Evolution: the color of blood (Introduction)

by David Turell , Saturday, April 27, 2019, 23:30 (2036 days ago) @ David Turell

Most animals carry genes for hemoglobin, but only vertebrates use it:

https://www.quantamagazine.org/icefish-study-adds-another-color-to-the-story-of-blood-2...

By looking at the genome of one icefish species, the researchers were able to peek at the evolutionary adaptations that allowed it to survive. Some were common to red-blooded fish that are also native to Antarctic waters, like the presence of extra genes for making blood proteins that act like antifreeze. Some were more distinctive to the icefish’s lack of red blood cells, such as a boost in the enzymes that protect tissues from the highly reactive free oxygen in its blood.

Most invertebrates carry genes for hemoglobins, but they generally use other metalloprotein pigments in their versions of blood. Insects, crustaceans and other arthropods use hemocyanin, a bluish copper-based pigment. Mollusks, ranging from clams to squids and octopuses, use hemocyanin, too, but they seem to have invented their version of it independently. Some worms use purplish hemerythrin; others use greenish chlorocruorin; some use a combination of pigments.

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From the very beginning of life, cells needed to move electrons around between molecules as part of their metabolism, explained Ross Hardison, a professor of biochemistry and molecular biology at Pennsylvania State University. As controls over these redox (oxidation-reduction) reactions, cells deployed ring-shaped molecules called porphyrins. When these porphyrins held a metal atom like iron or copper, they had a ferocious affinity for oxygen. “Once you have an iron in that porphyrin ring, it’s used throughout the biosphere,” Hardison said. He speculated that it “might be one of the earliest molecules that eventually got incorporated into cells.”

Hemoglobin arose out of four interlinked globin proteins, each holding a heme, and it rapidly became ubiquitous. “Hemoglobins predate the origin of animals and even predate the common ancestor of animals and plants,” said Mark Siddall, a curator in the division of invertebrate biology at the American Museum of Natural History.

The secret of hemoglobin’s success is collaborative bonding: With every oxygen molecule that the pigment binds, it can bind to the next one more easily, until all four vacancies are filled. This makes hemoglobin extremely efficient at collecting oxygen where it’s abundant (as in the open air and in lungs) and then releasing it again gradually in oxygen-starved tissues.

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Even if the alternative blood pigments are generally a poor second to hemoglobin at grabbing oxygen, they do have an advantage in terms of simplicity: They usually don’t need something like a red blood cell to hold them. In squids, lobsters and the other blue-blooded animals, for example, hemocyanin is dissolved directly in their plasma. This approach works because hemocyanin, hemerythrin and the other pigments are big, frequently polymerized molecules that keep their oxygen-binding metal atoms tucked away from casual interactions. Conversely, hemoglobin is small and its aggressively reactive heme is easily exposed, which makes it highly toxic — so much so that our livers make a protein, haptoglobin, to scavenge stray hemoglobin from broken blood cells out of our blood.

From a toxicity standpoint, hemoglobin is a triple threat, explained Pampee Young, the chief medical officer of biomedical services for the American Red Cross. Heme has even greater affinity for nitric oxide than oxygen, and the body uses nitric oxide as a signaling molecule to control blood pressure. Excess free hemoglobin will therefore rob the blood of nitric oxide, constrict blood vessels and potentially cause hypertension and reduced blood flow to the organs. Compounding the problem is that hemoglobin, when unprotected in blood plasma, decomposes into its component globin subunits. The naked heme molecules then randomly attack the lipid membranes and other structures in the tissues, damaging them. And as a coup de grâce, the isolated globin proteins can clog the filtration system of the kidneys and shut them down.

Packaging hemoglobin into red blood cells (erythrocytes) helps to contain the toxicity problems. It also makes the distribution of oxygen more efficient by keeping the hemoglobin inside the blood vessels: The molecule is otherwise so small that some of it would leak out into the tissues and fall out of circulation.

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The reason that vertebrates show less diversity in their blood pigments than invertebrates do is simply that invertebrates are a much more diverse group of organisms overall (all vertebrates fall within a single phylum, Chordata, while invertebrates are in more than 30).

Comment: Note that most early organisms carry genes for hemoglobin, even if not using it, as if pre-planning for future evolution. Further the toxicity of hemoglobin and oxygen require the contemporaneous design of protective mechanisms. The article doesn't mention that hemoglobin releases oxygen and immediately picks up carbon dioxide to take to the lungs. The use of hemoglobin and oxygen allows vertebrates to create and use much more energy to act in their activities than non-vertebrates, none of which are as active.