Translocation step

In the translocation step, the ribosome moves exactly three nudeotides (one codon) along the message. This moves the growing peptidyl-tRNA into the P site and aligns the next codon to be translated with the empty A site. 

EF-G seems to be the motor protein that drives translocation in the 30S subunit. When it is not attached to a ribosome, the EF-G GTP complex is very stable, but in its functioning location GTP is hydrolyzed rapidly. This occurs prior to translocation398 3983 and presumably causes an internal alteration in the ribosome that energizes it for the translocation step. 

One is consumed in the translocation step, plus additions for corrections. 

In the second frame (b), strong bridges form between actin and myosin. This is followed by a structural alteration in the myosin molecules and an effective translocation of the thick filament relative to the thin filament in (c). During this process the ADP is released. After the translocation step, the bridge structure is broken by the binding of ATP, which is rapidly hydrolyzed to ADP and Pj. Each thick filament has about 500 myosin heads, and each head cycles about five times per second in the course of a rapid contraction. 

This reaction is reversible when conducted in vitro, but under the conditions of pH and nicotinamide concentration that exist in the cell, it is irreversible. Thus, diphtheria toxin kills cells by irreversibly destroying the ability of EF-2 to participate in the translocation step of protein synthesis elongation. A number of other protein toxins have subsequently been found to ADP-ribosylate and inactivate cellular proteins involved in other essential cellular pathways. For example, cholera and pertussis toxins ADP-ribosylate and inactivate proteins important to cAMP metabolism. 

Ab initio calculations at the CIS, CASSCF and MRMP2 (second-order multireference Mpller-Plesset perturbation theory) levels have revealed that the reaction proceeds by H atom transfer via a series of Grotthus-type translocation steps [15]. Figure 3.35 gives an overview of these computational results. 

There are a number of sites within the sequence of protein synthesis where antibiotics can act. These include (1) inhibition of the attachment of mRNA to 30S ribosomes by aminoglycosides (2) inhibition of tRNA binding to 30S ribosomes by tetracyclines (3) inhibition of the attachment of mRNA to the 50S ribosome by chloramphenicol and (4) erythromycin inhibition of the translocation step by binding to 50S ribosomes, thus preventing newly synthesized peptidyl tRNA moving from the acceptor to the donor site. 

The macrolides bind irreversibly to a site on the 50S subunit of the bacterial ribosome, thus inhibiting the translocation steps of protein synthesis. Generally considered to be bacteriostatic, they may be cidal at higher doses. The binding site is either identical to or in close proximity to that for lincomycin, clindamycin, and chloramphenicol. 

C. Initiation and formation of the first peptide bond occurred therefore. A, B, and D were not affected. The antibiotic most likely affects the translocation step. 

FIGURE 9.13 Transport cycle and stoichiometry of excitatory amino acid transporters (EAATs). Simplified state diagram of the EAAT transport cycle. After glutamate and coupled ions (step 1) bind to the transporter (T), they are translocated (step 2) and released into the cell cytosol (step 3). Next binds from the intracellular side (step 4) and reorients the substrate-fiEe transporter (step 5). is released outside the cell (step 6). (From Jiang Amara, 2011. Reproduced with permission from Elsevier.)... 

The translocation step is probably the point at which prokaryotic and eukaryotic secretion differ most. The energy for this process may derive from different sources from the energy of protein synthesis in eukaryotes, and from protein synthesis and/or ATP hydrolysis and/or the membrane potential in prokaryotes. [In fact there is evidence for more than one secretion pathway in E. coli. The degree of coupling between translation and translocation may also be different in prokaryotes and eukaryotes (Section V,C).]... 

Biochemical site of action of erythromycin Inhibits bacterial protein biosynthesis at the translocation step of translation... 

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