Proteins and radiation. A new hypothesis on the mystery of life

Like so many students before and after me, during my university studies I came across one of those unsolved mysteries about the constitution of all living organisms which look really bizarre.

To explain it to the readers of this page, it is necessary to briefly mention our molecular composition, and precisely that of the machines that perform most of the functions of a biological organism: the proteins. These are chemical compounds made up of long chains of hundreds of smaller molecules linked together, like beads on a necklace: amino acids. All living organisms use about twenty different types of amino acids to make up each protein in their body, joining them into filaments which then fold up three-dimensionally to form “balls” whose shape is specifically determined by the precise sequence of amino acids joined together. The shape obtained is the one that then corresponds to the function of the protein produced: as for any human tool and machine, even for the protein nanomachines we are made of, it determines the specific function – a bit like a screwdriver, a hammer or any human tool.

Now, on this page we have already seen why, of the hundreds of possible amino acids, life uses are about twenty different types; however, a mystery remains to be faced, the one I mentioned at the beginning.

Each type of amino acid used to build proteins (except for one, glycine) can come in two mirror-like forms, called the L-form and the D-form. Like our hands, that is, a certain amino acid can appear in two superimposable mirror-like forms, made up of exactly the same atoms linked together just as the fingers themselves are mirror-linked in the structure of our hands. This property of amino acids, i.e. appearing in the L or D form, is called chiralityderived from the ancient Greek kheír kheirós for “hand”, as the right and left hands cannot be superimposed except in mirror image.

Now, it happens that all proteins, in all biological organisms, are made up exclusively of L-type amino acids; this despite the fact that amino acids with opposite chirality, that D, are abundantly widespread and easily obtainable in nature and have been found everywhere in the cosmos.

Why? In other words, what led to the selection of one chirality over the other, and precisely to the L form, given that the ordinary ways in which it is possible to obtain these products lead to the production of equal quantities of D and L amino acids?

This is one of those seemingly simple questions that have haunted biologists since the chemical structure of proteins has been known.

Now, since 1969 we have known that in carbonaceous meteorites such as the one mentioned by Murchison the L amino acids are more abundant than the D ones. This suggests that the mechanism which led to the selection of the former is much older than life itself and is connected to the very origin of amino acids in space. When I completed my university studies, it was hypothesized that the circularly polarized ultraviolet light observed in clouds in which young stars are forming, the same environment where the chemistry that gives rise to amino acids can take place, was responsible for the preferential formation of L-type amino acids in photosynthesis reactions; but, while for almost all the other amino acids this hypothesis was confirmed, at least in the case of one amino acid this light favors the formation of the compound with chirality D, instead of the biogenic one of type L.

In this context, a new work has tested an alternative hypothesis. Instead of polarized far-UV radiation, the researchers hypothesized the L-type shapes were specifically induced by a particular radiation of the hydrogen atom that permeated the early Milky Way (the so-called R-CP Lyα radiation). Furthermore, instead of focusing only on the photoreactions in amino acids, the researchers investigated the possibility of inducing the right chirality starting from the precursors that gave rise to amino acids in the cosmos. Well, by illuminating the precursors with this ancient hydrogen light and after reacting with water in the presence of heat, a strong prevalence of type L amino acids is obtained, and a smaller quantity of those of type D, in ratios similar to those found later in asteroids that sometimes affect our planet.

The study thus brings us one step closer to understanding the complex history of our own biochemistrysolving a conundrum that biochemistry professors usually nervously overlook when teaching their students, and demonstrating once again how much and to what extent the molecular properties of life on our planet have been determined by conditions occurring in deep space , billions of years ago.

Before biological evolution on planet Earth, a long chemical evolution preceded the birth of the first replicators, forming chemical varieties on which natural selection was then able to act, once the Darwinian process has begun; and we bear traces of that very ancient process not only in the kind of atoms we are made of, but also and perhaps even more importantly in the way they are joined together to form the molecular building blocks of life. Bricks which, having formed everywhere in the cosmos, are everywhere available for the triggering of life itself.