ACTH (corticotropin) 

controls the secretion of cortisol by the activation of adenylyl cyclase via Galpha s.

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.

TLS Online TPP Program

#Id: 7081

#Unit 3. Fundamental Processes

The tRNAAla structural elements recognized by then Ala-tRNA synthetase are unusually simple.  A single G=U base pair 3:70 is the only element needed for specific binding and aminoacylation

TLS Online TPP Program

#Id: 7082

#Unit 3. Fundamental Processes