tailieunhanh - Biochemistry, 4th Edition P103

Biochemistry, 4th Edition P103. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | How Are Proteins Synthesized in Eukaryotic Cells 983 Chloramphenicol Fusidic acid Tetracycline OCH3 Streptomycin Puromycin FIGURE The structures of various antibiotics that act as protein synthesis inhibitors. Puromycin mimics the structure of aminoacyl-tRNA in that it resembles the 3 -terminus of a Tyr-tRNA shaded box . b 5 3 I I C G G C 1406 U U1495 CG ak J XA1493 1408 Â1492 1409 C G1491 AU CG CG FIGURE a Structure of geneticin a representative aminoglycoside antibiotic. Note the characteristic 2-deoxystreptamine 2-DOS core structure in red. b The base sequence of the small RNA loop within the 16S rRNA decoding center. Note that unpaired adenine residues 1408 1492 and 1493 constitute the internal loop structure. Adapted from Figure 1 in Hermann 2005. Drugs targeting the ribosome. Current Opinion in Structural Biology 15 355-366. 984 Chapter 30 Protein Synthesis The most common effect of antibiotics that interact with the PTC is to occupy space within this center such that the amino acid or peptidyl chain linked at the 3 -end of a tRNA cannot be positioned properly for the peptide-bond forming reaction. This mode of inhibition is consistent with the catalytic role of PTC in precisely orienting the substrates so that the peptide bond-forming reaction can occur. This effect is more common for aminoacyl-tRNAs in the A site although some drugs can bridge the A and P sites and affect both aminoacyl-tRNA and peptidyl-tRNA orientation. Ribosomes with long peptidyl chains attached to the tRNA in the P site are less susceptible to macrolide antibiotics. SUMMARY What Is the Genetic Code The genetic code is the code of bases that specifies the sequence of amino acids in a protein. The genetic code is a triplet code. Given the four RNA bases A C G and U a total of 43 64 three-letter codons are available to specify the 20 amino acids found in proteins. Of these 64 codons 61 are used for amino acids and the remaining 3 are nonsense or stop codons. The .