Carbon cycle, nitrogen cycle, and oxygen cycle atoms circulate through photorespiration

TLS Online TPP Program

#Id: 3656

#Unit 3. Fundamental Processes

Certain proteins, such as the enzymes glutathione peroxidase and formate dehydrogenase, contain an unusual amino acid called selenocysteine, which is part of the catalytic center of the enzymes.

TLS Online TPP Program

#Id: 3657

#Unit 3. Fundamental Processes

Human proteome contains 25 selenoproteins. Selenocysteine, sometimes called the “twenty-first amino acid,”

TLS Online TPP Program

#Id: 3658

#Unit 3. Fundamental Processes

Selenocysteine 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: 3659

#Unit 3. Fundamental Processes

Instead, a dedicated protein (a special elongation factor) named SELB in complex with GTP is required to deliver Sec–tRNASec to the ribosome.

TLS Online TPP Program

#Id: 3660

#Unit 3. Fundamental Processes

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: 3661

#Unit 3. Fundamental Processes

The gene can be restored to normal function if a second mutation either (1)converts the misplaced termination codon to a codon specifying an amino acid or (2) suppresses the effects of the termination codon, such restorative mutations are called nonsense suppressors.