Williams–Beuren syndrome- Deletion on 7, long arm

TLS Online TPP Program

#Id: 7075

#Unit 3. Fundamental Processes

isoleucyl-tRNA synthetase has a nearby editing pocket (a deep cleft in the enzyme) that allows it to proofread the product of the adenylylation reaction. AMP-valine (as well as adenylylates of other small amino acids, such as alanine) can fit into this editing pocket, where it is hydrolyzed and released as free valine and AMP. In contrast, AMP-isoleucine is too large to enter the editing pocket and is therefore not subject to hydrolysis. As a consequence, isoleucyl-tRNA synthetase discriminates against valine twice: in the initial binding and adenylylation of the amino acid (discriminating by a factor of 100), and then in the editing of the adenylylated amino acid (again discriminating by a factor of 100), for an

overall selectivity of 10,000-fold (i.e., an error rate of 0.01%).

TLS Online TPP Program

#Id: 7076

#Unit 3. Fundamental Processes

TLS Online TPP Program

#Id: 7077

#Unit 3. Fundamental Processes

ThrRS has the opposite problem: It must synthesize Thr–tRNAThr but not Val–tRNAThr. Specificity is conferred by the aminoacylation site, which contains a Zn2+ ion that is coordinated by the side chain OH group of threonine. Valine cannot coordinate the Zn2+ in this way and hence does not undergo adenylylation by ThrRS.  A separate editing site deals with misacylated Ser–tRNAThr. TyrRS distinguishes between tyrosine and phenylalanine through hydrogen missing from the repertoire are GlnRS and asparaginyl-tRNA synthetase (AsnRS). To synthesize Gln-tRNAGln and Asn-tRNAAsn, these organisms possess distinct glutamyl-tRNA synthetase (GluRS) and aspartyl-tRNA synthetase (AspRS) enzymes that are nondiscriminating (ND). GluRSND synthesizes both Glu-tRNAGlu as well as misacylated Glu-tRNAGln; AspRSND synthesizes both Asp-tRNAAsp and misacylated Asp-tRNAAsn.

TLS Online TPP Program

#Id: 7078

#Unit 3. Fundamental Processes

Selenocysteine is generated enzymatically from serine carried on a special tRNA that is charged by serine-tRNA synthetase. The Sec residues of selenoproteins are thought to participate in redox reactions such as those catalyzed by mammalian glutathione peroxidase and thioredoxin reductase

TLS Online TPP Program

#Id: 7079

#Unit 3. Fundamental Processes

Selenocysteine, sometimes called the “twenty-first amino acid,”

is incorporated into proteins with the aid of a tRNA that interprets the UGA Stop codon as a Sec codon. tRNASec is initially charged with serine in a reaction catalyzed by the same SerRS that charges tRNASer.

TLS Online TPP Program

#Id: 7080

#Unit 3. Fundamental Processes

A dedicated protein (a special elongation factor) named SELB in complex with GTP is required to deliver Sec–tRNASec to the ribosome. SELB ∙ GTP ∙ Sec–tRNASec reads the UGA codon as Sec rather than “Stop,” provided that the ribosomally bound

mRNA has a hairpin loop on the 3′ side of the UGA specifying Sec.