6.3: Early Clues to the Protein Mystery

Shortly before Lavoisier made his revolutionary discoveries about the chemical character of life and food, a French scientist named Macquer made some of the first progress toward a scientific understanding about protein.

Protein isn’t physically obvious; seldom do we see it in pure form. We don’t really see it when we look at meat or cheese. This is in contrast to the other energy-providing nutrients: we see carbohydrate in quite pure form as table sugar or honey, or even the sap of trees. We are familiar with fat as we trim it from the leaner parts of meat, as we toss a salad with oil, or as we french-fry a potato.

We do see pure protein in the white of an egg. Macquer was struck by some of the now well-known characteristics of egg white. Heat it, and the viscous liquid coagulates to a slippery solid. Agitate it, and it assumes yet another form, as when we whip it for the meringue on a lemon pie. Macquer suspected that this coagulation-prone substance in food might be the single fundamental nutrient substance sought since the time of Hippocrates. So he examined other life substances.

---

The protein in a hen’s egg is divided between the white and the yolk. But all of the fat (including cholesterol) is in the yolk. The white is all protein.

---

Some very essential materials shared this characteristic coagulation of the egg white—blood and semen for example. Perhaps, Macquer thought, this was the stuff which had been sought for so many centuries. “The gelatinous matter of animals,” he wrote finally, “is the true animal substance. It constitutes almost entirely the bodies of animals; it is that which nourishes, repairs, and reproduces them.” It was thought that this gelatinous matter was the fundamental life chemical. So it was named protein, from the Greek meaning elemental or primary.

Essential in the diet (the body can't make these 9)

histidine lysine threonine isoleucine methionine tryptophan leucine phenylalanine valine

Not essential in the diet (the body can make these 11)

alanine cysteine proline arginine glutamate* serine asparagine glutamine tyrosine aspartate* glycine

*Aspartate and glutamate are also called aspartic acid and glutamic acid

Table 6-1 The 20 Kinds of Amino Acids Needed to Make Protein

Early scientists also found that all these protein-containing substances could be treated to give off ammonia. This wasn’t true of carbohydrates or fats, both of which have carbon, hydrogen, and oxygen as their basic structure. The ammonia showed that the basic structure of protein had something more—nitrogen, for ammonia is made of three atoms of hydrogen and one of nitrogen.

---

The chemical notation for ammonia is NH₃.

---

Nitrogen is hardly an uncommon element. It is, in fact, the largest constituent of the air we breathe—there is more than three times as much nitrogen as there is oxygen in air. But while we can use the oxygen in our body chemistry, we can make little use of the nitrogen we inhale. So our bodily nitrogen must come from food.