We need air (clean air!)

(Part 1 of 2)

There are a number of things that we need to stay alive. One is food; without food, we will starve to death (although it is said that we have enough fat stored around our waistline to last us a few weeks — even longer for some of us). Another is water; without water, we can survive only a few days. One that we direly need is oxygen; without oxygen, we will be dead in a few minutes. We are aerobic, that is, we require oxygen for life. Our cells use oxygen (molecular oxygen, dioxygen, O2) to produce the energy that keeps our body parts functioning, that is, energy that keeps us alive. Not just us, but other multicellular organisms also are aerobic. 

What do we do to make sure that we get an ample supply of oxygen? Simple, we breathe. We have lungs that we fill with (fresh) air and empty (of stale air) by the movement of certain muscles. Fish and some other animals have gills through which they absorb oxygen from the surrounding medium. Some could breathe through their skin (frogs, for example). 

How do we transport the oxygen in our lungs to all the cells that need it? We have our circulatory system. Our blood carries the oxygen (and food and other substances) and distributes its load to where it is needed. And our heart provides the mechanical push to move our blood around. 

(Insects have evolved other means of transporting oxygen to their tissues. They have hollow tubes (tracheae) throughout their bodies and gas exchange is directly between their tissues and those tubes. This essentially passive exchange prevents insects from growing very big — despite what they show in movies.)

While oxygen is being transported in our blood, it is bound to a molecule called hemoglobin — a protein (globin) with an iron-heme group (a mostly flat molecule that is built from four cyclic rings with chemical groups attached to them and which has an iron atom in the middle). In cells, there are molecules (also proteins with iron-heme groups) that hold oxygen in storage until needed. In muscle cells, the storage molecule is called myoglobin; the molecule in nerve cells is called neuroglobin; in other cells, we have cytoglobin. The term “hemoglobin” is often used as the generic name for all such molecules. 

All vertebrates (with a few exceptions) have hemoglobin. Some invertebrates also have hemoglobin, like some insects, worms, and bacteria; even some plants have hemoglobin. In some invertebrates, other molecules serve the same purpose as hemoglobin. Some have hemocyanin, which uses copper (but not with heme) to bind oxygen); some have hemerythrin, which uses iron (but also without the heme group); some have chlorocruorin, which uses an iron-heme that has a slightly different structure from that of the iron-heme in our hemoglobin.

These oxygen-binding molecules come in a variety of sizes and shapes. Some hemoglobins are monomeric (with only one subunit); some are multimeric (with two or more identical, or different, subunits). Our myoglobin and the hemoglobins of bacteria and some invertebrates are monomeric; the hemoglobin in our blood is tetrameric (with four subunits) and it is made up of two different types of subunits, called alpha and beta. Myoglobin and the other storage hemoglobins in our cells and the alpha and beta subunits in the hemoglobin in our blood have roughly 150 amino acids each; some globins present in lower animals like bacteria are “truncated” and have only about 120 amino acids. The hemoglobin molecules of some invertebrates are giants with 180 subunits, of which 144 are myoglobin-sized oxygen-binders; chlorocruorins also are giant molecules. Despite the variations in size and amino acid sequence, all hemoglobins have the same tertiary structure (the way the string of amino acids is folded to form a stable three-dimensional structure). The tertiary structures of the hemocyanins and hemerythrins are different from that of the hemoglobins.

(To be concluded)

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Eduardo A. Padlan is a retired research scientist formerly with the US National Institutes of Health and is currently serving as an adjunct professor in the Marine Science Institute, UP Diliman. He is a corresponding member of the NAST. His first research work was on the structure of the hemoglobin from a marine annelid worm. He can be reached at [email protected].