Abstract
Tryptophan (Trp) holds a unique place in biology for a multitude of reasons. It is the largest of all twenty amino acids in the translational toolbox. Its side chain is indole, which is aromatic with a binuclear ring structure, whereas those of Phe, Tyr, and His are single-ring aromatics. In part due to these elaborate structural features, the biosynthetic pathway of Trp is the most complex and the most energy-consuming among all amino acids. Essential in the animal diet, Trp is also the least abundant amino acid in the cell, and one of the rarest in the proteome. In most eukaryotes, Trp is the only amino acid besides Met, which is coded for by a single codon, namely UGG. Due to the large and hydrophobic π-electron surface area, its aromatic side chain interacts with multiple other side chains in the protein, befitting its strategic locations in the protein structure. Finally, several Trp derivatives, namely tryptophylquinone, oxitriptan, serotonin, melatonin, and tryptophol, have specialized functions. Overall, Trp is a scarce and precious amino acid in the cell, such that nature uses it parsimoniously, for multiple but selective functions. Here, the various aspects of the uniqueness of Trp are presented in molecular terms.
Highlights
Tryptophan (Trp, W) is one of three aromatic amino acids that minimally contain a six-membered benzene ring in their side chains, the other two being phenylalanine (Phe, F) and tyrosine (Tyr, Y).Whereas Tyr is a p-hydroxy derivative of Phe, the side chain of Trp is indole, which is more complex, as it is a six-membered benzene ring fused to a five-membered pyrrole ring with an integratedNH group
Even outside of the synonymous codons, single nucleotide changes in many codons may lead to only other amino acid, andimplications the only internal encoded by have a code single nonredundant codon, is Trp, encoded speculation and by research since the discovery of theacid, complete genetic
Trp was confirmed to occur at positions 3 and 16 (Figure 6A), which are near the beginning of the two signature pentatricopeptide (35 amino acid) repeats (PPRs) helices, designated as helix A and helix B [77]
Summary
Tryptophan (Trp, W) is one of three aromatic amino acids that minimally contain a six-membered benzene ring in their side chains, the other two being phenylalanine (Phe, F) and tyrosine (Tyr, Y). Even outside of the synonymous codons, single nucleotide changes in many codons may lead to only other amino acid, andimplications the only internal encoded by have a code single nonredundant codon, is Trp, encoded speculation and by research since the discovery of theacid, complete genetic. Base-pairing, as speculation and occurs research the position discovery complete genetic code [7,8,9].a The redundancy, to Thr, hydroxy amino acids that are oftenoffunctionally similar Trp in amino of as very different base-pairing recognition during translation, the third position is relatively tolerant to nucleotide This is largely due to will theacids structural genetic mechanism such that errors inmismatch. Sci. 2020, 21,obtained more expensive than the others, Trp being the highest of all
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