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Another Fine-Tuned Mechanism Gets Salmon Home


What is carbonic anhydrase and why does it matter? The presence of this enzyme in salmon hearts points to another case of intelligent design in these remarkable fish that were depicted in Illustra Media's film Living Waters. News via the Journal of Experimental Biology explains the challenge salmon face swimming upstream:

Fish plumbing is contrary. As the heart is the last organ that blood passes through before it returns to the gills, and with little direct blood supply to the ceaselessly contracting muscle, there are occasions when it could be on the verge of failure. 'We know this can happen under certain conditions like exhaustive exercise in combination with hypoxia or elevated water temperature', says Sarah Alderman from the University of Guelph, Canada. Added to the challenge of keeping the heart supplied with oxygen, Alderman explains that the haemoglobin that carries oxygen in fish blood is finely tuned to blood pH: the more acidic the red blood cells, the less able haemoglobin is to carry oxygen, which could prevent the red blood cells of exercising fish from picking up oxygen at the gills if they didn't have an effective pump to remove acid from the cells and restore the pH balance. [Emphasis added.]

Here we see a double challenge the salmon faces. It has to avoid excess acid so that the hemoglobin can carry oxygen, and it has to get the oxygen all the way from the gills through its entire body to the heart. Here's where carbonic anhydrase comes to the rescue:

But Alderman and her colleagues, Till Harter, Tony Farrell and Colin Brauner from the University of British Columbia, Canada, also knew that fish can take advantage of a sudden drop in red blood cell pH to release oxygen rapidly at tissues -- such as red muscle and the retina -- when required urgently. An enzyme called carbonic anhydrase -- which combines CO2 and water to produce bicarbonate and acidic protons, and vice versa -- lies at the heart of this mechanism. Normally there is no carbonic anhydrase in blood plasma; however, the enzyme has been found in salmon red muscle capillaries, where it facilitates the reaction of protons -- that have been extruded from the red blood cell -- with bicarbonate to produce CO2, which then diffuses back into the red blood cell. The CO2 is then converted back into bicarbonate and protons in the blood cell, causing the pH to plummet and release a burst of O2 from the haemoglobin. Could salmon take advantage of this mechanism to boost oxygen supplies to the heart when the animals are working full out? Possibly, but only if carbonic anhydrase was accessible to blood passing through the heart.

Well, what do you know! That's what Alderman's team found: the enzyme is present on the surface of the heart chambers. Using the heart itself as their "reaction vessel," they were able to see the pH plummet as the enzymes went into gear.

Working closely together, the duo painstakingly developed a technique where they could measure the pH in the beating heart with pH probes that were thinner than a human hair. Eventually, the duo's persistence paid off and the pH in the heart plummeted as they fed CO2 into the pulsating chambers. And when they added a carbonic anhydrase inhibitor (produced by Claudia Supuran) to the fluid, the pH fall slowed dramatically. Carbonic anhydrase was responsible for the drop in pH.

The Protein Data Bank 101 website shows pictures of this enzyme and describes its mode of action.

An enzyme present in red blood cells, carbonic anhydrase, aids in the conversion of carbon dioxide to carbonic acid and bicarbonate ions. When red blood cells reach the lungs, the same enzyme helps to convert the bicarbonate ions back to carbon dioxide, which we breathe out. Although these reactions can occur even without the enzyme, carbonic anhydrase can increase the rate of these conversions up to a million fold.

There's an evolutionary conundrum about this enzyme: functional equivalence without sequence similarity:

This ancient enzyme has three distinct classes (called alpha, beta and gamma carbonic anhydrase). Members of these different classes share very little sequence or structural similarity, yet they all perform the same function and require a zinc ion at the active site. Carbonic anhydrase from mammals belong to the alpha class, the plant enzymes belong to the beta class, while the enzyme from methane-producing bacteria that grow in hot springs forms the gamma class. Thus it is apparent that these enzyme classes have evolved independently to create a similar enzyme active site.

Further complicating the picture, there are different forms of the enzymes depending on the tissue or cellular compartment they are located in. These allow fine tuning of the enzyme's activity: "Thus isozymes found in some muscle fibers have low enzyme activity compared to that secreted by salivary glands," in the case of mammals.

Carbonic anhydrase (CA) was the first enzyme found to contain zinc, a biochemistry textbook says; now, hundreds are known. Zinc and other metals are often essential for function in metalloenzymes. The PDB-101 article explains,

Zinc is the key to this enzyme reaction. The water bound to the zinc ion is actually broken down to a proton and hydroxyl ion. Since zinc is a positively charged ion, it stabilizes the negatively charged hydroxyl ion so that it is ready to attack the carbon dioxide.

It's not surprising that this enzyme is present in the salmon, since it exists in all three domains of life. What's amazing is that the salmon's heart is studded with these enzymes that are "at the ready" when the large fish is fighting with all its might to leap above cascades and waterfalls, facing daunting challenges without the benefit of food. (Living Waters says, "The sockeye are so focused on their objective that after leaving the ocean, they do not eat again.")

If the pH dropped too early as the blood travels through the salmon's body, it would be less able to carry its precious oxygen cargo. But right when it is needed during that Olympic high jump over a waterfall or away from a hungry bear, the fish uses its CA enzymes in its heart to drop the pH and release the oxygen it needs. The spent blood then travels to the gills to load up with more oxygen. In the original paper in the Journal of Experimental Biology, the authors summarize what they found:

Combined, these results support our hypothesis of the presence of an enhanced oxygen delivery system in the lumen of a salmonid heart, which could help support oxygen delivery when the oxygen content of venous blood becomes greatly reduced, such as after burst exercise and during environmental hypoxia.

How many independent systems do we see at work in the remarkable migration of the salmon? The fish has a built-in map of its destination. It has the ability to memorize waypoints by smell. It can navigate by the earth's magnetic field. It has a sixth sense, the lateral line. It can distinguish odors by parts per trillion. Each of its body systems contain thousands of molecular machines like carbonic anhydrase that are located just where they need to be, doing what they need to do to support the whole organism.

If any one of these systems is a testament to intelligent design, how much more the composite? The only fish story here is the notion that all this came together by unguided natural processes -- sheer dumb luck.

Image: Salmon run, Brooks Falls, Katmai National Park, Alaska, by Dmitry Azovtsev, [CC BY-SA 3.0 or CC BY-SA 3.0], via Wikimedia Commons.