Ribosome prokaryotic, functional sites

The elongation cycle in eukaryotes is quite similar to that in prokaryotes. Three eukaryotic elongation factors (eEFla, eEFljSy, and eEF2) have functions analogous to those of the bacterial elongation factors (EF-Tu, EF-Ts, and EF-G, respectively). Eukaryotic ribosomes do not have an E site uncharged tRNAs are expelled directly from the P site. 

Translation is relatively rapid in prokaryotes. For example, an E. coli ribosome can incorporate as many as 15 to 20 amino acids per second. (The eukaryotic rate, at about 50 residues per minute, is significantly slower.) Recall that prokaryotic ribosomes are composed of a 50S large subunit and a 30S small subunit. The large subunit contains the catalytic site for peptide bond formation. The small subunit serves as a guide for the translation factors required to regulate the process. Figure 19.5 provides a three-dimensional reconstruction of a functioning E. coli ribosome. 

EF-Tu (Figure 19A), the protein factor that positions aminoacyl-tRNA complexes in the A site of prokaryotic ribosomes, is a well-researched example of a GTP-binding motor protein. Recall that motor proteins (Section 2.1) use nucleotide hydrolysis to drive changes in their own conformations that are often used to promote ordered conformational changes in adjacent molecules or subunits. In other words, motor proteins, often called NTPases, function as mechanochemical transducers. These NTP-hydrolysis driven conformational changes, which principally occur in localized structural units called switches, alter the affinity of the NTPase for other molecules. 

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