Translation and Protein Biosynthesis PDF

+ Amino acids  A. Amino acids All the amino acids that eventually appear in the finished protein must be present at the time of protein synthesis. [Note: If one amino acid is missing (for example, if the diet does not contain an essential amino acid)...

+ Amino acids  A. Amino acids All the amino acids that eventually appear in the finished protein must be present at the time of protein synthesis. [Note: If one amino acid is missing (for example, if the diet does not contain an essential amino acid), translation stops at the codon specifying that amino acid. This demonstrates the importance of having all the essential amino acids in sufficient quantities in the diet to ensure continued protein synthesis.  B. Transfer RNA B. Transfer RNA At least one specific type of tRNA is required for each amino acid. In humans, there are at least 50 species of tRNA, whereas bacteria contain 30–40 species. Because there are only 20 different amino acids commonly carried by tRNA, some amino acids have more than one specific tRNA molecule. This is particularly true of those amino acids that are coded for by several codons. B. Transfer RNA 1. Amino acid attachment site: Each tRNA molecule has an attachment site for a specific (cognate) amino acid at its 3'-end. The carboxyl group of the amino acid is in an ester linkage with the 3'-hydroxyl of the ribose portion of the adenosine (A) nucleotide in the —CCA sequence at the 3'-end of the tRNA. [Note: When a tRNA has a covalently attached amino acid, it is said to be charged; when it does not, it is said to be uncharged. The amino acid attached to the tRNA molecule is said to be activated.  B. Transfer RNA 2. Anticodon: Each tRNA molecule also contains a three-base nucleotide sequence—the anticodon—that pairs with a specific codon on the mRNA. This codon specifies the insertion into the growing peptide chain of the amino acid carried by that tRNA.  B. Transfer RNA C. Aminoacyl-tRNA synthetases This family of enzymes is required for attachment of amino acids to their corresponding tRNAs. Each member of this family recognizes a specific amino acid and all the tRNAs that correspond to that amino acid (isoaccepting tRNAs). Aminoacyl-tRNA synthetases catalyze a two-step reaction that results in the covalent attachment of the carboxyl group of an amino acid to the 3'-end of its corresponding (cognate) tRNA. The overall reaction requires adenosine triphosphate (ATP), which is cleaved to adenosine monophosphate (AMP) and inorganic pyrophosphate (PPi).  B. Transfer RNA C. Aminoacyl-tRNA synthetases The extreme specificity of the synthetase in recognizing both the amino acid and its cognate tRNA contributes to the high fidelity of translation of the genetic message. In addition, the synthetases have a “proofreading” or “editing” activity that can remove amino acids from the enzyme or the tRNA molecule D. Messenger RNA The specific mRNA required as a template for the synthesis of the desired polypeptide chain must be present. 16S ribosomal RNA (or 16S rRNA) is the RNA component of the 30S subunit of a prokaryotic ribosome ( SSU rRNA). It binds to the Shine-Dalgarno sequence  Shine-Dalgarno (SD) sequence: -The Shine-Dalgarno sequence, a purine-rich nucleotide base in E. coli, is located six to ten bases upstream of the initiating AUG codon on the mRNA molecule. -The 16S rRNA component of the 30S ribosomal subunit has a nucleotide sequence near its 3'-end that is complementary to the SD sequence. -This allows the small (30S) ribosomal subunit to position itself in close proximity to the AUG codon. - In eukaryotes, Eukaryotic messages do not have SD sequences. The 40S ribosomal subunit moves down the mRNA, requiring ATP for scanning.  Ribosomes E. Ribosomes are large complexes of protein and ribosomal RNA (rRNA). They consist of two subunits—one large and one small—whose relative sizes are given in terms of their sedimentation coefficients, or S (Svedberg) values. [Note: Because the S values are determined both by shape as well as molecular mass, their numeric values are not strictly additive. For example, the prokaryotic 50S and 30S ribosomal subunits together form a 70S ribosome. The eukaryotic 60S and 40S subunits form an 80S ribosome.]  Translation and Post-translational Modifications in Protein Biosynthesis Phosphorylation of glycogen phosphorylase enzyme result in increase enzyme activity. phosphorylation of glycogen synthase enzyme result in less