BCH Graduate Survey of Biochemistry

Transcription

1 BCH 5045 Graduate Survey of Biochemistry Instructor: Charles Guy Producer: Ron Thomas Director: Marsha Durosier Lecture 31 Slide sets available at:

2 David L. Nelson and Michael M. Cox LEHNINGER PRINCIPLES OF BIOCHEMISTRY Fifth Edition CHAPTER 27 Protein Metabolism 2008 W. H. Freeman and Company

3 The Nobel Prize in Physiology or Medicine 1968: A University of Florida Bachelor s (1948) and Master s degree (1952) Graduate in Zoology from Orlando, FL. His group added a synthetic poly-uracil RNA to a cell-free extract of Escherichia coli ribosomes and other cellular machinery that could possibly function in protein synthesis. DNase was used to remove DNA so that no additional proteins would be produced other than that from the synthetic polyu- RNA. They then added 1 radioactively labeled amino acid, and 19 unlabeled amino acids to each extract, varying the labeled amino acid in each sample. In the extract containing the radioactively labeled polyphenylalanine, the resulting polypeptide was also radioactive demonstrating that polyu contained the genetic code for phenylalanine. Next question, how big is the code word?

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5 Francis Crick postulated an adaptor hypothesis that could function as a decoder of the DNA through mrna sequences.

6 The genetic code for the protein amino acids. It turns out there is redundancy (degeneracy) in the genetic code. What about selenocysteine and pyrrolysine?

7 This slide not in lecture video The human genome encodes 25 selenoproteins, so selenocysteine is not a common protein amino acid. Selenocysteinyl-tRNA Sec requires a specific translation factor that allows a characteristic stem-loop structure in the mrna to redefine an in-frame UGA from a stop codon into a Sec sense codon. Serine is the precursor of Sec in bacteria. Serine is attached to the trna Sec by a seryl-trna synthetase enzyme. The Ser-tRNA Sec is converted to SectRNA Sec by the action of selenocysteine synthase that requires the selenium donor selenophosphate. In archaea and eukaryotes, another enzymatic step is required for Sec biosynthesis. O-phosphoseryl-tRNA kinase catalyzes the phosphorylation of Ser-tRNA Sec to form O- phosphoseryl-trna Sec. The Sep-tRNA:Sec-tRNA synthase then catalyzes the formation of the final product Sec-tRNA Sec from selenophosphate and Sep-tRNA Sec. Yuan et al. (2010) FEBS Lett. 584:

8 This slide not in lecture video Even more rare, pyrrolysine (Pyl) is found to be genetic encoded in about 1% of all of sequenced genomes to date. Genes encoding pyrrolysine containing methylaminemethyltransferases have an in-frame amber (TAG) codon, typically a stop codon for most organisms. During translation, a methylamine methyltransferase mrna is read-through to the next opal or ochre stop codon and then Pyl is inserted into the polypeptide chain in-response to an in-frame amber codon. Pyl is introduced into the genetic code by the catalytic action of a pyrrolysyl-trna synthetase enzyme that ligates pyrrolysine to the amber codon decoding trna Pyl. Yuan et al. (2010) FEBS Lett. 584:

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10 The genetic code has three possible reading frames. Frame shifting in the Rous sarcoma viral RNA. In a repeating tetranucleotide of GUAA, every fourth triplet is a stop codon.

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13 Posttranscriptional RNA Editing. RNA editing of apob-100 transcript occurs only in the intestine.

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