Discipline: Biological Sciences
Subcategory: STEM Research
Joseph Onyilagha - University of Arkansas at Pine Bluff
Co-Author(s): D. Machooka and M. Moorehead, University of Arkansas at Pine Bluff S. Freeland, University of Maryland, Baltimore County
The origin of genetic coding is intriguing. From a pool of available molecules, life ended up using four nucleotides and twenty amino acids to encode and build its proteins. By the time of the Last Universal Common Ancestor (LUCA), the process of protein translation was largely fixed in the form of the standard genetic code. The intention of this research is to determine whether metabolic pathways found in living organisms are indeed an accurate guide to ancient evolutionary events. The project goal is to provide additional insight into the emergence of a standard alphabet of 20 genetically encoded amino acids. Consequently, we investigated the assertion that the 20 standard amino acids of the genetic code consists of early and late members, and that the late amino acids are ‘inventions’ of early metabolism. In other words, genetic coding began with fewer than 20 amino acids. This ‘early’ alphabet (comprising prebiotically plausible amino acids) was then augmented as metabolism evolved new possibilities, and incorporated them into genetic coding. Contemporary data from databases were used. Pathways to amino acid biosynthesis were analyzed. The synquences of enzymes catalyzing the pathways were also studied. Our findings are consistent with earlier reports that the 20 amino acids of the standard genetic code comprise of two different groups: ‘early’ amino acids that were likely available at the origin for life through prebiotic syntheses, and ‘late’ amino acids that are best understood as inventions of biology itself. The results show that steps in the biosynthetic pathways of many of the late amino acids are longer than those of the early amino acids. Again, longer step means the involvement of many more enzymes. Additionally, late amino acid members are synthesized through several more pathways than the early amino acids.
However, our results are at variance with some of the theories surrounding the evolution or emergence of the 20 encoded amino acids of the standard genetic code, especially the ‘precursor-product’ assertion of the ‘Co-Evolution theory’. Firstly, the theory asserts that each new amino acid (product) is synthesized from a pre-existing precursor. For example glutamine is synthesized from glutamic acid; tryptophan and cysteine are synthesized from serine. Secondly, the theory asserts that this relationship is reflected in the assignment of amino acids to codons within the genetic code. These assertions are inconsistent with our findings. For example, there is no precursor-product relationship that connects Glycine (Gly) and Threonine (Thr) amino acids as claimed by the Co-Evolutionary theory. In other words, Thr cannot be synthesized from Gly and vice-versa. Nevertheless, our results show new pathways that facilitate the biosyntheses of Thr from Gly and/or vice-versa. Consequently, we conclude that the origin of amino acids of the standard genetic code is far from being resolved and that there is need for a critical evaluation of the theories surrounding the emergence of the 20 standard amino acids.
Funder Acknowledgement(s): Research is funded by the NSF HBCU-UP program.
Faculty Advisor: None Listed,