Proteins resurrected from reconstructions of prototypic forms may have existed in single-celled organisms that are the progenitors of all life.
The results of a study of one such protein were reported by Spanish and US scientists in the journal Structure.
Through computer analysis, gene sequences in a protein called thioredoxin, sampled from many modern organisms, were tracked backwards to those that may have been extant four billion years ago. Bacteria were utilized to create chemically active proteins using the ancient models. This process allowed scientists to determine the molecular structure and the properties of the predecessor protein.
The thioredoxin protein was selected because it is an enzyme with a variety of metabolic functions in cells, and is shared by almost all earthly life, from the simplest bacteria to human beings. It may be hypothesized that the single-celled ancestor of all life on Earth may have had the gene.
To avoid misidentifying mutations as originals in the evolutionary process, biochemically active reconstructions were created that folded up into three dimensional structures resembling those of modern proteins. This methodology was relied upon to validate the researchers’ approach. Molecular clocks were used to relate the reconstructions to established evolutionary branches and to geological changes over time.
Processive changes were indicated by a precipitant lengthening of the protein’s helical structure as cells began developing nuclei. Prokaryotes are cells that don’t have nuclei, while eukaryotes do. Nucleated cells are the basis of higher forms of life, including that of animals and plants. However, eukaryotes constitute a very small minority of all living things. The largest group of prokaryotes is archaea, single-celled organisms that can sustain themselves in extreme environmental conditions.
Within these organisms may have been proteins such as thioredoxin. Thus thioredoxin might be interlinked with the archetypes of life.
The changes in the helical structure of proteins over time suggest that biological systems may have evolved at the molecular level in fits and starts rather than in an orderly manner. Evolutionary studies have already established the concept of “punctuated equilibrium,” in which stasis is interrupted by rapid and unanticipated development.
Four billion years ago, ancient thioredoxin may have endured temperatures of more than 110° C, an acidic atmosphere and bombardments of meteorites. Proteins such as thioredoxin may have shared features with extremophiles, such as prokaryotic archaea, organisms that can thrive in physical or geochemical conditions that are detrimental to most other life on Earth.
Extremophiles can endure intense pressure and heat, and can flourish in environments without oxygen. They exist under the ocean, at high altitudes, inside rock and in hydrothermal vents (fissures in the planet’s surface from which geothermally heated water emerge). Extremophiles and progenitor proteins may have been the only forms of life that survived such hellish primal conditions.
There is also speculation that ancient protein rode on meteorites to Earth four billion years ago, emigrating from other planets such as Mars, as these planets experienced climatic changes that made them increasingly hostile to the protein. Mars may well have been a more conducive place for protein to be than Earth during the first 500 million years after the solar system’s formation.
The word protein is derived from the Greek proteios, or “primary,” and protos, meaning “first.” Proteins are the basic elements of living cells. They are required by the human body to build, maintain and repair body tissues, especially bone cartilage and muscle. The body uses proteins for energy when carbohydrates and fats cannot meet its energy needs. There are an estimated 100,000 different proteins in the human body.
Protein is vital to human life, and it may have existed even as life began to emerge on the Earth.
By: Tom Ukinski
Sources: BBC News Scientific American