Background: Prime mass amino acids residues, assigned based upon the nominal mass of their repeating structure in peptides and proteins, have been found to occur more often than by chance. These 9 such residues are predominantly hydrophobic in character and thus play an important role in protein stability and folding. This study investigates their prevalence in human proteins and the function of such proteins. Objectives: To identify human proteins rich in the prime mass residues and assess their functions. Results: Prime-rich proteins are found to proliferate among those important to transcriptional events, cell adhesion, and the formation of cell surface membranes. Eighteen prime-rich proteins are identified that are abundant in 3 or 4 prime residues, the latter notably contain alanine, cysteine, histidine or threonine, and proline. The percentage of prime residues in these prime-rich proteins exceeds one-half (>50%) of all residues in many cases, and these residues are found to be concentrated in particular regions of the protein. Conclusion: Proteins rich in prime residues are found to play an important role in biological events important to cellular life. Such prime residues represent those associated with the highest hydrophobicity, and those essential to protein folding and function. Many prime residues were recruited early into the genetic code according to the composition of the last universal common ancestor (LUCA) proteins. Consideration is given to whether the formation of evolution of these proteins is predicated by the primality of the residues or their physicochemical and structural properties. This remains an open question and awaits further inquiry and debate.
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