identify the missing link during hemoglobin evolution—a two-part complex, consisting of two copies of a single protein, which existed before the last common ancestor of humans and sharks. This ancient two-part complex did not yet possess any of modern hemoglobin's critical properties that allow it to bind oxygen in the lungs and deliver it to distant cells in the brain, muscles and other tissues.

By introducing into this missing link protein various mutations that occurred during the next historical interval, they found that just two mutations on the protein's surface triggered formation of the four-part complex and imparted the critical changes in its oxygen-binding function.

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Analysis of the ancient proteins' atomic structures showed how the two mutations took advantage of even more ancient features to assemble the intermediate two-part complex into the four-part complex. The mutations introduced two changes on the protein surface that allowed it to bind tightly to the surface of the other protein, which remained unchanged as it was recruited into the new interaction. Other ancient parts of the two surfaces also stuck together simply by chance, adding further strength to the interaction that was triggered by the two new mutations. Those older elements, Thornton pointed out, and even the two-part complex itself, must have existed then by chance, rather than because they enhanced the protein's final structure or function, because they evolved before those properties came into being. (my bold)

Perhaps the most surprising result was that the two critical mutations, by inducing formation of the four-part structure, also triggered the critical changes in the complex's oxygen-binding functions. Hemoglobin can perform its physiological function because its affinity for oxygen is high enough to bind oxygen in the lungs, but low enough to release it in the tissues elsewhere in the body. It also binds oxygen cooperatively: When one of the four components takes up a molecule of oxygen, the other components tend to do the same—and this happens in the reverse direction, as well—so the whole complex becomes even more effective at recruiting oxygen and releasing it in the right places.

Hemoglobin's ancient precursors—including the missing link two-part complex—bound oxygen too tightly and were not cooperative, so they could not have effectively performed the oxygen-exchange function. The researchers found that the two key mutations not only conferred the four-part structure but also imparted hemoglobin's critical oxygen-binding properties.

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"Imagine if those two mutations never occurred, or if the structural features that they took advantage of weren't in place at the time," Thornton said. "Hemoglobin as we know it would not have evolved, and neither would many of the subsequent innovations that depend on efficient oxygen transport, like rapid metabolism and the ability to grow much larger and move much faster than our ancient marine ancestors." (my bold)

Comment: Note my bolds. This is a Darwinist piece of research reporting where all must occur by chance. What are the odds for that? Instead I see God's purpose in the events knowing what is to come in the future. What is also important is hemoglobin carries CO2 out to the lungs and releases it faster than it picks up oxygen. We know we are short of breath from CO2 levels, not O2 levels. And another side gain is the development of myoglobin in muscles where oxygen is handled faster than other organs to allow rapid muscle use. It is easy to see the purpose. Why see chance?