Homocysteine-Thiolactone - Homocysteine in Protein Structure/Function and Human Disease - page 32

converts Hcy-thiolactone to Hcy, which is then detected by its ability to induce
aggregation of the FSN-AuNPs, quantified by measurements of absorption ratios at
610 and 550 nm, A 610 /A 550 . The selectivity of the gold nanoparticle probe is
significantly higher for Hcy-thiolactone than for aminothiols. The sensitivity of
FSN-AuNPs toward Hcy-thiolactone can be further improved by optimizing the
AuNPs concentration. The lowest detectable concentration of HTL is 100 nM. This
approach has been validated by quantification of Hcy-thiolactone in human urine
samples [229].
3.4 Biosynthesis
3.4.1 The Involvement of Methionyl-tRNA Synthetase
The first indication that Hcy-thiolactone is likely to form biologically in living
organisms came with the 1981 discovery of the enzymatic conversion of Hcy to
Hcy-thiolactone in error-editing reactions of aminoacyl-tRNA synthetases
(AARSs) [210]. AARSs are responsible for quality control [65] in the flow of
information from a gene to protein and carry out two basic functions in protein
biosynthesis: (a) provide chemically activated amino acids (in the form of
aminoacyl-tRNA) and (b) mediate the transfer of genetic information [6]. The
chemical activation involves a reaction with ATP, generating a high-energy inter-
mediate, AARS•AA-AMP (Reaction 3.6 ), which subsequently reacts with tRNA to
form a high-energy ester bond between the carboxyl group of an amino acid and a
hydroxyl of the 3 0 -terminal ribose of tRNA (Reaction 3.7 ).
The genetic information transfer involves matching amino acids with
corresponding tRNAs according to the rules of the genetic code. This in effect
translates the words from the nucleic acid language into the words in the protein
language. Thus, each of the twenty canonical AARSs translates its own set of
nucleic acid words (triplets of bases) into a protein word (amino acid). For example,
methionyl-tRNA synthetase (MetRS) translates the word “AUG” in the nucleic acid
language as “methionine” in the protein language. Similarly, CysRS assures that
words “UGU” and “UGC” in the nucleic acid language mean “cysteine” in the
protein language.
High accuracy of an AARS in the tRNA aminoacylation reaction is achieved by
preferential binding of a cognate amino acid and a quality control step in which
non-cognate amino acids are edited [6, 230]. Editing is used to achieve high
selectivity when the differences in binding energies of amino acids to AARSs are
inadequate [65, 231].
Because of its similarity to the protein amino acid methionine, during protein
biosynthesis, the nonprotein amino acid Hcy is often incorrectly selected in place of
methionine by MetRS. (The major role of MetRS is, of course, to catalyze the
synthesis of methionyl-tRNA Met , with Met-AMP as an intermediate.) Thus, in the