Senin, 13 Juni 2016

Translation I: initiation
Like amino acid activation (see p. 248), protein biosynthesis (translation) takes place in the cytoplasm. It is catalyzed by complex nucleo protein particles, the ribosomes,and mainly requires GTP to cover its energy requirements.
A. Structure of the eukaryotic ribosome
Ribosomes consist of two sub units of different size, made up of ribosomal RNA (rRNA) and nearly 80 proteins(the number of proteins applies to rat liver ribosomes). It is customary to give the sedimentation coeficients (see p. 200) of ribosomes and their components instead of their masses. For example, the eukaryotic ribosome has a sedimentation coefficient of 80 Svedberg units (80 S), while the sedimentation coeficients of its sub units are 40 S and 60 S (S values are not additive).
The smaller 40 S sub unit consists of one molecule of 18 S rRNA and 33 protein molecules. The larger 60 S sub unit contains three types of rRNA with sedimentation coeficients of  5S, 5.8S, and 28S and 47 proteins. In the presence of mRNA, the sub units assemble to form the complete ribosome, with a mass about 650 times larger than that of a hemoglobin molecule. The arrangement of the individual components of a ribosome has now been determined for prokaryotic ribosomes. It is known that filamentous mRNA passes through a cleft between the two sub units near the characteristic “horn” on the small sub unit. tRNAs also bind near this site. The illustration shows the size of at RNA molecule for comparison. Prokaryotic ribosomes have a similar structure, but are somewhat smaller than those of eukaryotes (sedimentation coeficient 70 S for the complete ribosome, 30 S and 50 S for the subunits). Mitochondrial and chloroplast ribosomes are comparable to prokaryotic ones.
B. Polysomes
In cells that are carrying out intensive protein synthesis, ribosomes are often found in a linear arrangement like a string of pearls; these are known as polysomes. This arrangement arises because several ribosomes are translating a single mRNA molecule simultaneously.  The ribosome first binds near the start codon (AUG; see p. 248) at the 5 end of the mRNA (top). During translation, the ribosome moves in the direction of the 3 end until it reaches a stop codon (UAA, UAG, or UGA). At this point, the newly synthesized chain is released, and the ribosome dissociates again into its two
subunits.
C. Initiation of translation in E. coli
Protein synthesis in prokaryotes is in principle the same as in eukaryotes. However, as the process is simpler and has been better studied in prokaryotes, the details involved in translation are discussed here and on p. 252 using the example of the bacterium Escherichia coli. The first phase of translation, initiation,involves several steps. First, two proteins,initiation factors IF–1 and IF–3, bind to the 30 S
subunit (1). Another factor, IF–2, binds as a complex with GTP (2). This allows the subunit to associate with the mRNA and makes it possible for a special tRNA to bind to the start codon (3). In prokaryotes, this starter tRNA carries the substituted amino acid Nformylmethionine(fMet). In eukaryotes, it carries an unsubstituted methionine. Finally, the 50 S subunit binds to the above complex (4). During steps 3 and 4, the initiation factors are released again, and the GTP bound to IF–2 is hydrolyzed to GDP and Pi.
In the 70 S initiation complex, formylmethionine tRNA is initially located at a binding site known as the peptidyl site (P). A second  binding site, the acceptor site (A),is not yet occupied during this phase of translation. Sometimes, a third tRNA binding site is defined as an exit site (E),from which uncharged tRNAs leave the ribosome again (see p. 252;not shown)